Environmental gradients and zooplankton distribution in a shallow, tidal lake

Gradients in environmental variables may promote heterogeneity in the distribution, abundance or composition of a zooplankton community. In tidal systems, salinity gradients strongly affect zooplankton communities, while chlorophyll-a is often important in freshwater ecosystems. Lake Waihola, a shallow coastal lake that receives tidal inflows of 0-4.8‰, is dominated by two species of calanoid copepod, Boeckella hamata and Gladioferens pectinatus, and one cladoceran species, Daphnia carinata. Multiple regression and redundancy analyses of environmental variables were used to relate salinity, chlorophyll-a and particulate suspended matter to the dominant crustacean zooplankton taxa. Salinity explained >50 % of the species-environment correlation, and chlorophyll-a was negatively correlated with B. hamata and D. carinata. Increases in salinity resulted in a change in dominance from the predominantly freshwater taxa, B. hamata and D. carinata, to the predominantly estuarine species, G. pectinatus. We predict that increases in salinity associated with reductions in freshwater flow and/or increased tidal inflows in the future may cause significant changes in the zooplankton community structure, and affect trophic interactions in this lake.