Ramsar site management plans -- Bulgaria, Srebarna

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Sofia, 2000


1.1. Date this sheet was completed/updated - March 2000

1.2. Country - Bulgaria

1.3. Name of wetland - Srebarna Lake

1.4. Geographic coordinates - 440 07 N, 270 04 E; UTM grid NJ 08

1.5. Altitude - 10-13,2 m a. s. l.

1.6. Area - 902.1 ha of the reserve proper; 542.8 ha of the buffer zone; 600 ha of the Ramsar Site

1.7. Overview - Date of designating the Site for the Ramsar List: September 1975

Legal Status: In 1942 the area was declared as a Protected Site. Since 1948 it has become a Nature Reserve. In 1975 it was designated a Wetland with International Importance for Waterfowl (a Ramsar Site). Since 1977 it has been a Reserve of the Biosphere. In 1983 Srebarna has been included in the List of the World Heritage Sites under the UNESCO Convention for the Conservation of the World Cultural and Natural Heritage. Access allowed only for research staff with research access permit issued upon request by the Ministry of Environment and Waters. In compliance with requirements of the Law for the Protection of Nature all tourist and other commercial activities in the reserve are prohibited. In 1993 after the reserve was listed as 'a wetland in danger', a Ramsar Monitoring Procedure was opened in accordance with a decision of the 1990 Montreux meeting.

The Site is a hypertrophic lake located on the Bulgarian (right-hand) bank of the Danube between km 391 and km 393. The Nature Reserve with a total surface area of 902.1 ha is a state property, adjacent lands being privately owned or belonging to the Municipality of Silistra. Until 1975 there was commercial fishing and mowing of Reed, Reedmace and Bulrush in the reserve. Since then all commercial activities there have been terminated.

Brief Physical Geographic Characteristics: The lake bowl lies in the Pliocene clays overlying the Barem and Apt limestones. In 1949 the lake was isolated from the Danube by a dike and since then the only source for feeding the lake with water remained the springs and surface run-off from neighbouring hills. Connection with the Danube was somewhat restored in 1978 but the river waters did not enter the lake every year. The lack of adequate connection with the Danube for prolonged periods of time set hydrologic conditions favouring the accumulation of bottom sediments (organic and inorganic sludge) at the same time diminishing the lake maximum and average depth to only 1 m (measured in 1993). In 1994 a canal connecting the Danube with the lake was built with the financial support of international organisations. It improved considerably the ecological conditions in the lake. An increase in the number and diversity of bird species typical for the region has been reported.

Flora and Fauna: The dominant plant association is that of the Reed (Phragmites australis) which covers about 2/3 of the Reserve's total surface area. At places Goat Willow (Salix caprea), Gray Willow (Salix cinerea) and Purple Osier (Salix purpurea) bushes grow. In terms of abundance the Lesser Reedmace (Typha angustifolia) holds the second place after the Reed and is more widely spread than the Great Reedmace (Typha latifolia) and other species. Of all the vascular plant species found in the Reserve 11 are rare or threatened with extinction. Till 1946 some 19 fish species occurred in the lake. Reptiles and amphibians number 21 species. Birds are the most diverse group in the Reserve. The total number of species found in and around Srebarna Lake is 179. Ninety nine of them breed there. The colony of the Dalmatian Pelican (Pelecanus crispus) is the pearl of the Reserve. For the period from 1950 through 1980 the number of breeding pairs in the colony had varied between 29 and 127. The great variety of bird species occurring in the region and the Dalmatian Pelican colony in particular makes us feel a deep concern for saving this invaluable Biosphere Reserve.

1.8. Wetland Type

Marine-coastal: A B C D E F G H I J K

Inland: L M N O P Q R Sp Ss Tp Ts U Va Vt W Xf Xp Y Zg Zk

Man-made: 1 2 3 4 5 6 7 8 9

According to Annex I of the Ramsar Guidelines the Reserve Srebarna pertains to the following types:

O - Permanents freshwater lakes (the lake's mirror or central open water area);

M - Permanent rivers (the arm of the Danube flowing between the river right-hand bank and the Danube island of Devnya);

P - Seasonal marshes/pools (excavation pits between the protective dike 'Vetren-Silistra' and the river's right-hand bank);

Xf - Freshwater, tree-dominated wetlands; seasonally flooded forests (the whole area of the island of Devnya and part of the riverside between the protective dike 'Vetren-Silistra' and the river's right-hand bank);

Zk - Subterranean karst and cavern hydrological systems (the spring "Kanarichkata" in the south part of the Srebarna Lake).

1.9. Ramsar Criteria

1a 1b 1c 1d 2a 2b 2c 2d 3a 3b 3c 4a 4b

See also 1.12 below.

1.10. Maps

The following maps are included in App. 1:

        1. Wetlands around Srebarna Lake
        2. Topography
        3. Geology and Geological Profile of the Srebarna Lake Catchment Area
        4. Catchment Area of Srebarna Lake
        5. Soils and forests around Srebarna Lake
        6. Hypsometry of Srebarna Lake
        7. Vegetation Map of Srebarna BR
        8. Long-term Changes in the Lake Mirror (1948-2000)
        9. Land Tenure around Srebarna BR
        10. Boundaries of Srebarna BR in the Past, at Present and Proposed
        11. Habitat Management
        12. Places for Dredging in Srebarna BR
        13. Nature Resources Usage of Srebarna BR
        14. Access to Srebarna BR
        15. Sampling Points

1.11. Name and address of the compiler of this form

A team with Ass. Prof. Georgi Hiebaum, PhD as a team/project leader, Central Laboratory of General Ecology, 2 Gagarin Street, 1113 Sofia, Bulgaria, E-mail: hiebaum@ecolab.bas.bg

1.12. Justification of the criteria selected under item 1.9

1c - Srebarna Reserve is a particularly good example of a wetland, especially one being with an across-the-border location, which plays a substantial hydrological, biological or ecological role in the natural functioning of a major river basin or coastal system.

Being situated on the Danube right-hand bank, the river also being the state border between Bulgaria and Romania, Srebarna Lake plays a substantial hydrological, biological or ecological role in the region as well. This role will be considerably enhanced with setting up of a bilateral protected area, both on Bulgarian and Romanian sides of the Danube.

2a - Srebarna Reserve supports an appreciable assemblage of rare, vulnerable or endangered species as follows:

According to the European List of the Globally Threatened Animals and Plants: According to the Red Book of Bulgaria:
Plants 2 Plants 13
Leeches 1 Animals 68, among them:
Snails 1 Fishes 1
Bivalves 1 Amphibians 1
Dragonflies 1 Reptiles 1
Beetles 3 Birds 59
Birds 9 Mammals 6
Mammals 4
Total   22 Total   81

3b - Srebarna Reserve regularly supports individuals from specific groups of waterfowl and other aquatic birds, which are indicative for the wetland values, productivity or diversity; Srebarna supports substantial numbers of the following groups of waterfowl:

Pelicans Pelecanidae
Cormorants Phalacrocoracidae
Herons, Spoonbills and Ibises Ciconiiformes
Ducks and Geese Anseriformes
Terns Sternidae

3c - Quantitative data on regularly supported 1% or more of the population of a given species or subspecies:



1% level






Phalacrocorax carbo

Black Sea / Med





Phalacrocorax pygmaeus*

Black Sea / Med



Pelecanus crispus*

All countries





Anser albifrons

Black Sea (winter)





Aythya nyroca*






1.13. General location

In the northeastern part of Bulgaria, on the right-hand bank of the Danube, District of Silistra, South Dobrudzha (App. 1, Map 1).

1.14. Physical characteristics

1.14.1. Geology, Geomorphology and Hydrology

Background Information

Geological structure and hydrogeological conditions in the area of Srebarna Lake have been characterized on the basis of existing information as well as on the results from investigations carried out by K. Shopova (Shopova, 1999) in recent years. Results from all geologic surveys and mappings carried out in the area under question have been summarized in the Explanatory note to the geologic map of scale 1: 100000, the map sheet Tutrakan & Silistra (App. 1, Map 3). Main archive sources on underground water for that area were the reports related to the hydrologic and geologic mapping on a scale 1: 250000 carried out by Iotov (1968) and Danchev, Manolov (1972). In the area of Srebarna Lake there are Lower Cretaceous, Neogen and Quaternary sediments.

The Lower Cretaceous sediments are represented by the Rouse formation (after the name of a Danube riverside town), exposed on the surface in small tracts located south of the lake. This formation is composed of strong, massive, light brown to white, porcelain or porcelain-like limestone; oolythic limestone; chalky and thick-layered organogenic limestone. Frequently these limestones are cracked and karstified.

Three formations represent Neogen materials. Sarpov’s formation lies as a transgression over the Rouse formation. It is composed of gray, gray-bluish to light brown calcareous, thin-layered, and at places, sandy clays. This formation makes up the basis of the Srebarna Lake and at places, in its southern end, the limestone from the Rouse formation tears it up from below. In the stand of the Aidemir formation lying upon the Sarpov’s formation participate fine- to medium-grained, gray, yellowish to light-brown quartz sands, lying in slanting layers at places. It is exposed on the slopes facing the lake and in the gullies that feed the lake with water. The third formation, Srebarna’s, is exposed at the higher levels of the terrain, over the Aidemir formation. It is composed of gray-bluish, strong marls (clayey limestone) in the base with lime clays over them. In the uppermost part lies light gray to white, compact and strong limestone, at places more clayey. The Quaternary is represented by Eolithic formations (loess) covering the greater part of the inter-fluvial massifs. It is by itself a beige-yellowish light, porous, fine-grained, loose, clayey-silt rock, enriched with calcium carbonate in the form of single grains, coatings or concretions. The flooded terrace of the Danube is made up of alluvial materials abutting the lake to the north. From top to bottom one can trace out fine-grained, gray-black sands, fine- to medium-grained sands mixed with medium-size rubble and gray sandy clays. Overall thickness is 20 m. Contemporary marsh silt and sludge covering the lake bottom should also be mentioned (App. 2, Geological Section).

Structurally the area under examination is within the limits of the Mysian platform. The whole region may be described as one of the platform type of development with almost horizontally laid out layers. The main structures are fault disruptions that delimit varying in size and leveling blocks. It should be born in mind that the area belongs to the Tutrakan grabenlike depression of northwest-southeast orientation.

The geologic conditions in the area of the lake pre-determine the presence of the following hydrology-geologic section:

  • Lower cretaceous (Apt) aquifer. The underground water forms in the unevenly karstified limestone of the Rouse formation. The aquifer here is 50 to 60 m thick and falls slightly towards the north-northwest. Under it lies the Hotrivian marlestone impermeable (watertight) layer while the top of almost all of it is covered by the Pliocene aquifer complex. South of the Srebarna Lake is the only place where the Apt layer (the aquifer) is exposed on the surface. Through hydrogeologic windows part of its water drains into the lake. Another part drains in the Quaternary aquifer. Surface water, mainly from rains, penetrates to it south of the area under examination. The aquifer is characterized by a relatively high abundance of water;
  • Pliocene aquifer complex. This complex is composed of three basic stratigraphic units, each one possessing its own hydrogeologic properties. The lowest one, the so called Sarpov’s formation, built-up of lime clays is impermeable (filtration coefficient equal to 0.8X10-6 m/d). It is in this watertight clay that the lake bowl is situated. The Aidemir formation that lies over it is the most permeable one because it is composed of sands in 2 to 16 m thick layers. It is exposed on the surface above the erosion basis determined by the Srebarna Lake water level. On its top the Pliocene aquifer complex is covered by the clayey-limestone Srebarna’s formation being also of low water abundance to impermeable. The Pliocene complex lies almost horizontally and is covered by the Quaternary Loess aquifer complex;
  • Quaternary aquifer. According to the type of deposits there are two main Quaternary aquifers;
  • Alluvial aquifer. Coincides with the Danube terrace deposits that build the Aidemir lowland. It is a two-layer aquifer with a more permeable lower part (built-up of sands and gravel) and a less permeable upper part (clayey-sandy). Total thickness of this aquifer reaches 29 m its lower part alone is up to 14 m thick. Water formed in it is semi-confined and non-confined. It is characterized by the highest water abundance in the whole region. The northern bank of the Srebarna Lake abuts to this aquifer;
  • Loess aquifer. This is the uppermost aquifer in the geologic section which is distributed almost everywhere in the inter-fluvial massifs. The existing information allows for making up only regional characteristics of the area under examination. The number of springs, boreholes and wells is rather small. Most of them are of unclear geology and structure. There is no systematic information on the qualitative and quantitative characteristics of the surface and underground waters. Available data do not allow for making an analysis and interpretation of individual elements of the water balance and for determining how the lake is fed and drained underground. From the analysis of the geologic and hydrogeologic conditions one can assume that Srebarna Lake water, apart from the surface run-off, comes also as Karst water from the Lower Cretaceous (Apt) aquifer through hydrogeologic windows while the draining goes into the alluvial aquifer. It follows from this fact that the role of the underground water is of great importance for the water exchange of the lake. Colmatage of the outcomes of the water from the Apt aquifer disturbs the underwater feeding of the lake with underground water. It is possible that this feeding of the lake with underground water has also decreased because of the pumping out of underground water thus disturbing the water balance.

