ZOOPLANKTON-PHYTOPLANKTON INTERACTIONS IN LAKES VORTSJARV, PEIPSI (ESTONIA) AND YASKHAN (TURKMENIA)

Zooplankton-phytoplankton interactions have been studied in three shal low lakes of different trophic state. In strongly eutrophic, large and very shallow Lake Vortsjarv the grazing does not play a leading role in controlling phytoplankton production and its standing stock. Small- size zooplankton can not eat filamentous blue-greens. The nutrient reg eneration by zooplankton has a weak impact on phytoplankton, the latte r being limited rather by underwater light than by nutrient availabili ty. In large moderately eutrophic Lake Peipsi the presence of concentr ated zooplankton in the experimental vessel mostly stimulated steady-s tate phytoplankton growth and negative grazing values were measured. M ost probably the nutrients (N, P), excreted by zooplankton in grazing chamber stimulated the growth of larger phytoplankton which dominated because of heavy grazing pressure on edible forms. In Lake Peipsi phyt oplankton seems to be nutrient-limited and heavily controlled by zoopl ankton community. In eutrophic, macrophyte-dominated Lake Yaskhan zoop lankton in grazing chamber also mainly stimulated the growth of phytop lankton community which seemed to be nutrient-limited because of stron g competition with macrophytes.