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|Title:||Critical N:P Ratio for Cyanobacteria and N2-Fixing Species in the Large Shallow Temperate Lakes Peipsi and Võrtsjärv, North-East Europe|
|Authors:||NOGES TIINA; LAUGASTE Reet; NOGES PEETER; TONNO I.|
|Citation:||HYDROBIOLOGIA vol. 599 no. 1 p. 77-86|
|Type:||Articles in periodicals and books|
|Abstract:||In the 1990s a sharp decrease of nitrogen loading occurred in Estonian rivers bringing about a reduction of the N:P ratio in large shallow lakes Peipsi (3,555 km2, mean depth 7.1m) and Võrtsjärv (270 km2, 2.8 m). The average mass ratio of total nitrogen (TN) and total phosphorus (TP) in Võrtsjärv (45) was about 2 times higher than in Peipsi (22) and the mass ratio of mineral forms of nitrogen (Nmin) and soluble reactive phosphorus (SRP) in Võrtsjärv (76) was 3-4 times higher than in Peipsi (22). In Peipsi N2-fixing Gloeotrichia echinulata, Aphanizomenon flos-aquae and Anabaena species prevailed in summer phytoplankton. In Võrtsjärv dominant cyanobacteria Limnothrix planktonica, L. redekei and Planktolyngbya limnetica are not able to fix N2 and the main N2-fixing species Aphanizomenon skujae and Anabaena sp. commonly did not dominate. In both lakes the biomasses of all cyanobacteria (CY) and N2-fixing species (Nfix), and also the share of cyanobacteria (CY%) and N2fixing species (Nfix%) in phytoplankton biomass correlated strongly with the water temperature achieving the maximum values in August at ~20oC. In May-October the critical TN:TP mass ratio below which N2-fixing cyanobacteria achieved high biomasses was in Võrtsjärv ~40 and in Peipsi ~30. In Peipsi both CY% and Nfix% achieved the maximum values at N:P mass ratio at or below 20, for both TN:TP and Nmin:SRP. In Võrtsjärv TN:TP supporting high Nfix% was between 30 and 40 and Nmin:SRP supporting high Nfix% was in the same range with that in Peipsi (<20), though the maximum Nfix% values in L. Võrtsjärv (69%) were much lower than in Peipsi (96%). Concerning the monthly average values of Nfix% and Nmin:SRP ratio, a common power function could be developed for both lakes, which allowed to describe Nfix% on the basis of Nmin:SRP in the extent of 42% if all monthly averages were considered and even by 77% if the period May-October was considered. The temperature dependence of Nfix% was rather of the type of maximum function, which limits the maximum possible Nfix% in certain water temperatures being again most distinct in the period of May-October.|
|JRC Institute:||Sustainable Resources|
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