Proposed measures

The main goal is to restore the natural water balance of the lake. To accomplish this it is necessary to find the exact points of feeding-up the lake with water, the quantities and the trends in its supply. It is inadmissible to deepen the lake by invading into the Sarpov’s formation without carrying out thorough geologic, hydrogeologic and hydrologic research studies beforehand. It is also considered that if the current man-caused disturbance of the lake water balance is diminished this would be of definitely positive significance.

1.14.2. Origin

Srebarna is a typical freshwater Danube lake of the river flood terrace. At the beginning of the Holocene, about 11 000 BC, after the so-called Flanders transgression, the river bed of the Danube underwent significant changes (Popov, 1987). According to the palinological evidence, Srebarna Lake has been formed some 8000 years ago following the inundation of the riverside terrace by the Danube (Borisova, Lasova, Stranchevska, 1989, Lazarova, 1990, 1994, 1995).

1.14.3. Hydrology (including seasonal water level and water balance, inflow and outflow)

Historical data review

Several stages can be marked off in the development of the Srebarna Lake ecosystems depending on the man-induced changes: natural state (until 1948); disturbed state (from 1949 till 1978); recovery I stage (from 1979 till 1994); recovery II stage (after 1994). In 1978 the periodical entering of the Danube water in the lake during the spring high water levels in the river was restored. This had a substantial impact on the water balance and the hydrologic characteristics of the lake. Prolonged drought during the period of 1988 through 1994 led to negative changes in the morphometric indices of the lake, as for instance the reduction of the central open water area (the lake mirror) and of the total volume of the lake. Putting into operation of the hydraulic system "the Danube - connecting canal - Srebarna Lake" in 1994 set the conditions for controlling the water level, the size of the flooded areas and the lake actual water volume. Studies of the lake hydrology were carried out in 1991 through 1993 in relation with the expected construction of the canal for the hydraulic connection of the lake with the Danube at high water levels in the river (Radev et al., 1993). Those studies showed the negative changes in the hydrologic and morphometric indices of the lake due to the prolonged drought.

Actual State

The lake water catchment area is drained by the rivulets Srebarnenska and Kalnezha. Water conditions in these rivulets vary greatly and in the summer-autumn period they are almost dry (App. 1, Map 4). By its morphometric indices Srebarna Lake can be assigned to the category of the smallest water bodies of up to 10 square km of surface area. An assessment of the hydrologic processes in the lake and its watershed has been made. Three to four times a month water level dynamics has been monitored with the use of a water leveling rod tied up to the main leveling at the CLGE field station. We have also used information from various literary sources on the hydrologic processes.

An integral index for the changes in the lake volume are the water level fluctuations resulting from a complex of factors like the morphometric properties of the lake itself, the inflow and the outflow, the internal dynamics of the water masses. Systematic monitoring of the lake water levels dynamics has been carried out since August 1990 at the Central Laboratory of General Ecology Field Research Station (Nikola Mikhov, personal communication).

Contemporary state of the lake may be characterized by the following hydrologic and morphometric parameters for the period 1998 through 1999:

For 1998 the water level elevation mark was from 11.91 m to 12.78 m and in November has reached the elevation 13.73 m after the Danube waters overflowed the dike:

  • The water column height changed from 1.10 to 2.90 at the point of water leveling rod;
  • Areas flooded at the above-mentioned water levels varied between 2.334 km2 to over 7.1 km2;
  • The water quantity retained in the lake varied from 2.82 mln m3 to 5.9 mln m3;
  • Thickness of bottom sediments was about 1.5 to 1.7 m; (Radev et al., 1993).

For 1999 water level changed within the limits of elevation 13.68 to elevation 14.06 m;

  • Water column height changed from 2.2 m to 3 m;
  • Flooded areas at these water levels varied between 7.137 km2 to 7.218 km2;
  • The water quantity retained in the lake varied from 10.67 mln m3 to 14.35 mln m3.

In order to determine the basic trends in the lake water level fluctuations for the period 1990 through 1999 a statistical model after the method of analysis of temporary seasonal rows was developed. The trend function is shown as a polynom of the fifth power looking as shown below:

h(t)=a0+a1.t+ a2.t2+a3.t3+ a4.t4+ a5.t5 where 't' is time.

This model characterizes well enough the trends in the lake water level dynamics.

The trend function shows variable progress with low amplitude when the level increases to 11.8 m for the period 1990 through 1991 and when it lowers in the period 1991 through 1994 within the 11 m to 11.5 m fluctuation. After the canal for hydraulic connection of the lake with the Danube was opened and put into operation in May 1994 a trend emerged towards water level increase for the period 1994 through 1996 followed by its stabilization in the past 2 years within the limits of 12 m to 12.4 m.

The results from putting the hydraulic connection canal into operation prove the possibility to regulate the water level as the main instrument for managing the biosphere reserve.

These changes in the water level, the size of flooded areas and the water quantities retained are all functionally related and one should bear this in mind when regulating the inflow of the Danube water.

Models have been developed to describe dynamics of the size of flooded areas and the water quantities retained depending on the fluctuations of the lake water level. The models are polynoms of the type shown below:

F (t)=a1+a2.h+ a2.h+a3.h2+ a4.h3
V (t)=a1+a2.h+ a3.h2, where ‘h’ is the water level elevation in meters.

Modeling the dynamics of these hydrologic and morphometric parameters is of considerable importance for managing the hydrologic, hydrochemical and hydrobiologic processes in the functioning of the lake ecosystems.

We have also worked out an equation of the lake water balance for a given time interval. This balance equation expresses the changes in the lake water level depending on the inflow, the outflow and the change of size of the flooded areas for that time interval. We have also looked over individual components of the lake inflow and outflow.

From the retrospective analysis of the pollution of the Danube waters (after data from the national system for ecological monitoring collected at a check point near the town of Silistra for the period 1986 through 1993) the basic trends in water quality evolution have been determined. It has been found that there is a trend towards decrease in the pollution as measured against the criteria of BOD5, oxidization, ammonia- and nitrate nitrogen, insoluble (suspended matter) and dissolved substances. This indicates that periodical inflow of the Danube water will not affect negatively the lake waters hydrochemistry.

Proposed measures

The studies carried out on the hydrologic and morphometric characteristics of the lake, its water catchment area and its water conditions will help to solve the problems of the Biosphere Reserve management. It should be stressed that the hydrologic processes being complex and contingent on a multitude of factors are not sufficiently investigated. Combining the inflow from the surface and subsurface (underground) run-off and the inflow of the Danube waters with the specific hydrologic and climatic conditions demands the set-up of scientific information system for regulating water conditions in compliance with the requirements for the management of this Biosphere Reserve.

All activities aimed at decreasing the influx of water from the water catchment area towards the lake will have a negative impact on the functioning of the lake ecosystems. Investigations completed so far show that the lake water balance should be optimized by combining processes of water exchange with hydrochemical and hydrobiological processes that take place in the lake’s ecosystems.

We have pointed out to ideal goals and ideal measures for opposing the negative factors in forming the lake water balance. These measures cover the area of the reserve and the lake water catchment area:

  • Reducing human impact in the lake water catchment area responsible for worsening the conditions in forming the surface run-off;
  • Optimizing the lake water balance depending on the ecological requirements for the functioning of the lake ecosystems;
  • Securing the influx of the Danube waters into the lake;
  • Systematically collecting information on the hydrologic processes in the lake water catchment area and in the lake proper in view of determining the lake water balance and conditions;
  • Optimizing water exchange processes in the lake and the export of bottom sediments.

The realistic measures refer to reducing the water consumption and the pollution of the water catchment area; increasing the influx of water to the lake; decreasing of the areas occupied by reed; improving the processes of water exchange and removal of bottom sediments from the lake.

1.14.4. Soil types and soil characteristics

Historical Data review

For the first time a map of the soil cover of the region was produced on a 1: 1000000 scale in a soil survey carried out by the 1948 Soviet-Bulgarian expedition (Antipov - Karataev et al., 1948). Since then a soil type determined as Leached Chernozem or Haplic Chernozem according to the FAO Legend (FAO, 1990) has been considered as the main type of soil for the region. A soil map on a scale 1 : 400000 provides a more detailed presentation of the soil varieties, with Carbonate Chernozem or Calcic Chernozem (after FAO classification) included (Angelov et al., 1975). Large scale maps (1 : 25000 and 1 : 10000) are quite representative for the existing soil units for the purpose of agricultural practice (Characteristics of the soils of TKZS Srebarna, 1963, 1978). Chernozems occupying any particular slope differ in chemical and physical properties and in their texture. These soils with their different fertility demand special agricultural treatment and conservation, which sometimes bring forth passing results or are of little use.

Actual state

In order to contribute to the project idea of monitoring the area a detail soil map on a scale of 1:5000 has been especially elaborated (App. 1, Map 5). Srebarna Reserve is situated in the northeastern part of the Danube hilly plain, in the region of Aidemir lowland. The relief of the area adjoining the Aidemir lowland is undulating dissected. Common slopes are cultivated thus provoking active soil erosion. At some slopes and banks the underlying limestone parent material (bedrock) is revealed.

The average annual rainfall is 500 mm; the average annual soil temperature is 12.4oC with the lowest value in January of 0.1oC at the depth of 2 to 5 cm. According to the Soil Taxonomy (FAO) temperature conditions are mesic and the moisture ones are ustic.

Main parent materials for the variety of the Chernozems in that area are the loess-like deposits. Alluvial deposits and clays prevail as soil forming materials in the lowlands. Soils have also been formed under the influence of deciduous forest and steppe grassy vegetation.

The following units of soil varieties characterize the area of the Srebarna Biosphere Reserve:

  • On the lands surrounding the Reserve prevail the Leached Chernozems (Haplic Chernozems, after the FAO classification). They cover the well-drained plateau and gently sloping areas. Moderate to severe erosion develops on the steep slopes;
  • Thickness of the upper Molic A horizon is 20 to 30 cm, abounding in fine roots. Structure is fine crumb, friable. Human influence is slight, confined to the plough layer. Humus content is 1.7% to 2.8%, gradually reducing in the depth of the soil profile. Metamorphic B-Horizon is lighter than the previous one, 30 to 40 cm thick, slightly compact with firm sub-angular blocky structure. Carbonates appear at the lower part of horizon as a mycelium-like accumulation. In the Ck horizon at the depth of 80 to 90 cm carbonates accumulate as small soft concretions. CaCO3 content is about 15% to 20%. pH is neutral in Molic A horizon and slightly alkaline deeper in the profile (pH of H2O is 6.5 to 8.0). Soil texture is loamy throughout the soil profile. The porosity is about 35% to 47%;
  • Meadows Chernozems cover the lowlands and blind creeks. Typically they have a well pronounced thick Molic A horizon, at places as a result of accumulation. Humus content is 2.0% to 2.9% but slightly changes deeper in the profile. pH is neutral to slightly alkaline (pH of H20 is 6.6 to 7.9). Carbonates can occur on the soil surface depending on the re-deposited materials. The soil texture is clayey-loamy.

The valley that separates the Danube from Srebarna Lake is formed of alluvial re-deposited materials. The soil cover is complex as it depends on the influence of ground water in forming the soil profile. The soil complex consists of a variety of meadow chernozems, alluvial and slightly swamped alluvial soils. Humus content is about 1% to 2% in the upper layers. The texture of different soils varies from loamy, clayey-loamy to silt-clayey. pH is slightly alkaline. In all soil profiles no presence of salt or alkali was detected. Lacustrine marsh soils periodically are under water.

