Lake Plessa 107 (Lusatia, Germany) – an extremely acidic shallow mining lake

Abstract

Lake Plessa 107 is an example of the older, relatively small and often shallow mining lakes of Lusatia which only have groundwater inflow. From a morphological point of view, the lake should be polymictic with short stratified periods. But besides temperature, mixing is also determined by chemical gradients in the water column that can lead up to monomixis. The lake water shows an extreme acidification with high concentrations of calcium, iron, aluminium, manganese and sulphate. Despite low TIC and TP concentrations allowing only a low primary production in the pelagial within the oligotrophic range, anoxic conditions can occur during stratification because of Fe(II) oxidation and anoxic groundwater inflow. The phytoplankton is dominated by phytoflagellates. Chlorophyll concentrations follow a yearly pattern determined by temperature and light availability. The zooplankton consists of two rotifer species, ciliates and heliozoans. Sediment analyses show contrary depth gradients of Fe and P with a very high fraction of Fe in the upper sediment layers (up to 60% of DW) which decreases with depth. Probably due to groundwater inflow, at some sites substantial decreases in redox potential and conductivity can be observed with increasing sediment depth accompanied by increases of pH, DOC, DIC and DIP concentrations. No correlations have been found between the available phosphorus or carbon concentrations in the sediment porewater and the phytobenthic biomass. Euglena mutabilis(Euglenophyceae) and Pinnularia acoricola(Bacillariophyceae) are the dominant phytobenthic species. Lake Plessa 107 has a benthic food-web that consists of benthic algae, chironomids and corixids and a pelagic food-web which is composed of phytoflagellates, rotifers, ciliates and heliozoans. The two food-webs are not coupled because larger prey organisms such as crustaceans are missing.