Changes in phytoplankton spatial and temporal dynamics in a Brazilian tropical oligotrophic reservoir after net cage installation

This study analyzed the phytoplankton community response and the spatial and temporal variation of abiotic variables caused by fish farming in net cages installed in Ilha Solteira reservoir, Sao Paulo, Brazil. Water samples were collected monthly (August 2011-July 2013), from the subsurface, at three sites: upstream from farming area (S1), farming area (S2), and downstream from farming area (S3) (n = 72). Multivariate analyses (PCA and CCA) were used for the data joint analyses. The activities related to tilapia farming in net cages promoted an increase in nutrient concentrations, as well as changes in the phytoplankton community, such as increased Cyanobacteria biomass, as it is hypothesized in this paper. Rhodomonas lacustris Pascher & Ruttner contributed to the highest biovolume along the studied period. However, 1 year after net cage installation, we recorded the highest biovolumes of Microcystis aeruginosa (Kutzing) Kutzing and Dolichospermum circinalis (Rabenhorst ex Bornet & Flahault) Wacklin, Hoffmann & Komarck. Both species were associated with higher concentration of ammonium and total phosphorus. The hydrodynamic characteristics of the analyzed system, such as short residence time (21.6 days) and outflow (172 m(3) s(-1)), probably generated the capacity to assimilate disturbances in water quality caused by the employed production process and are therefore instrumental in mitigating the impact of the organic load (from the feed and fish metabolism) in the studied aquatic system.