Trophic gradients and associated changes in the plankton community in two bays of Lake Ladoga

Simultaneous analyses of water quality parameters, and the composition of phytoplankton, crustacean and rotifer assemblages were done at two sampling areas in Lake Ladoga: the Sortavala Archipelago (August 1994 and 1995) and the Bay of Volkhov (August 1994). The samples were taken at stations located at different distances from the main nutrient-loading points in both areas. The plankton communities in the two study areas differed due to differing hydrological conditions and also with respect to the waste water influence. The effect of nutrient loading on the plankton community is evident along the Bay of Sortavala. However, wind-induced currents from the pelagic zone to the bay also affect the structure of the plankton community. In the Bay of Volkhov, high discharge and turbidity as well as the waste waters appear to decrease phyto- and zooplankton biomasses in comparison to those in the Bay of Sortavala.