Cyanobacteria and Toxic Blooms in the Great Mazurian Lakes System: Biodiversity and Toxicity

The Great Mazurian Lakes System (GMLS), located in the northeastern part of Poland, is an extremely valuable area in terms of natural environment value, tourism, and local economy. The system is divided into two parts – the northern meso-eutrophic and the southern eutrophic. GMLS are lakes with very high taxonomic diversity of phytoplankton, and cyanobacteria are very often predominant in the species composition and biomass. The presence of cyanobacteria belonging to 14 different families from the orders of Nostocales, Oscillatoriales, Synechococcales, and Chroococcales was recorded throughout the system. The GMLS has undergone significant changes over the recent decades which affected the taxonomic composition and dominant species of cyanobacteria. Particularly the southern part was subject to significant changes, from rapid eutrophication in the 1970s and 1980s, resulting in massive blooms of cyanobacteria, to a significant improvement in water quality in the 1990s and a reduction of cyanobacteria biomass. However, cyanobacteria are the dominant component of phytoplankton up to the present, although there are no dense blooms in recent years. Many of the cyanobacteria taxa in the GMLS can potentially produce toxins. Hepatotoxic microcystins are the most common cyanotoxins in freshwater, and in GMLS they sometimes reached significant concentrations in water. Studies have shown that the main producers of microcystins in GMLS are genera Microcystis and Planktothrix.