Environmental variables and plankton communities in the pelagic of lakes: enclosure experiment and comparative lake survey

Most phytoplankton production occurs in the turbulently mixed surface layer of lakes and oceans. Environmental factors such as mixed layer depth and background turbidity substantially influence the availability of essential, production-limiting resources, i.e., light and nutrients. We tested the predictions of a dynamic model concerning the effects of water column mixing depth and background turbidity on phytoplankton biomass, light climate and nutrients in a field enclosure experiment. The epilimnetic phytoplankton community of a low productive lake was exposed to high and low background turbidity along a gradient of mixing depths ranging from 1 to 15 m. The availability of light and thus the specific rate of light-limited phytoplankton production decreased with increasing mixing depth and background turbidity. Specific sedimentation loss rate decreased with increasing mixing depth, too, but was not influenced by background turbidity. Fast-sinking diatoms dominated the phytoplankton community. For this scenario, the model predicts that algal biomass should be most strongly limited by sinking losses at shallow mixing depths, by mineral nutrients at intermediate mixing depths and by a lack of light at high mixing depths. As predicted from this shifting pattern of limitation, phytoplankton volumetric and areal biomass showed unimodal trends along the mixing depth gradient and were negatively influenced by background turbidity. A predicted shift of the biomass maxima towards shallower mixing depths with increased background turbidity was also observed. Only partly in line with expectations the concentrations of dissolved and total phosphorus in the water column were positively affected by mixing depth but were only marginally affected by background turbidity. In addition, the standing stock of sedimented phosphorus did not conform to the expected unimodal relationship with mixing depth. A possible explanation for the latter deviations is that the ratio of seston carbon to phosphorus was not constant, but decreased with increasing mixing depth and background turbidity.