Human activities in the past were directed towards increasing the arable land instead of woodlands. The decrease of the woods (forested areas) and the increase of the arable land contributed to the increased erosion. Within the reserve boundaries comparatively large tracts of steep bank slopes have been terraced and afforested in order to prevent severe erosion. At the same time farmlands were put under intensive cultivation and modern technologies like ploughing and the application of organic and mineral fertilizer were implemented. However, the human impact on the soils in cultivated land was confined to the plough layer. Evidence of management practices such as drainage, terracing, embankment and melioration altered soil profile in a more pronounced way. Nevertheless, the problem of sheet and gully erosion is still an important one.

Proposed measures

The negative influences of the erosion and human activities are shown on the soil map of the scale 1: 5000 reflecting the soil varieties occurring within the Reserve Srebarna area. This soil map helped to focus on spots with negative factors as follows: erosion and sedimentation, accumulation and decomposition of organic matter, the surface run-off (lateral water flow) versus infiltration of water.

As an initial step in monitoring the reserve area one should note that a significant coincidence can be traced out between the terrain and soil peculiarities and that this manifests an approach for their conservation and management. It should be noted that potential run-off and soil loss increase under the inter-rill erosion conditions and that the wheel traffic grooves have greater effect especially on long or steep slopes. For this reason on the slopes facing the lake all activities should be forbidden. The tracks as well as temporary streams of potential run-off that brings about soil loss by causing soil masses to slide into the lake and form sediments there, should be broken by planting trees and shrubs.

The pressing task now is the soil conservation with minimum risk for run-off provocation on the nearest slopes towards the lake. The afforestation of the area has to be continued and the severely affected areas have to be fenced off by tree and shrub plantations. In this region the tillage practice demands special management.

One of the future goals to achieve in the management of this Reserve should be the introduction and use of innovative opportunities opened by the geographic information system (GIS) technologies and the high-speed computing capabilities as well as remote sensing to depict quantitatively the continuously changing nature of the terrain in a three-dimensional image to assist the organization of the information and to validate the soil-terrain modeling.

1.14.5. Nitrogen & Phosphorus Conditions in Soils & Lake Sediments

Historical Data review

Soils in cultivated land are the main source of N and P pollution of lakes and rivers due to soil erosion. Available forms of N and P are important part of the total amount of these elements in the soil. They have accumulated during the years of applying fertilizers and have very favourable effect on crop growing. Furrow crops near lakes and rivers increase erosion and pollution of waters. Soil particles and dissolved soil P in water run-off from agricultural fields are the most important source of lake and river pollution with phosphorus (Austin et al., 1996).

No earlier data on nutrient dynamics, either in soils or in sediments is available.

Actual state

The main soil type around the lake is the leached chernozem. Carbonates were found at a depth of 60 to 70 cm in non-eroded soils. Humus content varies between 1.7% and 2.8%. Average clay content is about 42.4%.

Investigations were carried out on the lands near the lake: the forest Gabritza on the east lake shore; the maize fields on the south lake shore and on the heavily eroded west shore. The north shore of the lake is low and cannot be a source of soil erosion material. Soil samples were collected from the 0 to 15 and 15 to 30 cm soil layers. Water samples were collected from the Danube, Kalnezha river, the canal connecting the Danube and the lake, and from three sampling points in the lake proper (App. 1, Map 15). Sediment samples were also taken from the same sampling points in the lake. The upper layer of the sediment was a semi-liquid, rich in organic matter ooze. The second layer was a well metamorphosed semi-liquid grey inorganic sediment. The third layer was a grey-white clay sediment. Total N was determined by Kjeldahl method, inorganic N by Bremner-Keeney method. Total P was determined by digestion in perchloric acid, available P by Petko Ivanov lactate method. Organic C was determined by Tyurin method. Isotopically exchangeable available phosphorous pools in sediments were determined by isotopic exchange kinetic method described by Fardeau et al. (1979)

Nitrogen mineralization of organic sediment was studied in laboratory incubation experiment. Four grams of wet sediment were added to 20 g of soil and incubated at 25oC for 28 days.

Main sources for N and P pollution of Srebarna Lake are: Kalnezha river; the crop-fields on the south shore of the lake and the west slope (heavily eroded).

Soils (App. 3, Tables 1, 2 and 3). Higher amounts of N total were found in the forest soil from the east shore of the lake. Agricultural practices have caused an almost two-fold decrease of the total soil nitrogen in the soils from the south and western lake shore. The lowest total nitrogen concentrations were found in samples from the eroded slopes of the west lakeshore.

Inorganic nitrogen content in soil is in the range of 3.5 to 20 mg N× kg-1. Nitrogen quantities are relatively small and do not pose a risk for a direct pollution of the lake water. Only nitrate nitrogen can be leached if favorable conditions are present. Available P content is nearly the same in all non-disturbed soils: from 7.3 to 7.9 mg P2Okg-1. Higher available P content was observed in the soils from the maize field on the south shore. Very high concentrations (50.4 to 52.5 mg P2Okg-1) depress the plant growth. For the method of determination the available P in excess of 25 mg P2Okg-1 can provoke depression in the growth of crops. Surface erosion from these fields has loaded Srebarna Lake with huge amounts of available phosphorous. Erosion material from the west shore of the lake contains high concentrations of available P, too. A study for the P bioavailability by isotopically exchangeable kinetics shows that readily available P contained in the soils of the south lake shore is several times higher than that in the soils from the west shore. High concentrations of bio-available P in wood soil are not a hazard for the lake water because of the low erosion rate in this soil. Positionally, the hazard from the west shore erosion is greater than that from the south shore. Available P transfer in the lake’s water favours eutrophication.

Sediments (App. 3, Tables 4 and 5). The net N mineralization is possible in the presence of organic matter with C:N ratio below 25, while the net P mineralization is possible in the presence of organic matter with C:P ratio below 200. All sediments have C N ratio below 25 and C:P ratio below 200. That means sediments will supply the lake water with ammonium and phosphate ions. Clay sediments have lower C:N and C:P ratios but overlaying sediments (rich in organic matter) isolate them and the rate of input of the ammonium and phosphate in the lake water will depend on the rate of the organic matter mineralization.

Sediments from Ribarnika have the highest P total content. Soils from the nearest shore are very rich of available phosphorus and this is an indicator of the soil erosion. The possible cause is the soil erosion from the west slope, which is due to the recent elimination of the vineyards above the lake and partially to the elimination of apricot orchard near this sampling point several years ago. The most important pools of readily available phosphorous (isotopically exchangeable phosphorous) were observed in the upper layer of the sediments from Ribarnika. High content of P in sediments from Kamaka indicates pollution from Kalnezha river, disgorging near this place.

High content of organic matter in the lake bottom sediments (about 30 cm of organic sediments in 1 to 1.2 m of water depth) is favourable for the denitrification of nitrate nitrogen present in the lake. Adding 10 mg C.l-1 reduced nitrate content in aquifer microcosms from 12 to 6 mg.l-1 for 35 days (Obenhuber and Lawrence, 1991). Denitrification prevents nitrate accumulation in the lake water.

Evaluation of fertilizing capacity of sediments. Comparing the sediment content of N and P to that in farmyard cattle manure is the fastest way to evaluate sediment fertilizing capacity. In well-composted cattle farmyard manure the average total N content is 0.5%, the average P total content is 0.25%. Results for N and P analyses are shown in Table 4, App. 3. Values for N total are higher in sediments than is the average content in farmyard manure. Use of sediments as organic fertilizer would be optimal in soils with high available P content, e.g. the maize field on the south shore. This assumption is supported by the incubation experiment with sediments and soil. The results show that mineralized nitrogen following 20 g dry weight sediment application in 1 kg of soil is about 50% higher than in the soil without sediment application (App. 3, Table 6).

High rate of nitrogen mineralization following sediment application is quite possible and fertilization using lake sediments will improve the nitrogen conditions in the soil. The application of large amounts of organic matter will improve soil texture and will act as a limiting factor for soil erosion.

Impact of the rivulet Kalnezha. Water with higher nitrogen content (as nitrates) was found in the river Kalnezha (1.84 mg/l). This rivulet runs through agricultural fields and near pig farms so pollution due to organic and inorganic fertilizer increases the nitrogen content in its water. However, as the water inflow from Kalnezha is negligible it could only temporary affect the nitrogen concentrations in the part of the lake called 'Kamaka'.

Reed analysis. Analysis of total nitrogen in reed samples shows a similar content compared to sediments from Ribarnika and Kamaka (sampling points close to the shores and the reed-beds). The total N content in the centre of the lake is twice as low as the total N content in the reed. This is due to the spatial remoteness from the reed. It means that conditions in the lake are favourable for maintaining low concentrations of inorganic nitrogen in lake water (App. 3, Table 7).

Proposed measures

Investigations carried out in Srebarna Lake area show that N and P content in soils pose certain hazard for polluting lake water. N and P total content and their available forms in the lake sediments could amplify the hazard of eutrophicating the lake. Sediment organic matter is a good organic fertilizer for the soils in the nearby eroded lands.

Reducing the rate of erosion will limit the N and P pollution to the lake. Rate of applying fertilizer in the agricultural fields surrounding the lake must be limited in order to avoid N and P pollution in the future. Better storage of farmyard manure and pig slurry is obligatory in Srebarna area and Kalnezha river watershed. Organic matter input from the reed must also be reduced. Organic sediments from lake bottom must be periodically removed and used as fertilizer in the nearest fields. Anti-erosion measures must be undertaken at places most affected by erosion - hill slopes, cart-roads and tracks. Long-term monitoring of the most endangered points for changes in soil surface layers is obligatory. A set of sites with spatial coordinates for soil sampling must be established in the Srebarna area as a research facility. It will be a very good practice if annual fluxes to Srebarna Lake of erosion materials (transferred soil particles) and dissolved N and P can be periodically evaluated. Quantitative and qualitative erosion evaluation will be helpful for better management of the Srebarna Biosphere Reserve.

1.14.6. Water Quality

Review of the Historical Data

Based on indirect information on the lake (Petkov, 1911) it was assumed that by the turn of the 20th century the lake was mesothrophic to slightly eutrophic (Stoyneva and Michev, 1998). Information from the 1960-s, after the natural connection with the Danube was broken in 1948, shows the transition of the lake water to eutrophic state (Rozhdestvensky, 1964). For the period from 1960 through 1985 an almost ten-fold increase in the concentration of phosphate and other nutrients was recorded (Tzankov, 1993). After that, as a result of the setting-in of a dry spell for the period from 1982 through 1994 the hydrochemistry of the Srebarna Lake underwent substantial changes. In 1964 the water was rated as bicarbonate-chloride type (Rozhdestvensky, 1964) while in 1985 it was described as bicarbonate (Radev et al., 1993). Drastic changes in the lake - the partial drying-up and shallowing during the period from 1990 through 1993 had caused a thorough change in the mineral composition of its water, the mineralization having increased almost two-fold and its chemical character having changed from bicarbonate to sulfate type with a 13-fold increase in the sulfate concentration recorded (from 38,1 to 487,3 mg. l-1 (ibid.). The setting-in of anaerobic conditions led to the appearance of hydrogen sulfide as a result of sulfate reduction. A surge in the concentration of nutrients was also recorded (Tzankov, 1993).

Actual state

The concentrations of inorganic forms of nitrogen and phosphorus (NH4+, NO2-, NO3-, PO43+) and of silicon (Si) in water were measured with the help of standrd analytical methods (Merck) and the spectrophotometer Ultrospec1000 of LKB/Pharmacia-Biotech. The temperature, conductivity and oxygen concentration were measured with the help of the field device MultiLine P4 of WTW. Samples were collected as average for the whole water column down to the interface between water and sapropel from 5 points in the lake (App. 1, Map 15).

At present Srebarna is in the initial phase of the classic type of a succession: lake – marsh – wet meadow. The first main factor affecting water quality is the periodic influx of river water and the partial drying-up in summer. Reed and plankton primary production is the second main factor that depends on the water quality and at the same time determines it to quite a great extend.

The gaps in the knowledge apparent in this section are related to the lack of more detailed data on the pollution of the lake with stable chemicals like oil derivatives, heavy metals, pesticides, chlorinated hydrocarbons, etc.

The lake mineral composition undergoes seasonal fluctuations corresponding to the processes of flooding and partial drying-up. The hydrocarbonate, chlorine, sulfate, sodium, calcium and magnesium ions make up the bulk of the minerals in the water. Results from lake water analyses in the summer and autumn of 1998 show again a decrease in the mineralization and a recovery of the bicarbonate type of the water. It is expected, with the Danube water flowing into the lake in 1998 – 1999, the water mineral composition to become similar to the Danube one.

