From littoral to pelagial: Comparing the distribution of phytoplankton and ciliated protozoa along a transect

Abstract

The seasonal dynamics of phytoplankton and ciliates were examined at several stations from littoral to pelagial over 2 years. Special interest lay on the evaluation of the interchange between the littoral and pelagic community. The phytoplankton succession showed recurrent features at all stations along the transect. In the annual mean total phytoplankton biomass decreased from littoral towards midlake. The algal assemblage of the littoral waterbody was strongly influenced by detached phytobenthos species, mostly pennate diatoms, which diminished rapidly after the slope. Chlorophytes and cyanophytes also decreased along the transect. Cryptophytes and dinophytes showed different distribution gradients. Species of the same class could reveal countercurrent distribution patterns. The mean annual diversity index reached highest values nearshore. The ciliated protozoa also revealed recurrent species succession at all stations. However, they exhibited far less prominent gradients along the transect. Among the numerically most important representatives were Oligotrichidae in the annual mean which reached similar abundances at all stations, whereas Prostomatidae increased towards the pelagial. A further conspicuous difference between phytoplankton and ciliates was the fact that benthic ciliate species could only seldom be found in the littoral water samples. Nearshore and offshore water masses and the organisms therein were in a continuous mutual exchange. Thus a per se littoral water body does not exist in this part of Lake Constance.