ELLS 2007 Critical N:P ratio for cyanobacteria and N 2 -fixing species in the large shallow temperate lakes Peipsi and Vo ˜rtsja ¨rv, North-East Europe Tiina No ˜ges Æ Reet Laugaste Æ Peeter No ˜ges Æ Ilmar To ˜nno Ó Springer Science+Business Media B.V. 2007 Abstract In the 1990s a sharp decrease in nitrogen loading occurred in Estonian rivers, bringing about a reduction of the nitrogen-to-phosphorus ratio (N:P ratio) in the large shallow lakes, Peipsi (3,555 km 2 , mean depth 7.1 m) and Vo ˜rtsja ¨rv (270 km 2 , 2.8 m). The average mass ratio of total nitrogen (TN) and total phosphorus (TP) in Vo ˜rtsja ¨rv (45) was about twice as high as that in Peipsi (22). In Peipsi, the N 2 - fixing Gloeotrichia echinulata, Aphanizomenon flos- aquae and Anabaena species prevailed in the summer phytoplankton, while in Vo ˜rtsja ¨rv the dominant cyanobacteria were Limnothrix planktonica, L. rede- kei and Planktolyngbya limnetica, which cannot fix N 2 ; the main N 2 -fixing taxa Aphanizomenon skujae and Anabaena sp. seldom gained dominance. In May–October the critical TN:TP mass ratio, below which N 2 -fixing cyanobacteria (Nfix) achieved high biomasses, was *40 in Vo ˜rtsja ¨rv and *30 in Peipsi. The percentages of both total cyanobacteria (CY) and Nfix (CY% and Nfix%) in Peipsi achieved their maximum values at an N:P mass ratio at or below 20 for both TN:TP and Nmin:SRP. In Vo ˜rtsja ¨rv, the TN:TP supporting a high Nfix% was between 30 and 40 and the N min :SRP supporting this high percentage was in the same range as that in Peipsi ( \ 20), though the maximum Nfix% values in Vo ˜rtsja ¨rv (69%) were much lower than in Peipsi (96%). The Nmin:SRP ratio explained 77% of the variability in Nfix% in May–October. The temperature dependence of Nfix% approximated to the maximum function type, with an upper limiting value at a certain water temperature, and this was most distinct in May–October. The critical TN:TP ratios obtained from our study (roughly 30 for Peipsi and 40 for Vo ˜rtsja ¨rv) are much higher than the Redfield N:P mass ratio routinely considered (7). Our results represent valu- able guidelines for creating effective management strategies for large shallow lakes. They provide a basis for stressing the urgent need to decrease phosphorus loading and to keep the in-lake P concentration low, and not to implement nitrogen reduction measures without a simultaneous decrease of phosphorus concentration. Keywords Large and shallow lake Á Nitrogen–phosphorus ratio Á Cyanobacteria Á N 2 -fixing cyanobacteria Guest editors: T. No ˜ges, R. Eckmann, K. Kangur, P. No ˜ges, A. Reinart, G. Roll, H. Simola & M. Viljanen European Large Lakes—Ecosystem changes and their ecological and socioeconomic impacts T. No ˜ges (&) Á R. Laugaste Á P. No ˜ges Á I. To ˜nno Institute of Environmental and Agricultural Sciences, Centre for Limnology, Estonian University of Life Sciences, 61101 Tartu County, Estonia e-mail: tiina.noges@emu.ee P. No ˜ges Institute of Environment and Sustainability, European Commission – Joint Research Centre, 21020 Ispra, VA, Italy 123 Hydrobiologia (2008) 599:77–86 DOI 10.1007/s10750-007-9195-x