When succession takes place in a flow-through type of a lake the water type changes from bicarbonate to sulfate in the final phases of turning of the lake into a bog (Wetzel, 1983). In the period from 1990 through 1993 the Srebarna Lake water started to change in this way but following the restoration of the connection with the Danube it has returned to the bicarbonate type (App. 4, Fig. 1). Water electric conductivity reflects its mineral composition. For the period under study it was within the normal limits: 420 to 520 uS/m or close to the electric conductivity of the Danube water (300 to 540 uS/m).

The concentration of nutrients depends, on the one hand, on the influx from the Danube and the surface and underground run-off from the water catchment area and, on the other hand, on the plankton production and the processes of recycling of nutrients. On their part, the nitrogen and phosphorus flows control the hydrobiological processes in the reserve. Disturbance of the lake hydraulic conditions in the period of 1990 through 1993 reduced the water layer thickness and accelerated the eutrophication - two processes that were at the root of most of the negative changes. For the period of 1985 through 1993 a significant increase was recorded in the nutrients concentration: nitrates (5- to 10-fold); ammonia (up to 6-fold for the minimum values), for the phosphate the increase was between 5- and 4-fold (App. 4, Fig. 2.1. to 2.3). This in turn led to a high production of organics and correspondingly – to insufficiency of dissolved oxygen. As a consequence food chains were disrupted leading to disturbance in the waterfowl food resources.

As may be seen on the same figures, after 1994 concentrations of nutrients had dropped drastically and in 1998 they were already within their normal ranges for a eutrophic environment: NH4+ (0.7 to 1 mg/l), NO3- (0.9 to 7.8 mg/l), PO43+ (0.1 to 0.55 mg/l) (App. 4, Fig. 3). In November 1998 with the Danube water rushing in the nutrients composition of the lake water underwent a drastic change. Large quantities of nitrogen compounds entered the lake after rushing water stirred the bottom sediments in the canal connecting the Danube to Srebarna and in the canal mouth in the area of Dragayka. The biogeochemical processes in the lake initially led to the re-mineralization of nitrogen and phosphorus compounds then to their consumption by the primary producers. The result was an almost five-fold decrease of nitrates and phosphates in the water (App. 4, Fig. 3).

During the period from January through July 1999 following the high water levels some 30% to 60% of the reed died out. The reed air-dry mass (assuming 1/3 of the mass in the roots) has been estimated to over 11,500 tons (see part At a nitrogen content in it amounting to 1.16% and that of phosphorus amounting to 0.0655% (1.14.5) it is possible, as a result of the biodegradation and re-mineralization, to have about 40 to 80 tons of nitrogen and 2.3 to 4.5 tons of phosphorus released in the water (see also the Project Removing of the sludge). These huge amounts could cause extremely powerful plankton blooms and of course, a jump in the nutrients concentration. In 1999 no such phenomena were observed – just the opposite: the nitrate, ammonia and phosphate concentrations remained extremely low and the primary productivity remained within normal limits (see Obviously the biochemical processes in the lake effectively block the nutrients in an inaccessible to the primary producers form.

The silicon concentrations determined as SiO2 exceed 3 mg/l and are not a limiting factor for the growth and development of the phytoplankton.

Oxygen concentration in the water column is one of the main elements of the water quality. Normally the gradient in water starts with the highest oxygen concentration (5 to 22 mg/l or 60 to 320% saturation) near the surface going quickly down to nearly zero close to the interface between the water and the sapropel. The 24-hour dynamics show that the greatest fluctuations in oxygen concentration occur during summer maximum in the plankton growth (August – September). In conclusion it may be noted that there was no depletion of oxygen concentration related to eutrophication in the water column, which could be dangerous for the aquatic life.

Results from analyses of the heavy metals (Cu, Pb, Zn, Cd, Mn, Co, Ni and Fe) content in the 1-meter thick layer of bottom silt show normal concentrations and are not a reason to assume the lake has been polluted. The same conclusions could be put on for the chlorinated organic compounds including pesticides and PCB’s.

Changes in the water quality and its relation to the lake trophic conditions during the period from 1990 through 1999 can be interpreted with the help of OECD classification system (Vollenweider, R. & J. Kerekes, 1982). Chlorophyll concentrations and the transparency measured with the Secchi disk show that the lake is within the range of the eutrophic state with a trend towards normalization.

Proposed measures

Under conditions of heavy eutrophication the negative factors are linked most of all with the balance of nitrogen and phosphorus. The lake is an end recipient for water from the Danube and from its own water catchment area and is a trap for nutrients and all kind of stable pollutants like heavy metals, chlorinated hydrocarbons, polycyclic aromatic hydrocarbons (PAH), etc. A second negative factor is the potential danger of releasing nutrients and stable pollutants from the sediments or from the reed biomass, including its root system. The latter forms in the end the peat-like material of which the floating reed islands are built as well as the surface of the bottom layers of sediments under the reed belt along the lake periphery. However, when biochemical conditions and the direction of the biogeochemical processes change as a result of intentionally undertaken biomanipulations or because of drastic changes in the lake general water conditions, as well as a result of dredging the bottom for the removal of a part of the sediments or the reed, then the danger of sharp decrease in the oxygen concentration becomes very real. A radical lowering of the lake water level during the coming 10 to 15 years following an overall drying-up of the climate can also cause undesirable changes. Potentially, the transportation of pollutants via the atmosphere may also become a negative factor not taken into consideration until now.

Taking into account the negative factors we can recognize some long-term objectives for recovering the natural state of the lake water:

  • Creating general conditions for water circulation;
  • Removing bottom sediments of silt and debris.
  • Eliminating sources of pollution of the water catchment area, the Danube and the atmosphere.

Operational goal and measures for opposing the negative factors: establishing a system for optimal lake water supply management; introducing an integrated management of the processes in the watershed in order to achieve:

  • An optimal use of the surface and underground water sources;
  • Affective management of the sources of pollution;
  • Implementation of a program for reducing the nutrients influx in the lake;
  • Implementation of a programme for decreasing the nutrients pool existing in the lake.

Programmes mentioned above include models, expert automated systems, automation and optimization of the system of sluices, land-use and water-use programmes, programmes for introducing organic agriculture.

1.14.7. Depth

The hypsometry of Srebarna Lake is shown on Map 6, App. 1.

The maximum depth of the lake is determined by the area where the contact with high waters of the Danube takes place. This is the northwest part of the Reserve where a fragment of the dike was removed and its level is 13.2 m a.s.l. At this water level the maximum depth of the lake is 3.3 m found at the mouth of the Dragaika canal and at another 3 places in the northern, western and eastern parts of the Reserve and the mean depth of the lake is 2.1 m.

1.14.8. Climate

Background Information

Individual components of the environment such as climate conditions, surface and underground water conditions, air and water quality and quantity, etc. form the living milieu of the reserve. Closely interrelated biotic and abiotic factors require the management plan that has been developed for the reserve area to pay serious attention to abiotic conditions in the area under examination.

Continental character of the climate there is well expressed by the cold winters and hot summers. In winter the weather is formed mainly under the influence of continental air masses. Summer is hot because sub-tropical air masses from southern latitudes predominate or either - they are formed here under the influence of the strong summer solar radiation under conditions of the slow-moving anticyclones. Here below is a short description of the climate by seasons including biological seasons as well (Agri-climatic Handbook, 1960; Climatic Handbooks of Bulgaria, 1978, 1979, 1982, 1983, 1990).

Winter: the biological winter is the period when the ambient temperature remains below 5oC. On the average the onset of winter is 26 November and its end is on 12 March. Its continuance is 110 to 120 days. The thermal characteristics of January: the mean temperature is within the limits of -1.5oC to -2.5oC; the minimum at snow cover and anticyclonic weather may vary between -20oC to -25oC; in extreme situations minimum temperatures may drop down to -30oC, -35oC; the microclimate variability is 5oC to 6oC. The number of days with minimum temperature: below 0oC (days of frost) is 26; the number of days with mean temperature below 10oC is about 8. The mean day temperature is higher than 5oC in 12 % of the winter days. Winter precipitation totals 105 mm. The snow cover onset is 13 November until the mid-December or an average of 48 days.

Spring: the onset is 12 March and lasts till 2 May. The frequency of early and late springs is 27% to 28%. Spring frosts are most often in the end of March and in the beginning of April. In spring on the average 1 or 2 days are with mean diurnal temperature below 5oC, but only in 4 to 5 days of the month the minimum temperature falls below 2oC. Precipitation total of the astronomic spring is about 122 to 128 mm, but for the biological spring it is only about 70 mm. Rainless periods of more than 10 days duration occur on the average in 2 of every 10 years.

Summer: Mean diurnal temperature steadily increases and exceeds 15oC about 2 May and lasts until 2 October. Mean July temperature is 23oC with maximum up to 39oC to 41oC. Eighty to eighty five per cent of the days in the period June through August are with maximum diurnal temperature higher than 25oC while 40% to 45% are with maximum temperature higher than 30oC. The precipitation total is 159 mm. All in all in summer and autumn there are 4 to 5 rainless periods lasting on the average between 16 and 20 days each.

Autumn: It sets in with the air temperature steadily passing below 15oC point and the season end comes when it drops down below 5oC: from 2 October till about 26 November. Precipitation total of climatic autumn is 113 mm and that of the biological – 66 mm. The mean circadian air temperature falls below 10oC by 20 or 25 October and below 5oC - between 15 and 20 November. The first autumn frosts set in as early as by the end of October or in the beginning of November.

Morphographic specifics of the region: the altitude of the flat-top heights surrounding the lake from west and east; the varying in area and depth water bodies as well as the type and thickness of the marshy vegetation thoroughly disturb the structure of the meteorological fields. The mean monthly minimum temperature there in summer differs by about 1.5oC from that measured on flat and open places and by 1.5oC to 2.5oC from that measured on the surrounding hilltops. Under conditions of night radiation cooling at no advection in clear and calm nights the above differences can reach 5oC to 6oC. Daily temperature amplitude at the feet of the surrounding hills is by 2oC to 3oC greater than on the hilltops. In winter this difference decreases by about 0.5oC to 1oC but is still significant. The duration of the frost-free weather in the region of the lake and the adjacent low shore is expected to differ by 7 to 12 days from that in the surrounding heights and from there the accumulated heat decreases with about 500oC. With advection present the coefficient of wind deviation reaches 2 to 2.5 and the percentage of calm periods increases several-fold. The differences in the rate of evaporation in the lake and on the shore during the warm half-year is of the order of 100 mm per month and more. These differences are not the same throughout the reserve area but have a rather mosaic distribution. Thus for example if in the foot of the western surrounding hill the minimum temperature is 6oC less than that on the top of the hill, in the eastern part this difference is still less, which is determined by the different air catchment.

One of the important features is the potential evapo-transpiration, which is expressed as the index of the possible evaporation from the natural ground surface at a given place (including the vegetation) under the existing climate conditions. The mean annual evapo-transpiration is calculated by Thornthwaite in mm. For Srebarna region it is 112.6 - 134.5 mm as computed on the basis of data from station Silistra in summer. During the driest part of the year for this region (December - February) it falls to 0 mm. The annual evapo-transpiration for the region is about 691 mm.

Another complex index on the basis of which the temperature and precipitation have been computed is the so-called "dry" or "aridity index" "I", developed by De Martone. The annual course of the aridity index "I" for the period 1931 through 1973 is shown graphically on Fig. 1, App. 5. The annual value of "I" is 23.2 - a fact that places the region in the category of the moderately dry ones according to the classification of Kirov and Kjuchukova (1955) with values of "I" under 20 during the four months from July through September.

We have also used Ped’s index to assess the aridity of the region under examination. The average Ped’s index for the region is below 1, which means that the region is not a very arid one.

Another important specific characteristics of the region’s climate that influences the state of the reserve are the processes of ice forming in the Danube. After 1986 ice events in Bulgarian section of the river have been observed more and more rarely. The physics of their manifestation shows that they are closely related to the meteorological conditions. A joint presentation of the time series of mean winter temperatures and number of days with ice events was made. Thus it was found that 1953 was the coldest winter with greatest number of days with ice phenomena. It deserves noting that in some comparatively cold winters there was not any ice phenomena while in other winters with mean temperature positive ice phenomena were present.

