Zooplankton as an indicator of trophic conditions in two large reservoirs in Brazil

Abstract

The physical and chemical variables of the water, and the composition and structure of the zooplankton communities, in Tres Marias and Furnas Reservoirs in Minas Gerais, Brazil, were compared to characterize these environments in rela- tion to their trophic state. Higher values of electrical conductivity and chlorophyll-a, total solids, suspended organic mat- ter and total nitrogen concentrations were recorded in Tres Marias Reservoir. Higher water transparency and nitrite and nitrate concentrations were observed in Furnas (P < 0.000). Higher zooplankton densities were always obtained in Tres Marias Reservoir and, during the rainy period (P < 0.000), with mean values in the dry and rainy periods of 23 721 and 90 872 org m )3 , respectively, in Tres Marias Reservoir and 9022 and 40 434 org m )3 , respectively, in Furnas Reservoir. Copepoda was the dominant group in both reservoirs, mainly the younger stages (nauplii and copepodids). Based on the absolute and relative values, the contribution of rotifers was higher in Tres Marias Reservoir than in Furnas Reservoir. Although the Trophic State Index, based on water transparency and chlorophyll-a and total phosphorus concentrations, indicated an oligotrophic state for both reservoirs, the higher densities of the zooplankton community in Tres Marias Reservoir, as well as the predominance of cyclopoids and smaller-sized species such as bosminids, characterized this environment as mesotrophic. Larger-sized species such as calanoids, daphniids and sidids in Furnas Reservoir character- ized the better water quality of this reservoir.