The status of the southernmost part (Lake Pihkva) of large Lake Peipsi: A purification pond or polluter?

Large Lake Peipsi (3555 km²) is divided between two countries, Estonia (44%) and Russia (56%). The southernmost part of the lake, Lake Pihkva, is almost entirely (99%) in Russia, and is continuously being polluted by the greatest inflow to Lake Peipsi, the insufficiently purified River Velikaya. Research on this lake (Lake Peipsi) in the growing season has been possible as a result of Estonian–Russian joint expeditions (2003–2010) in the month of August. Our study has demonstrated statistically significant variances in the hydrochemistry and zoo- and phytoplankton data between the lake parts. Several parameters used for characterizing the lake's ecosystem were considerably higher for Lake Pihkva than for the larger Lake Peipsi s.s. These include increases in TP concentration (3×); TN (2×); Chlorophyll a (3×); biomass of cyanobacteria (4×); Microcystis (4×); Aphanizomenon (5×); abundance of Chydorus sphaericus (5×); Keratella tecta (8×). However, several values were lower in Lake Pihkva than in Lake Peipsi: water transparency (3×); ratio between zoo- and phytoplankton biomasses (2×); cladoceran mean weight (3×); abundance of Eudiaptomus gracilis (1.5×); abundance of Kellicottia longispina (1.8×). Different natural conditions (topography, catchment area, relative depth) and different pollution loads in the lake parts have resulted in apparently different resistances in their ecosystems and different responses to human activity. At present, on the basis of TP data (up to 200 mg m⁻³), Lake Pihkva appears to act a polluter of adjacent lake part rather than a purification pond.