Climate characteristics described above undergo changes and fluctuations down the years. The long-term trend of temperature, precipitation and aridity index dynamics for the period from 1941 through 1997 was studied. The trend of the annual characteristics of the temperature, precipitation and indexes of De Martone and Ped is shown in Fig. 2, 3, 4, App. 5. No tendency towards changes in the mean annual temperatures was observed. The same refers to the Ped’s aridity index, the variations of which were found to be insignificant. Only in the annual precipitation totals and indexes of De Martone there was a slightly expressed tendency towards decrease of the humidity degree since the beginning of the 1970-s till 1994 after that there has been a tendency towards its increase. For a greater clarity the analysis was performed separately for winter and summer seasons.

Almost no long-lasting changes in the mean winter temperature and precipitation totals have been observed. However, the situation was not the same as regards the summer characteristics. From 1984 through 1996 there has been a well-expressed tendency towards an increase in the summer temperatures. To the contrary, the summer precipitation totals have revealed a slight tendency to decrease since the end of the 1970-s.

Since the end of the 1970-s there has been a marked trend towards drier summers. Thus for the period 1972 through 1995 the values of "I" have been under 20 in more than 50 % of the summers, which indicates an increased aridity of the region.

In conclusion we can say that in contrast to other regions of the country the tendency of climate warming and drying up here is rather poorly expressed or is altogether missing.

The climate characteristics as described above undergo significant changes within the reserve boundaries commensurable with a change of the climatic region. These changes may not be negative with respect to the climate proper, but may prove fatal for the reserve biota. Thus for example the decrease of the open water areas and their overall depth due to the accumulation of silt has had as a consequence a change in the thermal conditions of the individual water bodies. In the case of Srebarna in particular this may lead to the increase of the water temperature during the warm half-year, which in turn may cause the increase of the evaporation while during the cold half-year - to lower the water temperature and to the freezing of the lake throughout. This problem can be solved by undertaking measures for cleaning up the lake bowl by taking away the sediments as well as the vegetation of the floating reed-beds provided this will not harm the birds colonies.

Proposed measures

Obviously it is not possible to undertake conservation measures for the climate in the way one takes measures for the conservation of other components of the environment. In this case it is important to set up continuous monitoring. Taking into account that the reserve is within the influence of emissions from the industry in Silistra, Calarash and even Rouse under given synoptic conditions, the monitoring should include not only the basic meteorological elements but also the potential pollution of the atmosphere as well.

Regarding the component "Air and climate conditions" several realistic goals can be set forth:

  • Carrying out research experiments during representative periods (in the middle month for any one season, for the vegetation season or for another stretch of time, important for the existence of animal and plant species). The aim of these experiments will be to collect more complex and more detailed information from greater number of sample points in order to enable a more complete evaluation of the great variety of topographic-climatic conditions. Such investigations will open the possibility to find the statistical relations between the basic meteorological elements in important for the reserve sample points and the operating weather station. Results thus obtained will make possible the identification of interesting to us meteorological elements in the reserve by data available from the permanently operating automatic weather-monitoring station.
  • Evaluation of air qualities during investigations in the field;
  • Model simulation of the air pollution and deposition of pollutants emitted from the industrial sites in Silistra, Rouse and Calarash.

1.15. Hydrological values

Groundwater recharge: From the analysis of the geologic and hydrogeologic conditions one can assume that Srebarna Lake water, apart from the surface run-off, comes also from the Lower Cretaceous (Apt) aquifer through hydrogeologic windows while the draining goes into the alluvial aquifer. It follows from this that the role of the underground water is of great importance for the water exchange of the lake. Colmatage of the outcomes of the water from the Apt aquifer disturbs the underwater feeding of the lake with underground water. It is possible that this feeding of the lake with underground water has also decreased because of the pumping out of underground water thus disturbing the water balance.

Flood control: Areas surrounding the reserve are protected by a system of dikes erected along the Danube riverside close to the bank in the northern part of the reserve and east of it (App. 1, Map 2). When the Danube water level is very high the river water enters the reserve from its northeastern part which is outside the system of dikes. In that case the watershed is not additionally protected from flooding. If that happens after the level of the Danube water drops, the water will run off through the same spot from where it entered the reserve and through the canal connecting the lake with the Danube when the sluices are opened.

Sediment trapping: The Lake is a sediment trap for material eroded from the watershed. In fact eroded soil may enter the lake from the elevated western bank of the lake (see also parts 1.14.4 and 1.14.5).

Shoreline stabilization: There are no pre-conditions to advance soil erosion of any significance or to cause changes of the shoreline of the reserve.

1.16. Ecological characteristics

1.16.1. Main habitats and vegetation types

The Srebarna Biosphere Reserve vegetation, presented mainly by rooted hydrophytic, hygrophilic, hygromesophilic, mesophilic and mesoxerophilic communities (cenoses) plays significant role in maintaining an optimum ecological balance of the aquatic and land ecosystems in the lake area. Basically, its importance is expressed in the following:

  • A key role for the stability of the Reserve's ecosystems;
  • High scientific and cognitive value of a substantial part of the Reserve's higher flora;
  • Specific character of the cenotic combinations within the plant associations;
  • Main contribution to the ecological balance.

The habitat types were classified in accordance with the Corine Biotope Project. Srebarna Biosphere Reserve they are presented as follows:

22. Standing fresh water
22.2 Temporary fresh water
22.411. Duckweed covers (Lemna, Spirodela, Wolfia, Azola)
22.412. Frogbit rafts (Hydroharis morsus-ranae)
22.413. Water Soldier rafts (Stratiotes aloides)
22.415. Salvinia covers (Salvinia natans)
22.42. Rooted submerged vegetation (Potamogeton)
22.422. Small pondweed communities (Ceratophyllum)
22.43112. Northern Nymphaea beds (Nymphaea alba)
53.1111. Freshwater Reed-beds (Phragmites australis)
53.131. Great Reedmace beds (Typha latifolia)
53.132. Lesser Reedmace beds (Typha angustifolia)
38.251. Ponto-Panonia mesophyll hay meadows (Leucojum aestivum)
83.3112. Native pine plantations (Pinetum)
83.3212. Other poplar plantations (Populetum)
The habitats 22.412 - Frogbit rafts, 22.413 - Water Soldier rafts, 22.415 - Salvinia covers and 38.251 - Ponto-Panonia mesophyll hay meadows are included in the Bern Convention List of habitats in need of special conservation measures at European level. They are also listed in Annex I of the EU Habitat Directive.

The vegetation cover of the Srebarna Biosphere Reserve water catchment area consists of the following types of vegetation:

  1. Marsh and bog hygrophytic (at places also hydrophytic) vegetation dominated by the Reed (Phragmites australis), Lesser and Greater Reedmace (Typha angustifolia and Typha latifolia), Schoenoplectus lacustris, Sch. triqetra, Sch. tabernemontana, etc.
  2. Mesosclerothermal grassy vegetation dominated by Poa bulbosa, Rye-grass (Lolium perennae), Bermuda-grass (Cynodon dactilon), at places also by Beard-grass (Dichantium ischaemum) and more rarely Chrysopogon gryllus, mostly on the village pastures.
  3. Mixed oak forests composed of Quercus cerris, Quercus pubescens and Quercus virgiliana.
  4. Mixed forests composed of Quercus frainetto and Carpinus orientalis at places mixed with sucker growth of Mediterranean elements.
  5. Forests of Silver Lime (Tilia tomentosa), at places being a secondary growth.
  6. Arable land where oak forests used to stand in the past consisting of Quercus cerris and Quercus virgiliana frequently mixed with Quercus pedunculiflora.

The vegetation with its function of transpiration has a direct bearing to the wetland hydrological conditions. This necessitated to specially study the quantities of water transpired by the most water-consuming and widely spread within the Reserve and its buffer zone plant communities.

Experiments designed and carried out to determine the intensity of the transpiration of Reed (Phragmites australis), Grey Willow (Salix cinerea) in the reed-bed and that of poplar plantations in the reserve buffer zone showed that the transpiration intensity of the poplar for a two-season monitoring is the average of 1.79 g/m2/h (gram per square meter per hour) while water expenditure by 1 g of leaf mass (leaves) per day amounts to 1.81 g. From these mean values it is possible to compute the amount of transpired water by a poplar tree for a vegetation season: it is on the average 3000 l (varying from 2500 l to 3500 l). Transpiration intensity of Grey Willow (Salix cinerea) is 0.575 g/dm2/h (gram per square decimeter per hour) while water expenditure by 1 g of leaf mass per day amounts to 1.24 g. Transpiration intensity of Reed (Phragmites australis) is 0.478 g/dm2/h, the amount of water transpired by 1 g of leaf mass is 1.35 g.

On the basis of the above results and taking into account the surface area on which the plant communities under question grow, it was found that poplar plantations in the reserve buffer zone covering a total area of 18.1 ha transpire for a season 70,200 tons of water. Poplar plantations within reserve boundaries, which are designated as a "protective forest strip" along the Danube bank and cover a total area of 56 ha transpire 218,400 tons of water per season. Since the Grey Willow (Salix cinerea) distribution is random and it is found as small solitary patches within the reed-beds the amounts of water transpired by it have been computed together with those transpired by the reed proper. The surface area covered by the reed is 402 ha and it transpires approximately 1,413,000 tons of water per season.

1.16.2. Limnological characteristics

The main limnological features of Srebarna Lake are given below:




Altitude (m) According to Danailov (2000)

10.0 – 13.2

Water catchment area (km2) According to Part 1.14.3 of the MP


Length of the shore line (km), 1993 According to Map 1:5000, 1993


Area of the reserve (ha) According to State Gazette, No 97/ 1999


Area of the lake mirror (ha), 1993 According to Map 1:5000, 1993


Volume (km3), 1998 (low level) According to Part 1.14.3 of the MP


Volume (km3), 1999 (high level) According to Part 1.14.3 of the MP


Maximum depth (m) According to Danailov (2000)


Annual inflow (m3), 1998 According to Part 1.14.3 of the MP


Retention time (months), 1998 According to Part 1.14.3 of the MP

2.67 Phytoplankton

Review of Historical Data

Data on the phytoplankton as a community can be find in a small number of works. They are analyzed in the context of the overall evolution of the Srebarna Biosphere Reserve (Stoyneva & Michev, 1998). The most detailed information on the changes in the phytoplankton community and the trophic state of the water body for the period 1982 through 1995 can be found in the publications of Stoyneva (1994, 1998b).

The highest number of taxa was found in samples from the period 1987/ 1988 (up to 97 taxa per sample). In the extremely dry 1993 the depth of the lake shrunk to its most critical value - an average of 20 cm (Michev et al., 1993) and the number of species has dropped drastically (down to 7 species per sample). Heterocystic blue-green algae were excluded from the phytoplankton assemblages, but the number of chroococcal cyanophytes and pyrrhophytes increased; amongst some of the chlorococcal species appeared teratological forms. When the connection to the lake with Danube was restored in 1994 and the river water brought in its phytoplankton there was a new increase in the overall number of species (up to 43 species per sample).

Total annual average number of phytoplankton cells in Srebarna Lake varied between 1.9 x 108 cells× l-1 and 18.5 x 109 cells× l-1. For the period 1987 through 1988 the average total number of cells was 2.6 x 108 cells× l-1, and for the period 1989 through 1993 it was 4.8 x 109 cells× l-1. After the Danube water entered the lake in May 1994 and in March - April 1995, the numbers of algae began slowly to go down reaching 3.9x109 cells× l-1 on the average. The character of the lake changed progressively from eu- to eupolytrophic to hypertrophic until 1993 when the reverse process began, reaching again eupolytrophic conditions after 1994. For the period 1982 through 1993 the average value for the biomass tended to increase but for the next two years - 1994 and 1995 it began to decrease. Average values for given periods of time were as follows: 38.5 mg× l-1 for 1987 through 1988; 67.9 mg× l-1 for 1990 through 1993 and 40.8 mg× l-1 for the period 1994 through 1995.

Actual state

The report was based on the results from processing 35 samples collected almost monthly between May 1998 and April 1999. These samples were processed using the standard methods described in detail by Stoyneva (1998b). Phytoplankton is one of the most thoroughly studied communities of the Reserve (Michev et al., 1998). Nonetheless, because of its extremely dynamic characteristics, it should be studied permanently.

