THE TROPHIC STATE OF LAKE-LADOGA AS INDICATED BY LATE SUMMER PHYTOPLANKTON

As a part of the joint Russian-Finnish evaluation of human impact on L ake Ladoga, we studied the phytoplankton of the lake in order to find biological indicators for eutrophication. A second aim of the investig ation was intercalibration of sampling and phytoplankton counting tech niques between the Russian and Finnish laboratories. Phytoplankton sam ples were collected from 27 sampling stations in the lake and from the rivers Volkhov and Neva in 9-13 August 1993. In surface water samples the phytoplankton fresh weight biomass varied in the range 218-3575 m g m(-3). Highest biomass values were encountered in Sortavala Bay, and lowest ones in the western central part of the lake. Phytoplankton sp ecies composition varied considerably in the lake; blue-green and gree n algae predominated near-shore areas and Cryptophyceae in the offshor e stations. Canonical correspondance analysis revealed close grouping of eutrophy indicating communities, dominated mainly by greens and blu e-greens, in the most nutrient-rich parts of Lake Ladoga, the Volkhov and Svir Bays. Samples from the vicinity of the inflows of Vuoksi and Burnaya Rivers and off Pitkaranta formed a separate group, dominated b y diatoms, most of which were typical to mesotrophic or eutrophic lake s, As judged by phytoplankton biomass values and chlorophyll a concent rations, Lake Ladoga may generally be classified as mesotrophic. Eutro phicated areas are found in the northern archipelago of the lake and i n the areas influenced by large rivers.