Dynamics of autotrophic picoplankton in Lake Constance

Abstract

The vertical distribution, biomass concentrations and growth rates of autotrophic picoplankton (APP) were investigated during the growing season (March-October) in Lake Constance in different depths. Cell numbers determined by epifluorescence microscopy varied between 1.0 × 10 3 and 1.6 times; 10 5 cells ml −1 depending on season and water depth. Highest concentrations were recorded above the thermodine in late summer. Numerically, APP consisted almost exclusively of chroococcoid cyanobactena. During lake stratification several peaks of biomass concentrations occurred in epilimsietic waters at intervals of 6–8 weeks. In-situ experiments using a dilution technique and dialysis bags revealed that during summer APP population dynamics were primarily driven by combined changes of their growth and grazing rates, whereas temperature was less important. Gross growth rates varied between 0.006 and 0.051 h −1 , grazing rates between 0.002 and 0.053 h −1 . On average APP production was completely removed by grazing within the microbial community. Ciliates, heterotrophic nanoflagellates and rotifers have been identified as the major consumers of APP cells. APP biomass is small compared to larger phytoplankton, ranging from ito 5% of total phytoplankton biovolume. Due to its high gross growth rates, which are on the same level as those of free-living pelagic bacteria, APP contributes slightly more to overall primary production with maximum percentages of ˜15% in late summer.