Algal blooms in Estonian small lakes

The database of the Centre for Limnology includes phytoplankton material from 500 lakes: this represents approximately 21% the total number of the lakes in Estonia. A phytoplankton biomass value greater than 8 mg L-1 was set as the limit criteria for further investigation. Blooms of cyanobacteria have been the most frequent and intensive: 20 different species of cyanobacterial blooms in different lake types have been detected in approximately 15% of the Estonian lakes. Until 1970s, the most widely distributed phytoplankton mass occurrences were caused by genera Microcystis and Anabaena. After that period changes in plankton communities took place and chroococcal cyanobacteria were replaced by filamentous forms, e.g. Planktothrix, Aphanizomenon and Limnothrix. From that period onward cyanobacterial biomasses have diminished, probably because of a decreased nutritient load from agriculture. Raphidophyte Gonyotomum semen was firstly recorded in the beginning of 1980s and up to now this species has been found in more than 80 lakes, which belong to three different limnological type: dystrophic, semidystrophic and oligotrophic. In addition, blooms of chrysophytes, dinoflagellates, cryptophytes and chlorophytes were also documented.