A comparative study of microalgal production in a marine versus a river-dominated temporarily open/closed estuary, South Africa

Temporarily open/closed estuaries (TOCEs) account for almost 71% of the total number of South African estuarine systems. To date the dynamics of microalgal production and biomass in TOCEs is poorly understood. A survey was therefore conducted in two TOCEs along the KwaZulu-Natal coast: the river-dominated Mdloti and the marine-dominated Mpenjati. The aims of this investigation were: (1) to compare the variability of benthic and pelagic microalgal production and biomass in these two contrasting systems; (2) to determine the key environmental parameters influencing primary production in these TOCEs. Results show a similar pattern of primary production and biomass in both estuaries. High benthic microalgal biomass with low primary production and low phytoplankton biomass with high rates of pelagic primary production were observed in both estuaries. Possible explanations for this include: (a) optimum light conditions in the water column, compared to the sediment surface; (b) high impact of grazing by zooplankton in the water column; and (c) settling of phytoplankton. Significant correlations were observed between primary production and environmental parameters (Kd, PAR, temperature, DIN and DIP), during the open and closed phases of both estuaries. A BIOENV (PRIMER) analysis showed that patterns of primary production in both estuaries were influenced primarily by interactions of multiple independent parameters. Comparisons between measured hourly pelagic primary production rates at the Mpenjati and Mdloti and the composite parameter (Pc) of Cole and Cloern (1987) revealed highly significant relationships (r2 = 0.8; p < 0.001) between these differently derived variables.