The abundance and diversity of planktonic ciliates in 12 boreal lakes of varying trophic state

Abstract

The cell density, biomass and community structure of planktonic ciliates in 12 Finnish boreal lakes ranging from oligotrophic to hypereutrophic were studied in 1997. Ciliate numbers and biomasses showed a trend of increase toward eutrophication, and both were strongly correlated with the abundance of bacteria and phytoplankton. The mean cell densities for ciliates were: 2.9, 5.0, 9.8 and 20.3 cells/ml for oligotrophic, mesotrophic, eutrophic and hypereutrophic lakes, respectively. The ciliate communities were dominated by four orders: oligotrichs, haptorids, scuticociliates and prostomatids. The relative importance of oligotrichs and haptorids decreased along the trophic gradient from oligotrophy to eutrophy. In more eutrophic conditions these orders were progressively replaced by small scuticociliates and prostomatids. The diversity of ciliates in lakes was positively correlated (R 2 = 0.84) with the concentration of chlorophyll-a. The peak abundances, in terms of both cell numbers and biomasses, were recorded in spring and early summer (most eutrophic lakes) or autumn (oligotrophic and most mesotrophic lakes).