Changes in species composition during annual cyanobacterial dominance in a tropical reservoir: physical factors, nutrients and grazing effects

Abstract

Trends in recent years have indicated that cyanobacterial blooms in tropical reservoirs are increasing in frequency, magnitude and geographical distribution. Funil Reservoir in south- eastern Brazil has experienced eutrophication in the recent decades, resulting in lasting and intense toxic cyanobacterial blooms. As input of nutrients is high during the year, the aim of the present study was to evaluate the role of other variables related to changes in cyanobacterial biomass and compo- sition. The dominant group found over the entire study period was Cyanobacteria (97% of total biomass), which contributed to low diversity. A shift of nitrogen-fixing (Anabaena circinalis and Cylindrospermopsis raciborskii) and non nitrogen-fixing (Microcystis aeruginosa) cyanobacteria was observed. Redundancy analysis indicated that physical factors such as temperature, changes in the mixing zone and light intensity were the main driving factors of the seasonal succession. Nitrogen- fixing cyanobacteria dominated in periods of low light in the deepest mixing zone, and also seemed to have experienced stronger grazing effects as the density of the large zooplankton group was related to cyanobacteria biomass. M. aeruginosa bloomed in warm stratified waters, high water levels and during months with more daylight, when the zooplankton density was drastically reduced. Although the long-standing dominance of cyanobacteria may be related to high nutrient availability, the present study showed that under high and constant input of nutrients, other factors, especially physical variables, present a more plausible explanation to promote changes in species composition.