Environmental influence on planktonic cladocerans and copepods in the floodplain of the Upper River Parana, Brazil

The abundance of microcrustacean species recorded in plankton samples from the littoral and pelagic zones of the Upper Parana River, State of Mate Gro sso do Sul, Brazil, was evaluated in relation to abiotic and biotic environ mental factors. The cladocerans Bosminopsis deitersi and Bosmina hagmanni w ere most abundant. The presence or absence of aquatic macrophytes also infl uenced abundance and species composition. The true zooplankton species were most abundant at the pelagic stations, while species of Chydoridae and Mac rothricidae contributed markedly to microcrustacean abundance and species r ichness at littoral stations. Species abundance was also strongly affected by water level variation. Microcrustaceans, mainly bosminids, were most abu ndant during low water. During high water, bosminids decreased in abundance while most other species increased. The higher variation of the environmen tal factors during high water probably explains the shift in abundance patt erns. Changes in water level increased mixing of littoral and pelagic micro crustaceans between sampling stations.