GENERAL DESCRIPTION OF LAKE PEIPSI-PIHKVA

Lake Peipsi-Pihkva (3555 km(2), mean depth 8.3 m), consisting of three parts, (L. Peipsi, L. Pihkva, L. Lammijarv) is located on the border of Estonia and Russia. L. Peipsi belongs to unstratified eutrophic lak es with mesotrophic features, L. Lammijarv has some dyseutrophic featu res, while L. Pihkva is strongly eutrophic. The total annual nutrient load is 15.57 tons N km(-2) and 327 kgP km(-2) with 74% of N and 39% o f P originating from agriculture. The mean concentrations of total N a nd P in the lake are 876 mg m(-3) and 46 mg m(-3), respectively, both being the highest in L. Pihkva and the lowest in the northern part of L. Peipsi. Average pH is 8.14 and Secchi disk transparency 1.63 m. Dia toms and blue-green algae prevail in phytoplankton biomass. The blue-g reens Gloeotrichia echinulata and Aphanizomenon flos-aquae dominate in summer causing the water-blooms. The concentration of Chla was the lo west in the northern part of L. Peipsi (mean 14.7 mg m(-3)) and the hi ghest in the southern part of L. Pihkva (mean 47.9 mg m(-3), median 16 .3 mg m(-3)). An increase of Chla and decrease of Secchi depth could b e noticed in 1983-1988, while in 1988-1994 the tendency was opposite. The long-term average primary production is 0.8 g C m(-2) d(-1). Zoopl ankton is remarkably rich in species, the average biomass in the veget ative period being 2-3 g m(-3) and production 22 g C m(-2). The role o f rotifers in production is 53% followed by that of cladocerans (30%), copepods (16%) and Dreissena polymorpha larvae (1%). The total count of bacteria is 1-9 million cells per mi. Chironomus plumosus and Potam othrix hammoniensis are dominating in the profundal. The average abund ance of macrozoobenthos (without big molluscs) 2617 ind. m(-2), and th eir biomass 12.34 g m(-2) are considered to be the highest among the l arge lakes of North Europe. Macroflora occupies a small percentage of the total lake area but is rich in species. Taxa forming communities a re Potamogeton perfoliatus, Phragmites australis, Schoenoplectus lacus tris, Poramogeton lucens, Eleocharis palustris, and Polygonum amphibiu m. Submerged vegetation occupies the first place amongst different gro wth forms, followed by emergent plants. The main commercial fishes are lake smelt, perch, ruff, roach, bream, pike, vendace and pikeperch. T he stock of vendace has sharply decreased in the last years, while the amount of pikeperch has increased. Considering annual fish catches (9 000-12000 tons or 25-34 kg ha(-1)), L. Peipsi-Pihkva exceeds all large lakes in North Europe.