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The present study aimed at evaluating the influence of cyanobacterial summer blooms on the phytoplankton structure in a subtropical urban shallow lake used as a water supply (Guaiba Lake, southern Brazil). Samples were taken close to the water intake site at Belem Novo district, which is operated and processed for public distribution by the Porto Alegre Department of Water and Sewage (DMAE). Monthly samplings were collected from October 2010 to March 2011. The following abiotic parameters were considered: N and P, conductivity, pH and water transparency, besides thermal profile, dissolved oxygen and relative water column stability (subsurface, 1.20 m and 0.5 m from the bottom). Samples for phytoplankton were collected in the water subsurface. Strong influence of high concentrations of total phosphorus and water stability was verified in the establishment of Planktothricoides raciborskii (R² = 97%, p = 0.01). Decrease in diversity, evenness and species richness evidenced the replacement of phytoplankton species by cyanobacteria biomass, especially due to the low light availability and soluble reactive phosphorus limitation. Changes in rainfall patterns related to the La Nina phenomenon might have influenced the abiotic scenario, recruiting inocula deposited on nutritionally enriched layers, favoring bloom-forming species. Additional studies are needed in order to fully understand the mechanisms involved in the complex dynamics of ecological factors related to the success of this group of organisms, especially in ecosystems used as water supply, presenting recurrent episodes of cyanobacterial blooms, as is the case of the Guaiba Lake. Key words: cyanobacteria, nutrients, La Nina, diversity.

Effects of cyanobacterial summer bloom on the phytoplankton structure in an urban shallow lake, Guaíba Lake, southern Brazil