Light as a controlling factor of winter phytoplankton in a monomictic reservoir

Abstract

Factors affecting the seasonal succession of plankton communities in freshwater temperate lakes have been thoroughly studied for decades. However, there are still relatively few data describing the winter season patterns in detail, as the focus has been mostly on spring to autumn conditions. Ice cover is often the crucial factor limiting light availability for winter phytoplankton, but in warm monomictic lakes is usually lacking and the gradually increasing solar radiation should, theoretically, drive phytoplankton growth. In this study conducted in 2002–2010, we documented regular sharp increases in phytoplankton chlorophyll a, starting just after the winter solstice and lasting throughout the total circulation and/or unstable inverse stratification period in the monomictic Slapy reservoir (Czechia). Chlorophyll a concentrations analysed in one-week intervals reached their yearly minimum of 0.2–0.8 µg L−1 in the solstice period, and the spring peak occurred before the onset of stable thermal stratification. The regular pattern was slightly disrupted in some years, associated with short periods of ice cover. Winter phytoplankton were species poor and dominated by diatoms, cryptophytes, green algae, and cyanobacteria. Using semiparametric regression approach, we aimed to test if selected environmental parameters had a significant effect on the observed winter trend. The resulting model revealed that solar radiation and water temperature positively influenced log chlorophyll a concentrations, whereas water age had a significant negative effect. On the other hand, zooplankton density and ice cover effects were not significant. The shapes of the marginal effects of water temperature and solar radiation were nonlinear, and the interaction of these two major factors was significant. The model-based estimated chlorophyll a concentrations showed a shift from radiation dominance to temperature-positive effects along the temperature gradient. This might represent as yet neglected pattern of phytoplankton seasonal development in warm monomictic lakes worldwide.