LT(S)ER Lake Engure drainage area

Lake Engure drainage area (644 sq.km) includes the Lake which is a remnant of Litorina sea formed about 4000 years ago. Part of the territory is the Lake Engure Nature Park, the Ramsar site including unique inland and marine wetlands. Most of the drainage area is covered by pine forests, but there are also large areas of marshlands, meadows, deciduous forests, dunes and agricultural lands. Avifauna lists 186 species, vascular plants – 844 species. 44 species of birds, 5 fish species and 3 plant species are protected at European level, 23 habitats are in EU habitat directive. The ecosystems of the area have been changed by different human activities historically well documented. The most important activities in ancient times included regulation of water level, hay making, pasturing, hunting, and fishery. The traditional settlement type was the former fishermen's village, which is characterized by its linear structure along the seashore and farmsteads inland. During the past decade, such settlements have been subject to a wave of summer cottage, second home and guest house construction. The agriculture and main industry (fishing and fish processing) has sharply declined. Nowadays, the highest number of employees is in the service sector (wholesale, catering, tourism and leisure industries). It sets a new kind of pressure to the ecosystems of the region. LT(S)ER should serve as an instrument for solving the problem of sustainable development of the region.

There are 12 LTER sites where entomological and plant community studies are carried out since 1995. Sites include both specific and rare habitats of the Engure Nature Park and widely distributed habitats of the region. Sites include sample plots 0.1 – 0.2 ha. Each sample plot contains 3 permanent vegetation quadrats (2 × 2 m).

Investigation of long-term successional changes in different plant communities. Investigation of grass-dwelling arthropod communities on the background of climate warming.

Plant community structure and species diversity. Abundance and species diversity of grass-dwelling arthropods mainly Diptera and Coleoptera.

Plant communities within the permanent quadrats are described by using Braun-Blanquet five-level cover scale during midsummer. Grass-dwelling arthropods are collected by entomological sweep-net method 50 sweeps per plot 3 times per season following special sampling scheme.

Since 1995– up to now. Site manager Gunta Spriņģe.

Three sampling sites at river transect (at Mersraga canal latitude 57°N 14.987; longitude 23°E 04.454; at River Dzedrupe inflow latitude 57°N 15.214; longitude 23°E 05.175, southern part latitude 57°N 12.615; longitude 23°E 09.278).

Investigation of long-term processes in lake water and sediment communities on the backround of climate warming and changing land use in the drainage area of the lake.

Water chemistry (COD, BOD, nutrients, ionic composition), bacteriology, phytoplankton, benthic macroinvertebrates.

For water chemistry and biology „Standard Methods for the Examination of Water and Wastewater” (American Public Health Association/American Water Works Association/ Water Environment Federation, USA) were used.

• The increase of annual, spring (March-May) and winter (December-February) temperature, annual and winter precipitation, changes of water level show a good relationship between Western type of Atmospheric Circulation Index in general and between NAO indexes in March and lake water level in April.
• From the local point-of view the most important factors are historical development of the lake including canal built in the middle of 19th century and connecting Lake Engure with Gulf of Riga, high concentrations of calcium identifying the lake as hardwater lake, low level of reactive phosphorus and its speciation with calcium compounds and abundant macrophyte stands with charophytes.
• Since 1995 decrease in nitrites and reactive phosphorus has been observed that is clearly limiting element in the lake water whilst concentrations of ammonium and nitrates haven’t changed. Positive trends are found for pH and chlorides.
• Phytoplankton has a low biomass and significant changes are not observed. The abundance and biomass of benthic invertebrates have increased.
• In general, large scale impacts to Lake Engure are confirmed nevertheless local scale factors are even more relevant. At the moment Lake Engure has clear-water state mainly due to its particular internal factors.

Ornithological studies by the Institute of Biology on the Lake Engure and its surroundings were carried out since 1951, including stationary investigations since 1958 up till present. Engure Ornithological Research Centre provides logistics for researchers. Centre manager: Mara Janaus.

