Srebarna Lake

Category: Inland Aquatic Biomes.

Geographic Location: Eastern Europe.

Summary: Srebarna Lake is the only significant Danubian wetland in Bulgaria; it shelters a significant percentage of the endangered Dalmatian pelican.

At roughly 2.5 square miles (6.5 square kilometers), including wetlands and the lake, Srebarna is the largest river lake in Bulgaria. Surrounding and protecting the freshwater lake from intrusion is the Srebarna Nature Reserve, a World Heritage Site. The lake and reserve are located approximately 100 miles (160 kilometers) from the Black Sea to the southeast and 85 miles (140 kilometers) from Bucharest, Romania, to the northwest. The combined ecosystem incorporates former farmland, a belt of forest plantations along the river, three islands in the Danube River, and the water between the island and riverbank. Vegetation here is that of the Ukraine-Kazakh biotic province. Adjacent to Srebarna is the Pelikanite, another nearly 2-square-mile (5-square-kilometer) enclave of protected pelican environment. The main purpose of the reserve is wildfowl protection for half of the avifauna in Bulgaria; this area is on the Western Palearctic bird migratory flyway.

Srebarna Lake is quite close to the Danube River, which provides seasonal flooding of the lake. Surrounding hills serve as a natural boundary as well as a perfect vantage point for observing the waterfowl and other birds that nest, breed, or stopover here. The elevation is only 32–43 feet (10–13 meters). Temperature averages are 28 degrees F (minus 2 degrees C) in January and 73 degrees F (23 degrees C) in July. Average rainfall is 20 inches (50 centimeters), with summers increasingly drier since the 1990s.

Flora and Fauna

Srebarna is the only significant protected tract of Danube wetlands in northeastern Bulgaria. Wetland habitats here include standing and temporary open water with submerged vegetation, reed beds, swamp, seasonal marsh, river, hay meadows, and poplar stands. The wetland is of the sort that was once common across much of coastal Bulgaria, and it supports increasingly at-risk plant and animal species.

There are more than 130 plant species present in the Srebarna Nature Reserve; approximately one dozen are rare or endangered. Two-thirds of the area is covered in reeds, with varying amounts of lesser reed-mace, water lilies, willows, and osier. There are floating reed islands that serve as breeding and resting places for migratory birds. At the northern end of the lake, reedbeds gradually yield to wet meadows. The northwestern end of the lake, as well as the area along the Danube, includes riverine forest belts that include single old white willow trees.

Srebarna Lake is a significant breeding and wintering ground for a large number of bird species. Ninety-nine species of birds breed here, and some 80 migratory species from other parts of Europe and Africa winter here. Of the more than 200 bird species that have been identified in immediate lake area, nine are globally threatened, and 78 at risk in Europe. Twenty-four bird species that breed in Srebarna are endangered or rare.

The Dalmatian Pelican population is thought to be the only one left in Bulgaria. Globally threatened pygmy cormorants, ferruginous duck, white-tailed eagle, and corn crake found here are among the largest populations remaining worldwide. Other interesting species found in the lake biome include terns, egrets, herons, ibis, and the white spoonbill.

There are approximately 40 mammalian species supported by this wetland. Included are 18 rodents, four carnivores, three ungulates, and seven mustelids (weasels). Threatened mammals include otters, two types of polecat, and a wild cat. Invasive species include muskrats, raccoon dogs, and jackals, as well as wild boar, roe deer, red deer, and hares in large enough numbers to be hunted in nearby areas. Fish species number 18, including six endangered in Bulgaria. Reptiles total 15 species, amphibians 12.

Human Impact

Until creation of a flood-control dike for marshland drainage in 1948, the lake enjoyed annual periods of high Danube water. The dike stopped the annual inflow, causing the lake to deteriorate as vegetational succession accelerated. Organic mud filled the lake, causing it to become shallower, decreasing open water surface, and diminishing fish and bird populations and variety. Underground springs and runoff from the hills helped, but did not replace the Danube.

