The impact of changes in nutrient loading on cyanobacterial dominance in Lake Peipsi (Estonia/Russia)

The influence of changes in nitrogen (N) and phosphorus (P) loadings in 1983-2001 on cyanobacterial development in Lake Peipsi (3.555 km(2), mean depth 7.1 m) was studied. After the collapse of Soviet-type agriculture in the early 1990 s, total N loadings decreased sharply while the in-lake total P concentration increased, forming favourable conditions for harmful bloom-forming N, fixing (N(2)fix) cyanobacteria. The average percentage of the biomass of N(2)fix cyanobacteria in total phytoplankton biomass increased from 13 % in 1983-1991 to 29 % in 1992-2001. Gloeotrichia echinulata, a species capable of vertical migration and consequent transport of P from the lake bottom to the upper layers developed earlier in the season (June) at lower nutrient concentrations than other cyanobacteria in L. Peipsi (Anabaena, Aphanizomenon, Microcystis). The fixation of N-2 by G. echinulata as well as its ability to use sediment phosphorus could bring about the enrichment of the water column with nutrients. This enables the other N(2)fix species Anabaena and Aphanizomenon as well as Microcystis, which is not generally N-2 fixing but requires high nutrient concentrations, to flourish.