Investigation of spatial and temporal structure of microplankton community as well as its changes under specific abiotic conditions has been carried out in gypsum karst lakes of North Lithuania in 2000–2001. Data on abundance and species composition of predominant phytoplankton and heterotrophic protista is presented. High diversity of green algae, diatoms, golden-brown algae species, and cryptomonads is characteristic of karst lakes. In the biggest karst Lake Kirkilai, the abundance of phytoplankton varies from 0.023 × 106 to 8.0 × 106 cells/l, heterotrophic protista – from 350 to 4375 cells/l. Formation of anaerobic communities of certain cyanobacteria and heterotrophic protista species in sulphide strata is common in the investigated lakes.
Keywords: gypsum karst lakes, phytoplankton, heterotrophic protista, Lake Kirkilai, Lithuania.