Abstract
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.
Highlights
Cyanobacteria are a group of prokaryotic photosynthetic organisms dating from the Precambrian Period, who were able to colonize virtually every ecosystem on the planet (Fernandes et al, 2009; Chaves et al, 2009)
Observou-se forte influência das elevadas concentrações de fósforo total e estabilidade no estabelecimento de Planktothricoides raciborskii (R2 = 97%, Effects of cyanobacterial summer bloom on the phytoplankton structure in an urban shallow lake, Guaíba Lake p = 0.01)
The objective of this research was to evaluate the influence of cyanobacterial summer blooms in the phytoplankton structure in a shallow urban lake contributing to the comprehension of the driving factors involved in the recurrent summer bloom events in Guaíba Lake, southern Brazil
Summary
Cyanobacteria are a group of prokaryotic photosynthetic organisms dating from the Precambrian Period, who were able to colonize virtually every ecosystem on the planet (Fernandes et al, 2009; Chaves et al, 2009). Several strategies used by blue-green species may allow them to suppress other algae species, for example, creating a light limited condition due to excessive biomass generated by the bloom (Hubble and Harper, 2001) Many factors such as temperature, light, nitrogen, phosphorus, carbon, predation and microbial interactions may be involved in the establishment of these organisms (Dokulil and Teubner, 2000). In shallow lakes, mixing patterns and stratification of the water column can manage the availability of nutrients (mainly N and P) and light directly influencing the cyanobacterial blooms and species composition as shown by Bicudo et al (2007) In this context, considering the several environmental conditions that may favor cyanobacterial blooms and the strategies of blue green species that let them good competitors, the hypothesis that cyanobacterial blooms may reduce phytoplankton species richness, diversity and evenness during bloom events is tested in the present study. The objective of this research was to evaluate the influence of cyanobacterial summer blooms in the phytoplankton structure in a shallow urban lake contributing to the comprehension of the driving factors involved in the recurrent summer bloom events in Guaíba Lake, southern Brazil
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