Spatial and temporal characterization of cyanobacterial blooms in Salto Grande Reservoir.

O'FARRELL, I.; BORDET, F.; CHAPARRO, G.

Workshop; 16th Workshop of the Internacional Association of Phytoplankton Taxonomy and Ecology; 2011

Instituto Agrario de San Michele all'Adige

Spatial and temporal characterization of cyanobacterial blooms in Salto Grande Reservoir Inés O’Farrell(1), Facundo Bordet(2), Griselda Chaparro(3) (1) (3) CONICET, Lab. Limnología, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, Argentina, e.mail: ines@ege.fcen.uba.ar (2) Universidad Nacional de Entre Ríos, Área Ecología, Comisión Técnica Mixta de Salto Grande, Argentina, e-mail: bordetf@fcal.uner.edu.ar Salto Grande is a river–like reservoir with multiple arms located along 100 km of the main channel of the River Uruguay (29° 43’ to 31° 12’ S and 57° 06’ to 57° 55’ W). The river discharge largely varies in dry and rainy periods. The reservoir, filled in 1979, is polymictic with short lasting stratifications under low flow conditions. Historical data show that phytoplankton was initially dominated by diatoms, with the exception of two arms where Microcystis aeruginosa thoroughly developed when water level was low. This species was rare in the main channel during periods of high water turnover. In this study we analyse the spatial distribution of cyanobacterial summer blooms from 2007 to 2011; these are mainly represented by scum-forming and nitrogen-fixing eco-strategists of the Anabaena and Microcystis genera. The use of eco-strategists approach, based on ecophysiological work and field observations, allows predicting which cyanobacteria can be expected to occur under certain conditions. The Microcystis spp. complex achieves higher density than the coiled Anabaena spp. complex. Moreover, the temporal dynamics of both complexes differs during each of the summer period studied. Spatial differences are produced due to much higher and fluctuating cyanobacterial abundances at the right margin of the reservoir and at the littoral areas closer to the dam. The sharp variations of the hydrological cycle contribute to explain the inter-annual fluctuations of the intensity and frequency of cyanobacterial blooms, whereas spatial differences between the reservoir arms, its margins and the main channel mainly respond to morphometrical and hydrological characteristics. Total cyanobacteria abundance usually exceeds World Health Organization alert level 2 (i.e. 1 105 cell mL-1) in summertime, especially at sites located at the right margin.