Synopsis: Watershed of L. Võrtsjärv (3 104 km 2 ) belongs to the basin of L. Peipsi (47 800 km 2 ). Regular investigation of these large, shallow, nonstratified lakes (L. Peipsi: area 3 555 km 2 , mean depth 7.1 m; L. Võrtsjärv: 270 km 2 , 2.8m) started in the 1960s. In the 1980s, the riverine nutrient discharge increased drastically while in the early 1990s a sharp decrease occurred, especially in the loadings of nitrogen. As nitrogen loading decreased faster than phosphorus loading, the N/P loading ratio decreased. There has been an increase in cyanobacterial dominance in the late 1960s and early 1970s, decline in the late 1970s and 1980s, and a new increase in the late 1990s in both lakes. Since the beginning of the 1990s, the biomass of N 2 -fixing species has clearly increased in L. Võrtsjärv but much less in L. Peipsi. The increase of cyanobacterial dominance and the occurrence of water blooms can be caused by reduced N loading and decreased N/P ratio. The ecosystem of L. Võrtsjärv is very sensitive to water level fluctuations, which follow the North Atlantic Oscillation index refelecting climate changes in the northern hemisphere. In deeper L. Peipsi the direct influence of the water level is not so obvious.

1 Institute of Zoology and Botany, Estonian Agricultural University, Võrtsjärv Limnological Station , 61101 Rannu, Tartumaa, Estonia. E-mail: tnoges@zbi.ee

2 Institute of Geography &
3 Institute of Zoology and Hydrobiology, University of Tartu, Vanemuise 46, 51014 Tartu, Estonia. E-mail: ajarvet@ut.ee

4 Tallinn Technical University, Department of Environmental Engineering, Ehitajate tee
5, 19086, Tallinn, Estonia. E -mail: ennloigu@edu.ttu.ee