Zooplankton fluctuations in Jurumirim Reservoir (São Paulo, Brazil): a three-year study.

During three consecutive years, monthly samples of zooplankton were taken in the lacustrine (dam) zone of Jurumirim (São Paulo, Brazil). The seasonal effect on basic limnological features (thermal regime, oxygen distribution, phytoplankton biomass, etc.) was also examined. The influence of the seasonality on the fluctuation of the zooplankton composition and abundance was not clearly detected (low degree of recurrent patterns). Rotifers (32 taxa) were the most abundant organisms during almost the entire study period with some seasonal alternations in the maximum abundance peaks of the main taxa (Conochilus unicornis, Keratella americana, K. cochlearis and Hexarthra spp.), except for Polyarthra (mainly P. vulgaris). Only occasionally copepods were numerically dominant. Higher copepod abundance was positively associated to periods of increase in the water retention time. Among the Copepoda (10 taxa) the calanoids (mainly Notodiaptomus iheringi) were more abundant, especially in warmer periods. Conversely, cyclopoids had higher abundance in autumn and winter. The species Thermocyclops minutus and T. decipiens co-occurred, but the first attained higher abundance. Some evidence of co-existence strategies between both species are considered. Cladocera (17 taxa) was never numerically dominant and the main taxa (Bosmina spp., Ceriodaphnia spp. and Diaphanosoma spp.) occurred almost the whole study period and did not present a seasonal pattern of fluctuation. Diaphanosoma (mainly D. birgei) attained the highest abundance among cladocerans. Most organisms were always found at the surface, but they also occupy the whole water column, even in periods of stratified conditions and low oxygen concentration in the bottom layers. Among the main zooplanktonic taxa, only Hexarthra avoids deep layers. An exceptionally high concentration of Copepoda nauplii on the surface was influenced by low transparency, high concentration of phytoplankton at this layer and low oxygen concentration at the bottom. In periods of higher retention timevariability there was a more heterogeneous distribution of the zooplankton in the water column. The increase in the retention time seems also to favor the copepod development. Finally, some inter-decade changes are considered on the basis of zooplankton assemblage structure observations.