Collected sample material contained a total of 145 taxa (species, varieties and forms) from 9 divisions and subdivisions: Cyanoprokaryota - 52, Euglenophyta - 3, Pyrrhophyta - 4, Chrysophyta - 15 (Chrysophytina 5, Xanthophytina 1, Bacillariophytina 9), Cryptophyta - 3 and Chlorophyta - 74 (Euchlorophytina 70, Zygnemophytina 4). Samples from the lake central part contained a total of 123 taxa while in samples from the separate pool called 'Kamaka' their number was 94. Cyanoprokaryotes and green algae are the main groups forming the phytoplankton qualitative pattern in almost all periods of this study (App. 6, Fig. 1). Both in the central part of the lake and in the pool 'Kamaka' green algae, chlorococcals in particular, were the most numerous taxa (49 and 42 taxa respectively). Next were cyanoprokaryotes, among which chroococcals (28 and 12 taxa respectively) and non-heterocystic filamentous representatives (12 and 14 taxa respectively) prevailed. The number of species in samples collected at any given sampling site and period of time was different. The curve in the diagram showing the total number of species in the central part of the lake has a spring-summer peak while the curve showing the total number of species in the separate pool 'Kamaka' has two peaks, a summer and an autumn one (App. 6, Fig 2). Compared to the period 1990 through 1994 it could be seen that the number of species increased in the years 1990, 1991 and 1993. However, the number of species found in the period 1998 through 1999 has been 4 times as less compared to their number for the period immediately after the canal connecting the lake with the Danube had been set into operation. Then studies carried out at intervals from October 1994 till December 1995 found totals of 131, 106, 206, 165 and 62 taxa (Kovachev et al., 1995). The highest number of species per sample found during the period of the study (53) was considerably lower compared to the period of 1987 through 1988 when it was 97 (Stoyneva, 1998b).

Phytoplankton abundance in the largest (central) open water area (the lake mirror) had a summer-autumn peak and spring-winter minimum, the dynamics of cell numbers and biomass being similar (App. 6, Fig. 3, 4). In the 'Kamaka' pool the number of cells had its peak in the autumn, while the biomass had a summer-autumn peak. The number of cells in the central part of the lake varied from 1.2x107 cells× l-1 (April 1999) to 7.1x1010 cells× l-1 (September 1998) and in 'Kamaka' pool - from 2x105 cells× l-1 (January 1999) to 1.17x1010 cells× l-1 (October 1999). Values of the biomass in the central part of lake varied from 2.45 mg× l-1 (March 1999) to 153.73 mg× l-1 (September 1998) and in "Kamaka' pool - from 3.31 mg× l-1 (January 1999) to 141.07× l-1 (September 1998). The peak/mean ratio for the number of cells and the biomass had the following values: 3.86 and 3 respectively for the central part and 5.1 and 3.3 respectively for the 'Kamaka' pool. This ratio is higher compared to the one for the period 1994 through 1995 (Stoyneva, 1998b).

These peaks in the phytoplankton abundance were due mainly to the bloom of cyanokaryotes. It is a typical characteristic for eu- and, most of all, for hypertrophic water bodies. The winter-spring minimums in phytoplankton abundance with the prevalence of golden, diatom, cryptophyte and fusiform coccal green algae is quite normal when one takes into account the low winter and spring temperatures in 1999 combined with the sharp increase of water level after the Danube water entered the lake. In spite of the fact that diatoms were the main group forming the composition of the Danube phytoplankton during cold part of the year (Kusel-Fetzmann, 1998; Stoyneva, 1998c), their quantities found in Srebarna Lake after the Danube water entered in 1998/1999 period were notably lower compared to the quantities typical for the river.

The dominants and co-dominants of the phytoplankton communities in the central part of the lake and in the 'Kamaka' pool are listed below separately. Phytoplankton assemblages in the central part of the lake were more frequently monodominated, while these in the 'Kamaka' pool were more frequently oligo- or polydominated with bigger differences between dominants depending on the method they had been calculated - by numbers or by biomass. These data coincide with the results obtained after Margalef's index was calculated. Only three of the dominant species found in 1998/1999 period have also been found as dominant species in Srebarna before: Scenedesmus acuminatus (found once in the period 1980-1988 - Stoyneva, in litt.), Leptolyngbya foveolarum (found several times in the period 1989-1993 - Stoyneva, in litt.) and Planktolyngbya sp. (found once in the period 1994/1995 - Stoyneva, in litt.).

Dominant species in the central part of the lake (b - calculated by biomass, n - by numbers): May 1998 - Limnothrix planctonica (n, b); June 1998 - Scenedesmus acuminatus (b); Planktolyngbya subtilis (n); July 1998 - Cylindrospermopsis raciborskii (b); cf. Achroonema angustatum (b); August 1998 - Cylindrospermopsis raciborskii (b) cf. Achroonema angustatum (n0; September 1998 - cf. Achroonema angustatum (n, b); October 1998 - cf. Achroonema angustatum (n, b); November 1998 - Limnothrix planctonica (n, b); January 1999 - Leptolyngbya foveolarum (n, b); March 1999 - Nitschia acicularis (n, b) April 1999 - Synedra sp. (n, b).

Dominant species in the 'Kamaka' pool: May 1998 - Actinastrum hantzschii (b), cf. Achroonema angustatum + Planktolyngbya subtilis (n); July 1998 - Planktolyngbya subtilis + Euglena sp. (b), Phormidium circumcretum + Planktolyngbya subtilis (n); August 1998 - Cylindrospemopsis raciborskii (n, b) + Phormidium circumcretum (n); September 1998 - cf. Achroonema angustatum (n, b) + Anabaena cf. variabilis (b) + Planktolyngbya subtilis (n, b); October 1998 - cf. Achroonema angustatum (n, b) + Anabaena cf. attenuata (b); November 1998 - Leptolyngbya foveolarum (b), Planktolyngbya sp. (n); April 1999 - Cryptomonas erosa (b), Scenedesmus ecornis (n); March 1999 - Uroglena sp. (n, b).

Generally, the indices of diversity of Shannon-Weaver (H) and of evenness of Pielou (E) had lower values in the central part of the lake compared to those in the 'Kamaka' pool. According to the value of the index of dominance of Margalef (c) the phytoplankton complexes in the central part of the lake were mainly monodominant, while these in the 'Kamaka' pool were polydominant. According to data on structural parameters, the phytoplankton in both places was in a rather unstable state and far from its optimum. The values of H, E and c were lower than the same values found for the Danube (Stoyneva & Draganov, 1994b; Stoyneva, 1998c) and for Srebarna Lake before 1996 (Kovachev et al., 1995; Stoyneva, 1998b) - (App. 6, Fig.5).

According to Nygaards index of trophic conditions Srebarna Lake could be referred to the highest category of Nygaards system, the polytrophic one. Apart from the species composition a criterion for the trophic condition is the annual average value of the phytoplankton abundance. According to this criterion Srebarna pertains to the highest category in the OECD classification - the hypertrophic one. This is valid for the central area of open water of the lake where the annual average number of algae was 18.4x109 cells× l-1 and the average annual biomass amounted to 51.18 mg× l-1 as well as for the 'Kamaka' pool where these values were 2.28x108cells× l-1 and 42.72 mg× l-1. Although the low abundance levels in the spring-winter period show a temporary mesotrophic state, according to the OECD criteria these levels cannot be accepted as indicative in determining the trophic conditions of the lake.

The comparison of values obtained in this study with those from previous studies shows that the average number of cells has been significantly higher for all periods since 1987 on, while the average of the biomass has been lower than it was in the most critical period of the existence of Srebarna Lake - that of 1990 through 1993 when it was 67.9 mg× l-1 - but higher than it was in the period 1994 through 1995 (40.8 mg× l-1) after the Danube water entered for the first time via the newly built connecting canal (Stoyneva, 1998b).

As one could have expected after the Danube water entered the lake for the first time in 1994, there was a significant improvement of the ecological conditions there (Kovachev et al., 1995; Stoyneva, 1998b; Stoyneva & Michev, 1998). However, as this first "life-saving" step was not followed by other restoration activities, the positive trend observed then, has recently turned somewhat back and the lake ecosystem can be considered as being in an unstable state. All data on the phytoplankton show that the limnic system not only remains in a hypertrophic state, but that this condition has worsened compared to that in the period 1994-1995. Nutrients accumulated in bottom sediments are the main cause for algal blooms. A supporting factor was the extremely high temperatures of the lake water in the summer-autumn period of 1998. Phytoplankton species, which caused massive blooms are not edible for the zooplankton and fish and the great amounts of the produced and accumulated biomass cannot move along the food-web. Resting stages of many species of algae have accumulated in the bottom sediments, which had already caused algal blooms in the past and, as this study also showed, could still cause them. This is an extremely negative and dangerous factor for the future development of the Srebarna Biosphere Reserve. Most of these species have developed toxic strains. The limnic system is in an extremely fragile state and if restoration activities are not carried out continuously, the best prognosis will be that the limnic system will continue to be in the same as before hypertrophic state.

Proposed measures

The ideal, or long-term, objective is to fully restore and maintain the water in the central part of the lake and in all surrounding pools in the state, which was typical for it in the period before 1987. That same long-term objective is also the realistic, operational one. To achieve it it will be necessary: 1) to maintain the inflow of the Danube water into the lake; 2) to stop the influx of nutrients and to reduce their influx from the lake water catchment area; 3) to stop pumping underground water by the pumping station near the village; 4) to maintain an average depth of the lake at least 2.5 m; 5) to make what is necessary to prevent new land-slides into the lake; 6) to dredge the lake bottom sediments; 7) to restore the native vegetation in the area of the Reserve and to destroy the nearby Poplar plantations; 8) to control the inflow of the Danube water (it is strongly recommended to introduce and apply the bio-monitoring tests with bivalves developed by Salanki which are widely applied in Europe); 9) to carry out permanent monitoring of the lake water. Zooplankton

Historical Data Review

The first information on zooplanktonic Rotatoria from Srebarna Lake was in the works of Konsulov (1912). Under the conditions of a dike-protected right-hand riverside Naidenov (1965) reported 21 species of Branchiopoda and Copepoda. Later on the same author (Naidenov, 1984) found some Rotatoria species. In 1992 Kraeva determined the species composition and seasonal dynamics of the zooplankton in the reserve and found 9 species of Rotatoria and 8 species of Crustacea. She also gave for the first time quantitative data on the community.

Actual State

Qualitative and quantitative samples for determining structural characteristics of the metazoan plankton were collected monthly from May through November 1998 and from January through May 1999, from 5 stations in the lake. Collection stations (sampling points) were both on the open water area of the lake and amidst the lake vegetation (App. 1, Map 15). Because of the little depth samples were taken by directly drawing in 50 l water from beneath the surface and filtering it through a flour sieve with size of openings 100 µm. The residue was then fixed, then brought to 100 ml by adding water, then from this volume the zooplankton content in 1 ml was counted. Then the value thus obtained was computed for 1 m3 by a coefficient of 2000 for the numbers. The biomass was determined by the standard individual weights dependent on the average body length of the plankters multiplied by the number of individuals per m3.

During the present studies 40 taxa, including species and genera, of Rotatoria, 12 of Cladocera and 12 of Copepoda were found in the lake. In 1998 the most widely spread perennial species in the reserve were Brachionus diversicornis, Brachionus calyciflorus, Bosmina longirostris, Acanthocyclops robustus and Thermocyclops crassus (excluding the subadult stages of Cyclopoda which are impossible to determine). In 1999 3 more species were added to the above list: Asplanchna sieboldi, Keratella quadrata and Filinia terminalis. What impressed the authors in 1998 was the complete lack of some very common and widely spread in Bulgaria species, genera and even families, which in the past studies were always well presented in the pelagic parts of the lake. This statement refers most of all to families and genera of the suborder Calanoida found almost in every marsh on the Bulgarian riverside or on islands in the Danube (Naidenov, 1965; Naidenow, 1968, 1979, 1998a). In 1999 some of the taxa, namely Sididae, Daphniidae, Calanoida and Polyarthra, mentioned above as absent, began to reappear, sometimes with high frequency. In general during the second year of the study 23 taxa of Rotatoria, 8 of Cladocera and 10 of Copepoda were found, i.e. significantly less than in 1998.

The year-round surveys show more or less clearly the seasonal dynamics of the zooplankton composition. In winter months of January and February the zooplankton is dominated by representatives of Rotatoria like Keratella quadrata (over 50% of the total numbers), Brachionus calyciflorus and Polyarthra dolichoptera. In March diversity of the zooplankton components increases sharply with Rotatoria still dominating (57.2% of the total numbers and 25.7% of the biomass), later into the spring there is noticeable increase in the quantitative indices for Crustaceans, the numbers of which reaches 63.16% of the total with subadults of Copepods prevailing. The biomass of Cladocerans is particularly high and in May it averages 65.5% of the total biomass mainly due to the high population density of Daphnia galeata and Bosmina longirostris (App. 7, Fig. 1, 2).

