Cool winters versus mild winters: effects on spring plankton in Lake Peipsi.

The influence of two winter periods with a different duration of the ice cover on Lake Peipsi (Estonia) on plankton and nutrient content was analysed. The winters of 2005 and 2006 were cold with ice duration of 140 days, whereas the winters of 2007 and 2008 were mild with about 50 and 15 days of ice duration, respectively. Total phosphorus (TP) concentration was lower, while silicon content and the nitrogen-phosphorus ratio (TN : TP) were markedly higher in the springs after the short winters of 2007 and 2008. The high Si concentration and TN : TP ratio persisted throughout the growing season of those years. Unicellular centric diatoms showed a sharp increase in April 2008, while the large filiform diatom Aulacoseira islandica dominated in the cool winters and after ice break-up. The high spring peak of diatoms was followed by their low biomass in summer. In May 2008, total zooplankton biomass, cladoceran biomass, and rotifer biomass were two times and that of copepods three times as high as in the Mays after the long-lasting ice cover. In the Junes after the mild winters the biomasses of total zooplankton and both crustacean groups (Cladocera, Copepoda) were about two times as high as the corresponding indicators for the Junes after the cool winters. The biomass of rotifers, on the contrary, was two times lower in the Junes after the warm winters because the numerous cold stenotherms Polyarthra dolichoptera and Synchaeta verrucosa had totally disappeared from zooplankton. The influence of ice duration on phyto- plankton is most likely indirect, acting through nutrients, and on zooplankton direct, acting through water temperatures. The springs after warm winters related positively to zooplankters' mean weight, zooplankton-phytoplankton biomass ratio, and the timing of the clear water period.