Zooplankton: A Valuable Environmental Indicator Tool in Reservoir Ecological Management?

Abstract

The trends in zooplankton community were assessed in response to the variations in the trophic state and water quality, in Serra Serrada, a shallow reservoir, with seasonal water level fluctuations, located in the Portuguese part of River Douro Basin. Various ratios were also calculated: (1) the ratio of crustacean abundance to rotifer abundance (NCrust/NRot); (2) the ratio of large (>10 μg) cladoceran abundance to total cladoceran group abundance (NLarge-Clad/NClad); (3) The trophic state indices of rotifer abundance. The reservoir’s hydrological cycle was characterized by three phases. The maximum level phase lasted from January to the beginning of June, the emptying phase from mid-June to the beginning of September and the minimum level phase between mid-September to the beginning of the first autumn/winter rain events. The highest values for total phosphorus, soluble reactive phosphorus, nitrate, and ‘chlorophyll a’ were found during the minimum level phase. Rotifera was the most abundant taxa, except in summer and in autumn where Cladocera and Copepoda were dominant. The ordination space defined by the first two RDA axis accounted for 84.2% of species–environment relations and represented 25.4% of the variation in species data. The ratio NCrust/NRot have shown the highest role of rotifers in the zooplankton community in the low level phase. The ratio NLargeClad/NClad displayed a decreasing trend in the abundance of large-sized cladocerans for the same mentioned period. Redundancy analysis (RDA) revealed a strong influence of temperature, chlorophyll a, soluble reactive phosphorous and total phosphorous to the observed significant associations between the zooplankton assemblage and environmental variables. Therefore, these indices are suitable to be used in reservoir management as rapid tools to evaluate the effects of environmental disturbances with a concern to the reservoir ecological integrity.