GOMES, Luiz Carlos; MIRANDA, Leandro Esteban. Hydrologic and climatic regimes limit phytoplankton biomass in reservoirs of the Upper Paraná River Basin, Brazil. Hydrobiologia, Dordrecht, v.457, no.1-3, p.205-214, 2001.In reservoirs of the Upper Paraná River Basin, Brazil Paraguay, phytoplankton biomass is generally low relative to reservoirs in other parts of the world. To investigate what might be limiting phytoplankton biomass in the Upper Paraná reservoirs, we used an empirical approach wherein climatic conditions in the basin as well as chemical, hydrological and morphometrical characteristics of 13 reservoirs, were compared against those prevailing in 58 reservoirs of the south-centralUnited States. In both regions, phytoplankton biomass was correlated with chemical, hydrological andmorphometrical characteristics of the reservoirs. Therewere no differences in phosphorus concentrations between the two regions, nitrogen levels were higher in the Upper Paraná, ionic concentrationswere higher in the south-central United States and there were major differences in all physical characteristics considered. In reservoirs of the south-centralUnited States, hydraulic retention time increased and discharges decreased during the post-spawning period allowing increases in phytoplankton biomass; in the Upper Paraná River Basin, discharges peaked during the post-spawning period, delaying the peak of phytoplankton blooms until after completion of the post-spawning period. Our comparison between the two sets of reservoirs suggests that hydrologic regimes dictate differences in phytoplankton biomass and that hydrology of the Upper Paraná River Basin, exacerbated by climatic patterns, deter phytoplankton production despite suitable levels of essential nutrients.