Drivers of cyanobacteria dominance, composition and nitrogen fixing behavior in a shallow lake with alternative regimes in time and space, Laguna del Sauce (Maldonado, Uruguay)

Laguna del Sauce, one of the main drinking water sources in Uruguay, is an eutrophic shallow lake with high temporal variation of inorganic turbidity caused by extreme wind events. During low turbidity periods, high phytoplankton biomass can be reached, frequently associated to cyanobacteria blooms, which can cause interferences in the water supply. In this study, we assessed the environmental drivers of cyanobacteria dominance, composition, and nitrogen-fixation behavior. For this, we analyzed the spatial and temporal phytoplankton composition, physical and chemical variables performing weekly samplings during two summers: 2015-2016 and 2016-2017. When inorganic turbidity washigh (above 30 NTU), phytoplankton biomass was controlled, below this threshold, temperature, secchi depth and nutrients played key factors controlling cyanobacteria biomass and composition. Blooms of N-2-fixing cyanobacteria (Dolichospermum crassum, Aphanizomenon gracile, and Cuspidothrix issatschenkoi) were promoted by low N:P ratios (average 11.5) and wide TN range (286-1300 mu gl(-1)). Non-heterocystous cyanobacteria blooms occurred above TN 1000-1200 mu gl(-1). The N-2-fixing behavior (heterocytes:vegetative cell ratio) depended on TN, it was highest at low TN (<700 mu gl(-1)) but null above ca. 1000 mu gl(-1). While low inorganic turbidity allowed cyanobacteria blooms in Laguna del Sauce, its composition and N-2-fixation behavior depended on the TN and TP levels.