Studies of dynamics of breeding bird fauna (species richness and abundance) in relation to different environmental both natural and anthropogenic factors, with the main accent on ducks, larids, waders and coot. Waterbird habitat management and maintaining.

Species composition, population abundance, demographic parameters (mortality/survival, philopatry/dispersal, nesting success), migration.

• Annual nest counts and following their fate on sample plots (up to 110 ha) on islands and emergent vegetation stands (ducks, waders, larids).
• Periodical evaluation of the nesting population size on the whole lake (larids, coot, grebes, common crane, etc.).
• Population demography studies based on mass-scale ringing, including original ringing method of day-old ducklings and capture-recapture analysis (ducks, mostly Tufted Duck, Pochard, Shoveler, Mallard).
• Studies of breeding success of the Black-headed Gull using method of fenced sampling plots and mass-scale ringing of chicks.
• Studies of feeding flights of the Black-headed Gull using registration of dyed specimens.
• Experiments with waterbird breeding habitats to increase their carrying capacity – fragmentation of emergent vegetation, vegetation control on islands, predation control.
• Registration of numbers and species/sex/age composition of harvested waterbirds at the beginning of hunting season (August).

• Altogether 187 breeding bird species have been found on the Lake Engure Nature Park. In 71 species major quantitative changes have been observed: negative trends in 44, and positive trends in 27 species. Habitat succession was found as the most important cause for negative alterations. Southern origin species prevailed among newcomers, and northern origin species – among diminishing/vanished ones. Especially pronounced negative changes were observed since 1992/1993 till the late 1990s’.
• Black-headed Gull population changed from 200 pairs in late 1940s up to 34000 pairs in 1986, and declined to 4200 pairs in 2002. Increase was promoted by switch to anthropogenic food and its abundance, decline – by successional changes of vegetation, sudden disappearance of anthropogenic food, predation by American mink.
• Black-headed Gull feeding flight distances changed according to local population size. At maximum population size (mid-1980s) gulls flew up to 70 km, mass-scale feeding flights – up to 40 km.
• Studies of the Black-headed Gull allowed to describe population model in early 1970s which correspond to the intensive growth period: 2,3 hatched chicks/pair, fledging mortality rate 40%, first year mortality 60%, annual adult mortality 15% which provides annual growth rate of 22% (correspond to observed value). Following factors have been found responsible for fledging survival: hatching date, clutch and brood size, hatching sequence and weight, weather conditions. Post-fledging survival depends upon hatching time and weather conditions during breeding season.
• Following activities have been approved and proposed for maintaining and improvement of breeding conditions for waterbirds:
  • regulation of emergent and meadow vegetation;
  • predation control (especially alien species);
  • establishing of closed areas for breeding and moulting, and refuges during hunting season;
  • artificial nest sites elevated above water with predator guards (for Mallard).
• Number of annually shot ducks during first three hunting times in 1993-2010 fluctuated between 32 and 513 (average 190) and negatively correlated with water level. Number of harvested ducks declined by years reflecting both decline of local breeding populations (especially in Pochard) and number of active hunters.
• Due to ringing of ca. 86000 day-old ducklings and 12000 incubating females different demographical parameters of breeding duck populations are described: age structure (e.g. in Shoveler and Tufted Duck: old residential females – correspondingly 45% and 61%, first nesting local recruits – 44% and 20%, immigrants – 11% and 19%), amount and pattern of philopatry/dispersal, density dependence of duckling survival and breeding philopatry, compensatory mortality, etc. Detailed patterns of seasonal distribution and migration of Mallard, Pochard, Tufted Duck, Shoveler, Garganey, Pintail and Gadwall are described.
• Duck populations have declined dramatically since early 1990s from 2620 pairs (all species combined) to 625 pairs in 2010. Causes of these changes are following:
  • successional changes of vegetation (merging of separate stands into huge reed dominated massives, decline of meadow covered areas on islands and coasts);
  • dramatic decline and dislocation of Black-headed Gull colonies (preferred nesting places of ducks);
  • increased predation (mostly alien predators – American mink and raccoon-dog, as well as red fox).