Publication

Summary

Eutrophication of shallow lakes is often considered as the main reason for algal blooms, and especially cyanobacteria. Recent studies on cyanobacteria ecology point out the importance of climate change and variability of meteorological conditions on inter annual and seasonal variation of cyanobacteria community in lakes. In addition to deterioration of water quality, cyanobacteria produce toxins and various bioactive compounds harmful to wildlife, livestock and humans. The objective of our study was to examine inter annual and seasonal variation in cyanobacteria forming blooms, cyanobacteria composition and their toxicity in Lake Årungen, that is a shallow lake (mean depth of 8 m) located in the southeast of Norway. The mesotrophic state of the lake changed in the 1950?s from mesotrophic to eutrophic and in 1980?s to a highly eutrophic lake with severe summer algal blooms. Since then, several countermeasures have been taken to reduce the external loading of nutrients. A study on phytoplankton composition from the early 1970?s showed only the presence of Microcystis aeruginosa from cyanobacteria species. However, Planktothrix agardhii became the dominant algae in the period after external reduction of nutrients loading and improved water transparency. We analysed fieldwork data of cyanobacteria biomass and composition together with chemical data from the lake for the period from 1988 to 2002. Heterocystous species, Anabaena sp. and Aphanizomenon sp., were registered for the first time in 1988. Their occurrence and periodical dominance cannot be explained with lowering in-lake nitrogen concentration, but rather with their ecological strategies. Warm and wet Norwegian winters with early ice out and high input of nutrients from the catchment, as well as changed pattern of lake thermal stratification, seems to be most responsible for different cyanobacteria qualitative and quantitative response in the lake. Comparing year-round data of water chemistry, qualitative and quantitative phytoplankton, and chemical toxins analysis will provide more insights on the effect of seasonal variation on cyanobacteria composition and their toxicity. We collected such data from 2007 and 2008, and found that 2007 was a year with severe toxic cyanobacteria bloom and co-dominance of Microcystis aeruginosa and Anabaena sp.. Algal bloom and toxicity were not exhibited in 2008, and Aphanizomenon sp. was the dominant species. Our result emphasize the need for investigation on why the same morphotypes but different genotypes of cyanobacteria species occurs in certain years, and to what extent the combination of environmental factors, and to what extent anthropogenic influence respond on development of certain cyanobacteria and their strains in the nature.