Salto Grande's reservoir (31-degrees-S 50-degrees-W) is a river-like reservoir of 780 km2 with multiple arms. The limnological behavior of the impoundment shows that excessive fertilization (eutrophication) in the lower half of the reservoir is of sufficient magnitude to cause impairment of beneficial use of recreational areas. The reservoir presents different subenvironments: several differentiated arms plus a main channel. The central zone has a high rate of water renewal, with physical and chemical characteristics similar to those of the Uruguay river before being dammed, except for suspended solids. The high variability of the system makes possible that the main channel sometimes behaves as a river, and at other times as a lake. These differences have a significant influence in phytoplankton development. Under normal or elevated-flow conditions, water travel time is such that less than the maximum possible algae develop near the dam, compared to the norm in other lakes and reservoirs; therefore of particular concern are the consequent low-flow conditions. These conditions increase the time for algae development in the reservoir, and more importantly, allow for the sedimentation of inorganic turbidity, which under most conditions limits available light reducing algal growth below what normally occurs in waterbodies. This characterization as a 'hybrid' lake is confirmed by the differences in algal development, which reaches its maximum expression in the Gualeguaycito arm, and is not explained by differences in nutrient concentrations. At the same time some physical factors operate with unequal intensity in both systems. While in the lateral arms lentic conditions prevail, in the main channel the intensity of the turbulence and the horizontal transport establish lotic conditions. The nutrient conditions of the water characterize this reservoir as mesotrophic, and even sometimes eutrophic, which indicates that the nutrients are not the limiting factor in phytoplankton development. This fact has been confirmed by blue-green algae blooms that occurred during March and April of 1980 and 1982. Dissolved oxygen concentration is acceptable in the lake, albeit showing a tendency to decrease towards the bottom, especially in the lateral arms, and presenting the minimum values in summer. The relationship between Secchi depth and the hydraulic residence time clearly shows that, when the water is slow, the turbidity decreases, Secchi-depth measurements increase and, therefore, more algal growth takes place. The reservoir may be considered as a system composed of two markedly different subsystems regarding their biological manifestations: the system of lateral arms where the phytoplankton is exclusively dominated by Cyanophyceae, particularly Microcystis sp., and the main channel characterized by diatomous algae, with absolute dominance of Melosira sp. The evolution of algae concentrations and species, of chlorophyll-a concentrations, and of some physico-chemical parameters, emphasize the unequal behavior of both subsystems, which implies the development and persistence of different environmental conditions. Our results indicate that the phytoplankton of the reservoir is limited by the inorganic turbidity, except at the end of the arms, and is mainly determined by water renewal in the center of the reservoir and by the washing effect due to rainfall on the lateral arms.
