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.
