The effect of extreme water level decrease on hydrochemistry and phytoplankton in a shallow eutrophic lake

A drought-induced drastic decrease in mean depth and volume brought about several effects in the ecosystem of L. Võrtsjärv (270 km2). Differences in alkalinity, Si and inorganic N in two consecutive winters can be attributed to the concentration effect resulting from different ice/water volume ratios in the lake. Smaller initial amount of oxygen that remained under the ice in winter 1995/96 accounted largely for the observed anoxia. Increase in phytoplankton biovolume represented another concentration effect, as a similar areal primary production in 1995 and 1996 resulted in about twice a higher density of algal crop in the summer of 1996, when the average amount of water in the lake was 1.7 times smaller. Due to a better mixing of shallow water, integral oxygen concentration during the ice-free period in 1996 was systematically higher than in 1995. Stronger sediment resuspension in 1996 enriched the water with suspended solids and nutrients, resulting in a slight increase in the trophic state. Lower water level in 1996 improved light conditions by `cutting off' less illuminated and less productive deep layers. This was probably one of the main factors which caused the collapse of the common dominants Limnothrix redekeiand L. planctonicaand initiated mass development (max. 36 g m−3) of Cyanonephron styloides.The observed changes in the phytoplankton composition in 1996 could be caused also by the selective grazing of larger zooplankton.