Abundance changes of autotrophic and heterotrophic picoplankton in the Zingster Strom, a shallow, tideless estuary south of the Darß-Zingst Peninsula (Southern Baltic Sea)

Abstract

The hypertrophic Darss-Zingst estuary had high abundances of autotrophic picoplankton (0.2-15.2.10 6 cells.ml -1 ) and heterotrophic bacteria (5.4-46.0.10 6 cells.ml -1 ) in the early nineties. Their contribution to total plankton biomass varied between 15 and 33 %. At most times of the year bacteria dominated the heterotrophic biomass. During the annual cycle, autotrophic picoplankton showed a clear spring peak and their production exceed bacterial production by up to 33 times. Bacterial secondary production was, on average, less than 50 % of carbon uptake by autotrophic picoplankton. The bacteria in 1990 showed high summer values which were not repeated in the following years. A long term trend of decreasing bacterial abundances was observed over 5 years. The spring peaks of picocyanobacteria of 1990-1992 disappeared in 1993/94. Nutrient limitation of bacteria could not be ascertained. Increasing grazing pressure on picoplankton could not be proven, too.