¹1Centre for Limnology, Institute of Agricultural and Environmental Sciences, Estonian University of Life Sciences, 61120 Rannu, Tartumaa, Estonia
²Rural, Water and Ecosystem Resources Unit, TP-483, Institute for Environment and Sustainability, EC Joint Research Centre, Via E. Fermi 2749m 21027 Ispra (VA), Italy

*Corresponding author: tiina.noges@emu.ee

Abstract

   From 1961-2004, surface water temperature in large and shallow Lakes Peipsi and Võrtsjärv in Estonia increased significantly in April and August; respectively 0.37-0.75 and 0.32-0.42 degrees per decade reflecting the changes in air temperature. Also the average annual amount of precipitation in the catchment increased significantly. Reflecting practices in agriculture and wastewater treatment, nutrient loadings to the lakes increased rapidly in the 1980s and decreased again in the early 1990s. As total nitrogen (TN) loading decreased faster than total phosphorus (TP) loading, the TN/TP ratio in the loadings decreased. Both the increased temperature and low TN/TP ratio favoured the development of cyanobacteria blooms in Lake Peipsi. In Võrtsjärv, where the TN/TP mass ratio is about two times higher than in Peipsi, blooms did not occur. Recently, the TN/TP ratio has shown a tendency of increase in both lakes suggesting a certain reduction of blooms to be expected also in Lake Peipsi. Nutrient dynamics in the lakes followed the changes in loadings, showing the ability of shallow lake ecosystems to react sensitively to changes in catchment management as well as in climate.