Relationships between environmental variables and the abundance of cladocerans and copepods in the Atchafalaya River Basin

The relationship of species abundance to eight environmental variables was tested for 24 common species of crustacean zooplankton collected in the Atc hafalaya River Basin during the summer of 1994. Stepwise regressions (alpha = 0.05) revealed significant relationships between zooplankton abundance a nd at least one environmental variable for is species (R-2 = 0.14 - 0.61, p < 0.0435 - 0.0001). The majority of these species' peak abundances were co rrelated with variables indicative of seasonal changes in floodplain habita t, as the Atchafalaya river receded, water temperature increased, and/or ph ytoplanktonic photosynthesis increased. Surface water temperature and the p ercent saturation of dissolved oxygen showed the most significant relations hips, but specific conductance, current velocity, and Secchi disk depths we re also related to abundance patterns of certain taxa. A principal componen ts analysis of species abundances provided further insight into the partiti oning of temporally-distinct zooplanton assemblages, showing that several s pecies (Bosmina longirostris, Daphnia parvula, Eurytemora affinis, and Ceri odaphnia quadrangula) predominated during early summer, and were supplanted by a distinct late-summer assemblage (Diaphanosoma birgei, Moina micrura, Mesocyclops edax, and Daphnia lumholtzi) as time progressed. The transition al assemblage was dominated by Simocephalus serrulatus, Macrocyclops albidu s, Microcyclops rubellus, and Thermocyclops inversus, all of which were mos t abundant in the hypoxic conditions characteristic of the latter stages of the Atchafalaya River flood-pulse.