Dominant cyanobacterial genera in Lake Peipsi (Estonia/Russia): effect of weather and nutrients in summer months

Hydrochemical and phytoplankton data from L. Peipsi (3555 km 2 , mean depth 7.1 m) for July-September 1997-2011 (two lake basins) and for August 2003-2011 (three lake basins) were analysed. Our aim was to explain the impact of nutrient content and weather factors on the composition and species dominance of cyanobacteria. The share of cyanobacteria was on average 64% of the total biomass, maximum values amounted to 93% and 38 g m -3 in the areas of open water. Close to the lake shores these values reached 99% and 100 g m -3 in some cases. The most prevalent taxa affecting cyanobacterial biomass were Gloeotrichia echinulata in the littoral areas and Microcystis species in the open water. Principal component analysis placed all dominant genera (Aphanizomenon, Anabaena, Gloeotrichia, and Microcystis) separately from each other. Stepwise multiple analysis showed G. echinulata to be fairly independent of nutrients and related to the days with water temperature over 22 °C. The biomass of the genus Microcystis was found to have evidently the strongest positive connections with phosphorus and also with iron, as well as with all potentially toxic (vacuolated) forms of cyanobacteria. The other group of cyanobacteria, mainly small-celled colonial forms (Aphanocapsa, Aphanothece, Cyanodictyon, etc), constituted on average up to a fourth of the cyanobacterial biomass; its biomass showed a reasonably positive correlation with nitrogen and a negative correlation with water level. In summer, the succession of cyanobacterial genera in the lake started with Anabaena, then Gloeotrichia appeared (in the larger and deeper moderately eutrophic northern part of the lake), followed by Microcystis and Aphanizomenon.