THE SEASONAL DYNAMICS AND TROPHIC RELATIONS OF THE PLANKTON COMPONENTS IN LAKE PEIPSI (PEIPUS)

The seasonal dynamics of the biomass and production of phyto-, zoo- an d bacterioplankton was investigated during the vegetation periods (fro m May to November) in 1985 and 1986 in the pelagial of the large eutro phic lake Peipsi (Estonia). The average values of productions per vege tation period for the investigation years were as follows: phytoplankt ion - 203.5 gC . m-2; bacterioplankton - 37.9 gC . m-2; filter-feeding zooplankton - 20.6 gC . m-2 and predatory zooplankton - 1.5 gC . m-2. The herbivorous zooplankton production constituted 10.1 % of primary production. This ratio indicates a direct relationship between zoo- an d phytoplankton in the food chain - filtrators are feeding mostly on l iving algae and the detrital food chain seems of little importance. Th e dominance of large forms (Melosira sp., Aphanothece saxicola), in th e phytoplankton during the major part of the vegetation period is assu med to be a result of high grazing pressure on small algae. Zooplankto n grazing was investigated in situ in a specially constructed twin bat hometer. Experimental measurements revealed, that zooplanktion presenc e in the experimental vessel actually stimulated the phytoplankton gro wth in many cases - the negative grazing values have been registered. That could be caused by the stimulation effect of nutrients (N, P), ex creted by the concentrated zooplankton in the grazing chamber, which l ed to an increase of the nongrazed phytoplankton production. Bacteria have satisfied the zooplankton food requirements on average by 11 %. G razing on bacteria increased, when grazing on phytoplankton was someho w disturbed.