It should be stated with regard to the horizontal distribution of the species that, more frequently and all over the lake, prevail the perennating components together with the temporally limited Asplanchna sieboldi, Brachionus forficula, Alona rectangula. Chydorus sphaericus, Daphnia galeata, Brachionus diversicornis.

In 1998 the situation at individual stations for collecting samples showed rather close values regarding species abundance. The least number of species was found at stations 1 and 5 (a total of 27), the greatest one was found at station 4 (a total of 39) but the numbers and biomass of any single species found there were rather low. In 1999 the number of participating species remained almost the same but the highest numbers of species was found at station 5 (a total of 38) while at the rest of the sample stations (points for collecting samples) the total number of the species found varied between 25 and 28.

The average annual values of the summary numbers and biomasses found in samples from any single station in 1998 (App. 7, Fig. 1) allow us to divide them into two groups. While the density at stations 1, 2 and 3 varies between 826,000 and 1,706,000 ind. per m3 and the biomass was between 3.2 and 6.7 g/m3, the zooplankton at stations 4 and 5 was with density of about 500,000 ind./m3 and a biomass 2.3 to 2.5 g/m3. The horizontal distribution of the zooplankton varied considerably from month to month. For instance in May the highest parameters of population density and biomass were found at station 3 (1,900,000 ind./m3 and of 6.7 g/m3 respectively) and the lowest ones (population density of 276,000 ind./m3 and a total biomass of 1.1 g/m3) were found at station 5. In June the difference was considerably reduced while in July and August these parameters showed highest values at station 1 and lowest, at station 4. From September till November station 3 took again the leading position with the highest values and stations 4 or 5, with the lowest ones.

In 1999 the horizontal distribution of the quantitative indices was traced out in March, April and May. The average summary values of the population numbers and biomass at any individual station were considerably more equalized compared to the previous year (119,200 to 184,300 ind./m3 and 600 to 1169 mg/m3). The highest values of these parameters were found in samples from stations 2 and 4. During different months in which studies were carried out there were noticeably greater fluctuations. The extreme values for numbers were found in samples from station 1 in March (42,680 ind./m3) and at station 5 in April (372,000 ind./m3) The lowest biomass for the period was found at station 2 in March (210 mg/m3) while the highest one was found at station 4 in April (2092 mg/m3).

The big difference between quantitative indices for 1998 and 1999 is obvious, even without taking into account the results obtained during the period when the lake was under ice cover. Available data indicate that the best conditions for the appearance of the secondary production in the pelagic parts of the lake exist both in the open water areas and in areas overgrown with macrophytes. The antagonism between the higher aquatic vegetation (vascular plants) and the phytoplankton is a scientific fact known for a long time (Sirenko, 1975). Indirectly, through the food chains, this antagonism also affects the phytophagous and detritophagous zooplankton as more of the dominant species in the lake are. Contrariwise, the massifs of higher standing aquatic vegetation and particularly its submerged and emerged components favourably affect the development of the phytophilic zooplankters among which a number of benthic plankton species occur.

For the period May through November 1998 the summary seasonal curves for the numbers and the biomass have a very well expressed maximum in August (App. 7, Fig. 1) – something typical for the polymictic water bodies in our country. This maximum was mainly due to the depletion of the soluble inorganic compounds of phosphorus and nitrogen being released in the surrounding water by the decomposition of organisms from the probable April under-ice maximum (bloom). The peak values were amongst the highest ones found so far in Bulgaria – 10.2 g/m3, but they were very high during individual months too, as is the case with other water bodies of this type. For instance for the lakes Shabla-Ezerets and Durankulak, which are similar in their morphometry and trophic conditions, the maximum of their biomass was respectively 10.4 and 10.2 g/m3 (Naidenov, 1981; Naidenow, 1998b).

The situation becomes clearer when the main zooplankton groups are analyzed separately. In 1998 the numbers and biomass of zooplankton in the Srebarna Biosphere Reserve were built up only of Rotatoria and Copepoda which is a typical property of the potamoplankton but not of the zooplankton of stagnant water bodies. Cladocerans are in very small quantities or they simply are not present (their average relative participation is 0.23% for the numbers and 0.1% for the biomass).

The absolute values are lower than those typical for the eutrophic water bodies. This substantial difference may be explained with the presence of a multitude of newly hatched fishes in the period of March through May that later turn to feeding on benthic animals or become predators. This opinion of ours was supported by the trend in increasing of the percentage of the big crustaceans in May and their secondary increase in October and November. The Rotatoria preserve their high numbers almost throughout the year as the fish-press is of much lesser importance to them than to larger crustaceans.

The retrospective analysis, which we made using the data of Kraeva (1992) and from our studies in the 1998/1999 period allowed us to conclude that the species composition of Rotatoria and Copepoda in recent times was significantly richer and the dominating species from order Cladocera, with the exception of Bosmina longirostris, previously belonged mainly to the Daphnidae family, which in 1998 was absent and was again well presented in 1999, mainly by Daphnia galeata and Bosmina longirostris. In March and April the Copepods were 70 to 90% of the summary biomass.

In quantitative aspect during the period of 1990 through 1992 there was a winter-spring and a summer (in August) peaks having absolute values of 6.8 mln ind./m3 and 10.17 mln ind./m3 for the numbers and 90 and 102 g/m3 for the biomass respectively, while the average annual ones for the two amounted to 4.3 mln ind./m3 and 47.6 g/m3. It was a curious fact that both peaks owed their existence to the Cladoceran Bosmina longirostris. Cyclopoida prevailed only in April and Rotatoria (Brachyonus diversicornis), in June. Both numbers and biomass decreased substantially which obviously may only be explained with entering of large quantities of the Danube water in the lake. The river water caused a change of the dominant species by bringing into the lake lighter plankters.

The lack of information for the last several years since the connection between the lake and the Danube has been restored do not permit us to assess the actual changes the zooplankton quantitative and qualitative composition has undergone. As a blank spot in our information on this group of aquatic animals remains the almost complete lack of information on the protozoan component in the zooplankton structure and composition.

The dominating complex of species as well as a number of indicator species allow us to refer Srebarna Lake to the eutrophicated ß-mesosaprobic stagnant water bodies of the Southeast Europe. High summary numbers and biomass for the period May through October are indicative for the comparatively smaller relative share of the predatory plankters and the weak pressure from the plankton-eating fishes. A strong influence of carnivorous fishes and fish-eating birds is also quite possible, an indirect evidence of which are the considerably lesser quantities of zooplankton for the period November through April.

The rather big differences found for the qualitative and dominating species composition in 1998 and 1999 are indicative for unstable, mainly trophic relations within the ecosystem, which affect negatively its homeostasis.

Proposed measures

Among the major negative factors affecting zooplankton development are the substantial fluctuations in the water level and the pollution of the lake with pesticides and fertilizers applied in agricultural practices in the lake water catchment area. Saprobic loading of the lake waters with organic matter (both native and imported from the water catchment area and from the nearby village) are also among the threatening factors.

Preventive actions may include stopping of the influx of water polluted with toxic substances from the lake watershed or the import of biodegradable organic matter with waste water. In our opinion it seems recommendable to put an end to the practice of burning the reed in the winter but we will recommend the removal of part of the bottom sediments outside the lake watershed. A must for the successful conservation of the Srebarna Biosphere Reserve is to provide for a regular ‘ventilation’ of the lake at the time of the Danube high water levels. Reduction of the area of the reserve buffer zone or any infringements of the conditions of its maintenance should be considered as highly unacceptable.

To ensure these measures are implemented correctly it will be necessary to work out projects related to:

  • Implementation of alternative (ecologically friendly or organic) agricultural techniques excluding the use of pesticides and fertilizers within the lake water catchment area;
  • Treating and/or taking away of the waste waters formed in the nearby village outside the lake watershed within the framework of a water quality management programme;
  • Rational and efficient management of the reserve water volume with view to maintaining optimal levels/volumes for the development of the zooplankton. Fishes (see Part 1.18.4) Macrozoobenthos

Historical Data Review

Regarding the macrozoobenthos of Srebarna Lake, it should be emphasized that the community has never been a subject to systematic benthological investigations until now. All available data were summarized in Michev et al. (1998) and concerned the record of species from various taxonomic groups, some of them being a component of the bottom communities. This synopsis contributed to the better knowledge of the species diversity of the reserve as a whole.

Most of the available data were findings during episodic visits to the lake and/or additional samplings, obtained mainly from phytophilic communities along the lakeshore. In general, one may assume that there are 136 species registered in the faunistic inventory of the reserve, which are or could be a component of bottom communities (Michev et al., 1998).

The first investigations on an ecosystem level have been undertaken by Assistant Prof. Dr. St. Kovachev from the Sofia University, School of Biology, Chair of Hydrobiology (personal communication) in the period of 1994/1995, when the first effects of the canal connecting the Danube to Srebarna Lake were expected. Unfortunately, at that time results were entirely negative: there was no macrozoobenthos at all. According to Dr. Kovachev's personal opinion, the state of the lake ecosystem could be described as an ecological regress, when (concerning at least the benthic communities) the ecosystem has gone into a passive state and is not able anymore to consume efficiently the incoming organic matter, and for this reason, was degraded under microaerobic, even anaerobic conditions close to the lake bottom.

The prediction about future restoration of the macrozoobenthos under the conditions of regular inflow of fresh water from the Danube and maintenance of relatively high water levels in Srebarna Lake was ascertain by this research, carried out in the period of 1997 through 1999.

Actual state

Within the framework of the Project Monitoring Programme quantitative bottom samples were collected monthly with the help of an Eckmann's drag of 225 cm2. Single preliminary quantitative observations were performed in August-September 1997, but samples were collected mainly from May through November 1998 and from March through July 1999 at 5 permanent sampling points in the central part of the lake (a total of 60 samples (App. 1, Map 15). In parallel with collecting quantitative samples, qualitative ones were also taken on three occasions (in August, September and October 1998) along the lake shore and at the connecting canal with the Danube (App. 1 Map 15) - a total of 16 qualitative samples.

As a result of the present investigation, some 34 species were found as newly recorded for the bottom fauna of the lake, the finding of another 40 species was confirmed. Thus the number of all species recorded so far has increased from 136 (after the summarized data from "Biodiversity of the Srebarna…") to 170. It is a fact, however, that by the number of recorded species the diversity of the shoreline areas is considerably higher compared to that from the lake open water area at the sampling points (a total of 45 taxa).

This study found that Segmentina nitida (Gastropoda) was the only globally threatened species in the lake. Other species of the same conservation status like Hirudo medicinalis (Hirudinea) and larvae of Gomphus flavipes (Odonata) were not found during the period of this study. The dragonflies Brachytron pratense and Leucorrhinia pectoralis (Odonata), also globally threatened species, have not been confirmed since the late 1960-s (Beshovsky, Marinov, 1993).

Special attention should be paid to the fact that, even as poor in the number of its species as was found, the macrozoobenthos was presented all the time while the biological season in the lake lasted. This is a sign for some stability of the lake environment as compared to the period before 1995. Important characteristics of the lake ecosystem recovery was the participation of secondary aquatic insects, among them the main dominant species being Chaoborus crystallinus, together with several representatives of Chironomidae and other Diptera larvae. The participation of primarily aquatic species (like worms, molluscs, crustaceans, etc.) is still slight as regards their population density. The most common species are the moluscs A. lacustris, B. tentaculata, V. piscinalis, P. corneus, D. polymorpha, and the oligochets L. claparedeanus, L. hoffmeisteri, P. bavaricus, etc.

It could be inferred that after completion of the connecting canal in 1995, the bottom invertebrate communities have started slowly to recover their species diversity and that the macrozoobenthos could be found during the whole vegetative season in the lake.

Data on the density and biomass of the bottom community are distinguished by low values: on the average 190 ind/m2, with a maximum at station V in July 1999 (1188 ind/m2). As has been already mentioned the benthos here is composed of permanently presented but single larvae of Chaoborus cristallinus, some chironomid and other insect larvae and single molluscs. The "soft" biomass has also been built up by the larvae of Chaoborus cristallinus as well as by those of Chironomus plumosus and other chironomids, single Dragonflies (Odonata), Mayflies (Ephemeroptera) and Water Bugs (Corixidae). Its maximum value was recorded at station V in April 1999 (14.7 g/m2), while the average value for the zoobenthos for the period under study was 0.903 g/m2 - significantly lower compared to other, already studied, wetlands (Kovachev & Uzunov, 1981) (App. 8, Fig. 1, 2).

