Reflection of the changes of the North Atlantic Oscillation Index and the Gulf Stream Position Index in the hydrology and phytoplankton of Võrtsjärv, a large, shallow lake in Estonia

The North Atlantic Oscillation Index (NAO) was correlated with winter and spring air temperature in Estonia while wind speed dynamics were mediated by the position of the Gulf Stream. In the low-NAO years, the large (270 km2), shallow (mean depth 2.8 m), polymictic lake, Võrtsjärv, was significantly shallower than in the high-NAO years. Phytoplankton biomass was higher in the springs after high-NAO winters, and it was inversely related with the depth of the lake in summer and autumn but not in spring. In Võrtsjärv, the North Atlantic Oscillation directly influences spring phytoplankton via its effects on the ice cover dynamics. The climate signal `recorded' in the water levels results in lower summer/autumn phytoplankton biomass in the years when the water level remains high throughout the year. The effects of light and phosphorus limitation on phytoplankton at higher water levels are discussed.