The two Iron Gates Dams built in Romania, with construction beginning in 1972 and completed in 1984, damaged the lake environment further, as did the increased pollution from neighboring farmlands. Drought between 1982 and 1994 decreased average water depth of the lake; nitrogen and phosphorus from agricultural runoff created a hypereutrophic condition in which the over-abundant nutrients supported the growth of unhealthy algal blooms, leading to lower oxygen content in the water, and, ultimately, to lower biodiversity. The turning of the lake into a marsh adversely affected phytoplankton, fish, and birds.

Reconnection of the lake to the river began in 1979. By 1994, two canals allowed renewed river water flow, and all agricultural and residential activity around the lake was halted. In 1999, the mean depth was reestablished at 7 feet (2 meters), with a maximum depth of 9 feet (3 meters). The current lake contains much of its former open water areas and reedbeds. It is still surrounded by marsh, with low hills and farms beyond that. The 1994 canal slows the deterioration—but continues to restrict flow from the lake to the river. Lake maintenance requires two-way flows.

While the dense reeds are a barrier around the lake, as well as the foundation for reed islands on which birds nest, thickening of this vegetation has also caused its own set of problems. The extension of these reedbeds has attracted predators such as fox, jackal, wild boar, and wild cat that prey on the nests and on the 100-year-old Dalmatian pelican colony. This colony is the gem of all the avian life here, with breeding pair counts ranging from 29 to 127 between 1950 and 1980. The Srebarna Nature Reserve was established as a United Nations Educational, Scientific, and Cultural Organization (UNESCO) World Heritage Site in 1977.

During recent decades, the Srebarna Lake ecosystem has undergone significant changes toward hypereutrophication (when an abundance of nutrients sets off a chain reaction of algal bloom and plant growth, reducing the oxygen and leading to dieoff) and degradation because of the disrupted natural connection with the Danube River water, due to both natural and man-made factors. These include the Iron Gates Dams, the dike built in 1948, the drought during 1984–93, and the new canal built between the lake and the Danube in 1994. These phenomena have led to mainly negative changes in the lake and wetland ecosystems, which depend largely on annual flooding from the Danube.

The latest restoration programs, such as canal dredging, have provided successful interventions that have led to a partial recovery of the wetland’s functioning. However, the Lake Srebarna biome now faces additional pressures from global warming. Unlike some far-upstream segments that may see average flow diminish as glaciers retreat and rainfall patterns shift, this lower section of the Danube River is expected to experience surface level rises in coming years. Flooding of Lake Srebarna lasting beyond historic seasonal cycles is therefore an anticipated threat. However, great uncertainties in such projections remain; those who wish to help conserve this biome must also plan for the opposite: a climate change outcome where the Danube River flows into the lake are far reduced, leading to a diminishing of the depth, extent, and habitat richness of Lake Srebarna. Wildlife experts have noted that climate change is affecting species as well. The Dalmatian pelican, for example, has traditionally followed a short migration within a range of wetlands. However, in modern times many populations have limited their range to areas close to breeding grounds. The importance of maintaining and improving the conditions in these areas is thus vital. Pelicans breed in small colonies of up to 20 nests over large masses of stone, mud, sticks, reeds, grasses, and feathers, but flooding endangers their nests. Artificial nesting platforms that allow them to breed above flood levels have proven successful in encouraging the pelicans to establish breeding colonies despite low-quality natural habitats.

Bibliography

Michev, T. Biodiversity of the Srebarna Biosphere Reserve. Sofia, Bulgaria: Pensoft Publishers,1998.

Nikolova, Mariyana, Rumiana Vatseva, and Valentin Nikolov. GIS Assessment of Global Change Impacts on the Dynamics of the Srebarna Lake Ecosystem. Sofia: Bulgarian Academy of Sciences, Institute of Geography 2010.

Riley, Laura and William Riley. Nature’s Strongholds: The World’s Great Wildlife Reserves. Princeton, NJ: Princeton University Press, 2005.

"Turning the Tide for the Dalmatian Pelican." Rewilding Europe, 18 July 2021, rewildingeurope.com/blog/turning-the-tide-for-the-dalmatian-pelican/. Accessed 2 Sept. 2022.

UNESCO World Heritage Centre. “Srebarna Nature Reserve.” 2012. http://whc.unesco.org/en/list/219.