Agnieszka NAPIÓRKOWSKA-KRZEBIETKE¹, Julita DUNALSKA², Jolanta GROCHOWSKA², Michał ŁOPATA² & Renata BRZOZOWSKA^2
1Department of Hydrobiology, The Stanisław Sakowicz Inland Fisheries Institute, Oczapowskiego 10 Street, 10–719 Olsztyn, Poland, e-mail: akrzebietke@infish.com.pl
²Department of Water Protection Engineering, University of Warmia and Mazury, Prawocheńskiego 1 Street, 10–957 Olsztyn, Poland

INTENSITY AND THRESHOLDS OF CYANOBACTERIAL BLOOMS – AN APPROACH TO DETERMINE THE NECESSITY TO RESTORE URBAN LAKES

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Cyanobacterial blooms are broadly recognized as a global problem for aquatic ecosystems, health and economics. This study focused on determining the intensity and main contributors of cyanobacteria blooms, and identifying the cyanobacteria-environment based recovery potential of impaired urban lakes. Based on physicochemical parameters, nutrient-rich (especially excess of nitrogen and phosphorus) conditions were recognized in the majority of lakes. In summer Cyanobacteria formed biomass on average from 2.5 to 88.8 mg L^-1. The WHO-designed low risk threshold (summer average > 2 mg L^-1) for cyanobacterial blooms was significantly exceeded in all lakes, while the WHO-designed moderate risk threshold (> 10 mg L^-1) was exceeded in five lakes. In two lakes, cyanobacterial biomass exceeded even the WHO high risk threshold specified for surface scums or the newly-proposed very high risk threshold of 40 mg L^-1 connected with possibly high potential toxin risk. Cyanobacteria assemblages were then dominated by potentially toxic species, mainly from the genera Microcystis, Cuspidothrix (Aphanizomenon), Dolichospermum (Anabaena), Limnothrix and Planktothrix, which considerably restricted the recreational use of these lakes. The response of cyanobacteria and dominant species to environmental variables (especially Microcystis), positively correlated with N and P enrichment, should also be very useful to determine the main water management targets for urban lakes.