Influence of light and mixing regime on bloom-forming phytoplankton in a subtropical reservoir

The joint influence of local climatic and hydrological conditions on the vertical distribution of bloom-forming phytoplankton was analysed for the Salto Grande Reservoir, a large and enriched subtropical system on the Uruguay River (South America). Threshold of physical factors hindering or impeding blooms was obtained and then contrasted with worldwide observations in reservoirs at similar latitudes. Inflowing discharge, water level, and wind velocity intensity interacted with temperature, producing mixing and light regimes with overriding influence on the vertical distribution of Microcystis spp., Dolichospermum spp., and Ceratium furcoides, hence affecting their maximum abundance and biomass. Cyanobacteria (Microcystis>Dolichospermum) showed the most heterogeneous distribution in the depth profile during strong thermal stratification, showing surface scums prone to horizontal displacements. C.furcoides was evenly distributed in the water column in correspondence to windy periods. Blooms of both Cyanobacteria and dinoflagellates did not occur when inflowing discharge exceeded 10,000m(3)s(-1). Nutrient influence on phytoplankton vertical distribution appeared strongly subordinate to the effect of light. Highest microcystin concentrations (>WHO alert Level 2) occurred especially after blooms collapsed during highly turbulent situations.