Based on the "soft" biomass only, a preliminary estimation of the annual benthic production was made of 5.148 g/m2 (at an average for the period 0.903 g/m2) or 8.125 t/y per 150 ha of open water area of the lake. If the fish populations could utilize one third of this production (about 2.7 t/y), the expected growth of the fish mass would be only 387 kg/y, or 4.8% of the annual production of the macrozoobenthos. These approximate estimates are actually lower than the real figures could be, when one takes into account the share of the moluscs in the fish diet, as well as the trophic capacities of the phytophilic invertebrate communities along the lake's shore for fishes.

Proposed Measures

It seems that the factor affecting in the most negative way the biodiversity and species abundance of the bottom dwelling communities is the overloading the lake with autochthonous organic matter, together with the accompanying changes of the quality of water and sediments. The rates of organic matter influx (crude plant debris and detritus) exceed the capacity of the macrozoobenthos to utilize it. Dynamics of the hydrochemical parameters within the process of utilization and mineralization of the incoming organic matter can also be regarded as a negative consequence of the organic loading. All anaerobic and/or microaerobic processes near the bottom interface, leading to reduction with forming of NH3 and HS2, have a real negative potential. Hydrochemical data do not support this assumption but one can smell the hydrogen sulfide in bottom samples taken from the middle of the lake or from the slime taken between the stems of the emergent aquatic vegetation along the lake shore, or between the floating reed-beds.

Amongst factors, threatening the macrozoobenthos the lake water quality may be of substantial importance for regulating the biodiversity and the abundance of benthic communities. Apart from the possible import of waste waters from the nearby village, the basic allochthonous (or external) sources of pollution are the non-point influx from the agricultural activities in the lake water catchment area where intensive agricultural techniques are implemented including the application of fertilizers and pesticides as well as point loading with polluted Danube water via the Dragayka canal.

Actual risk of polluting the lake water by the farming practices may be estimated as a low one because of the current level of implementation of intensive farming techniques. In the future, however, the risk may grow as a result of a possible intensification of the agricultural activities in the watershed. It should be recommended to stick strictly to the adopted farming techniques, treating the crops with chemicals on written instructions and, possibly, by introducing alternative agricultural practices in the Reserve water catchment area.

The risk of polluting the lake via the Danube water is greater, particularly at accidental flooding, but as a rule the Danube water can be controlled by the sluices of the connecting canal.

A serious negative, even fatal factor for the macrozoobenthos could be the planned dredging of the bottom sediments. This kind of remedy may have some positive effect on the aquatic ecosystem as a whole, but with respect to the recovery of the macrozoobethos, it may turn out to be a disaster. It seems likely that several years should pass before restoring back the minimum parameters of the bottom communities after such a treatment.

There are no special measures undertaken for the protection of the macrozoobenthos. The main salvation step for the lake ecosystem was the construction of the canal connecting the Danube with the lake. It ensures a regular inflow of fresh river water thus having a positive effect for the recovery of the macrozoobenthos.

On a short-term basis, it is not necessary to undertake any special protective measures in the near future in order to maintain the trend of species enrichment and quantitative development of the bottom invertebrate communities. High water levels of the lake may have some positive effect, as will do the cut-down and/or complete removal of all sources of pollution from the arable lands surrounding the lake, from the village proper and from the Danube polluted waters. Limiting the sediment removal activities, the introduction of alternative farming methods (without the use of fertilizer and pesticides, etc.) will also have a favourable effect.

On a long-term basis, one should think about the construction of the so called "Western canal" in order to restore the flow inside the lake that will ensure the washing away of organic sediments, the import of suspended matter from the Danube, the improvement of mechanical and chemical parameters of the bottom sediments, etc. Production & Destruction of Organic Matter

Historical Data Review

Functional structure (i.e. the basic biological processes of producing and biodegrading of the organic matter) of Srebarna Biosphere Reserve ecosystems was studied only by G. Baeva (1994) and Vasilev et al. (manuscript). Baeva determines the primary productivity of the dominant species like Reed (Phragmites australis) and Lesser Reedmace (Typha angustifolia), forming comparatively pure associations in the so called "reed-belt", for the period of 1986/1987 by the annual increase of the plant biomass. Vasilev et al. studies were carried out in the period of 1990 through 1991 and were focused on the production and degradation processes in the water column of the lake using an oxygen modification of the "dark and light bottles" method.

Three main groups of producers are responsible for the synthesis of the primary organic production in the Srebarna ecosystems. The main share is produced by the so called "reed belt" where the dominant associations are composed of Reed (Phragmites australis) and Lesser Reedmace (Typha angustifolia). According to data supplied by Baeva (1994) the net amount of organic matter produced by the Phragmites australis association, which covers 402 ha of the reserve surface area, is 1926 g of absolute dry biomass per square meter per year (1926 g/m2 a year). A substantial part of the produced organic matter accumulates on the lake bottom.

The other group of producers with substantial share in forming the total gross primary production of Srebarna ecosystems is the phytoplankton. According to our yet unpublished data (Vasilev et al., manuscript) the gross primary production per square meter of water column (SGPP) for the period 1990 through 1991 fluctuates between 1 and 4 g of organic carbon per square meter per day (on the average 2.33 g/m2 a day of organic carbon) or 850 g of organic carbon per square meter per year. For the same period the destruction of organic matter for the whole of the water column (SR) was 10% less on the average. Bottom sediments metabolism was not assessed. If we assume that the average surface area of the lake's central part (mirror) is 1.5 km2 (150 ha) then the total gross primary production of the phytoplankton for the whole area of the lake will amount to 1275 tons of organic carbon (or 2560 tons of organic matter) per year while the net production will be about 256 tons of organic matter.

To put it in other words then 7742 tons of organic matter accumulate annually in Srebarna Biosphere Reserve reed belt of which 85 tons fall to the lake bottom (taking into account only a 20 m width of the belt around the lake's mirror) if the reed is not mowed and/or burnt away. Another 256 tons of organic matter synthesized by the phytoplankton also settle there. As a result, for the past 30 to 40 years a layer of sapropel and sludge over 1 m thick has been deposited on the lake bottom. As a whole the submerged macrophytes played a subordinate role as producers for the period of 1990 through 1991.

Actual State

The aim of the recent monitoring study is to trace out the trophic status of the lake as well as to assess the balance between the processes of production and degradation in Biosphere Reserve Srebarna. Phytoplankton primary production and destruction in the water column have been assessed by the classic oxygen modification of the ‘dark and light bottles’ method. The only difference between the studies in 1990 through 1991 and those carried out at present is that while in 1990-1991 the content of dissolved oxygen was measured by the chemical method of Winkler, in the current studies we have used the WTW-Multiline P4 measuring device. The diurnal oxygen cycle in the water column was measured as well. The concentration of chlorophyll "A" was measured by the spectrophotometric method. The water transparency was assessed by the Secci depth. The integral assessment of the light conditions in the lake has been made on the basis of the maximum depth of colonization. The primary production and the diurnal cycle were measured monthly at the sampling station No. 1 while chlorophyll A and Secci depth were measured at sampling stations 1, 3 and 5 (App. 1, Map 15).

Monitoring studies carried out in 1998 and 1999 on the production and destruction processes, the concentration of assimilation pigments and the photic conditions in the water of Srebarna Lake show a picture that has considerably changed in the course of time. The total (gross) primary production by the phytoplankton per unit area during this period has been 3.3 times less compared to that production for the same months in the period of 1990/1991. The maximum gross primary production per unit water volume has dropped even more drastically, the decrease being by an order of magnitude. There is also a trend for restoring the balance between production and destruction processes. The correlation production/destruction (SGPP/SR) = 1.05 (from 0.56 to 1.63). All data synonymously speak for a decrease in the lake productivity (App. 9, Fig. 1).

Lowering of the trophicity has been confirmed by data on the chlorophyll A concentration. For the period 1990 through 1991 the concentration of the total of chlorophyll A has been without any exception over 100 to 140 mg× m-3, i.e. the water body was indisputably classified as hypertrophic one. In 1998 the chlorophyll concentration (110 to 116 mg× m-3) is typical for the dividing line between hyper- and eutrophy and only in individual occasions (September, October) it has been of higher values. The large-scale rush of the Danube water in the autumn of 1998 led to the decrease in the chlorophyll A concentration by another order of magnitude, and in a number of cases it was even getting within the limits typical for mesotrophic water bodies (App. 9, Fig. 2). The maximum values in 1999 were 50 to 60 mg× m-3. A more or less permanent trend has been apparent for the decrease in the trophic levels from hyper- towards eu- and even mesotrophy. This trend has also been confirmed by the data for the transparency (Secci depth) which has increased by 2 to 10 times, 4 to 5 times on the average (App. 9, Fig. 3).

Changes in the maximum depth of colonization as a long-time integral indicator that characterizes the light conditions in the lake confirms the already recorded favourable trends. In 1998 through 1999 colonies of Ceratophyllum were also found in the main part of the lake at 1 to 3 m depth. This, by itself, shows that irrespective of the much higher water level the photic zone spreads through the whole of the water column.

Monitoring studies on the intensity of the production and destruction processes in the lake shall most likely continue in this way even after the project is over. Collection of bigger series of data will allow, on the one hand, a more accurate assessment of whether the balance between production and destruction processes has been restored and, on the other hand, will reflect in due time the possible alarming trends as for instance an increase in the trophicity towards hypertrophy or significant disturbance of the balance between the basic biological processes.

A blank spot in our information currently remains the production of bottom macrophytes, although we can reasonably assume for the time being that it is negligibly small (especially in the main open water area). The exchange between water and sediments has not been measured directly.

Proposed measures

Because of its hydrologic peculiarities Srebarna Lake has act as a trap for nutrients. This has brought about an over-eutrophication.

The huge amounts of organic matter synthesized in Srebarna ecosystems cannot be utilized by consumers along the food chains, neither can it be reduced by reducers. The disturbed balance between the processes of producing and degrading organics has accelerated incredibly the natural succession of the water body and the accumulation of a thick layer of sapropel and silt on the lake bottom. A large part (approximately 2/3) of the basin former area has overgrown with Reed, Reedmace, Bulrush, willows and other plants and practically have ceased to be a lake surface proper. This alarming trend has continued, though considerably slowed down, in recent years too. The Srebarna Lake turns into a marsh and the prospect to disappear, as an aquatic ecosystem in not so distant a future is rather realistic.

Changes in the hydrochemical conditions as a consequence of the eutrophication and particularly those changes that go hand by hand with the phenomena of "blooms" like dying out of fish due to oxygen depletion in water have led to disturbances in the trophic chains by most often eliminating the higher trophic links. Disturbed relations in the community deprive the pelicans and other fish eating birds from their food and force them to search for food elsewhere all the time.

The decrease of the lake depth makes the access to rare birds breeding colonies much easier for wild boars, jackals and other predators thus reducing the breeding success in some cases almost to zero.

Taking into account the negative factors we can recognize some long-term objectives for restoring the lake water natural state:

  • Limiting the influx of nutrients into the lake;
  • Taking away part of the nutrients;
  • Management of the relations within the community by biomanipulations.

Based on the above some operational objectives and proposals for counteracting the effect of negative factors are suggested. Measures for limiting of nutrient influx consist of:

  • All waste waters from the lake water catchment area to be caught by a peripheral canal and taken directly to the Danube after adequate treatment;
  • Limiting the use of fertilizers and manure on arable land in the region to be limited. To use instead sapropel and sludge from the lake if acceptable from ecological point of view.

Taking away part of the nutrients may be done by:

  • Pumping out the sapropel;
  • Dredging the bottom sediments;
  • Mowing and removing the reed, bulrush, etc.;
  • Taking away part of the fish production.

Biomanipulations in managing the relations within the community consist of natural or man-induced restoration of:

  • Populations of end consumers like the big predatory fishes – Wels (Silurus glanis), Pikeperch (Stizostedion lucioperca), etc.;
  • Populations of consumers of the 1storder like some zooplankters, zoobenthic organisms, etc.;
  • Populations of rare and endangered species;
  • Selective catching of substantial part of the populations of some species.

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