Critical N:P ratio for cyanobacteria and N2-fixing species in the large shallow temperate lakes Peipsi and Vortsjarv, North-East Europe

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 Vortsjarv (270 km(2), 2.8 m). The average mass ratio of total nitrogen (TN) and total phosphorus (TP) in Vortsjarv (45) was about twice as high as that in Peipsi (22). In Peipsi, the N-2-fixing Gloeotrichia echinulata, Aphanizomenon flosaquae and Anabaena species prevailed in the summer phytoplankton, while in Vortsjarv the dominant cyanobacteria were Limnothrix planktonica, L. redekei 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 similar to 40 in Vortsjarv and similar to 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 Vortsjarv, the TN: TP supporting a high Nfix% was between 30 and 40 and the Nmin: SRP supporting this high percentage was in the same range as that in Peipsi (<20), though the maximum Nfix% values in Vortsjarv (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 Vortsjarv are much higher than the Redfield N: P mass ratio routinely considered (7). Our results represent valuable 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.