Ecosystem functioning of selected estuaries on the east coast of South Africa.

Abstract

River inflow is one of the most important factors influencing the density and biomass of estuarine biotic communities. The aim of this study was to obtain an understanding of the ecosystem functioning of selected estuaries on the east coast of South Africa and to represent their dynamics through ecosystem models. The responses of water column nutrients, plankton density and biomass to inlet phase changes for 16 temporarily open/closed estuaries (TOCEs) in South Africa were first determined. This analysis demonstrated that inlet phase and the duration of mouth closure were the most important factors determining plankton density and biomass of the analysed TOCEs. Estimates of planktonic standing stocks for four of these estuaries revealed that stocks can be from 26 to 10 000 times higher during the closed compared to the open phase. Also, slightly higher variability of planktonic density and biomass was recorded during the closed phase of TOCEs. The second major thrust of this study was to analyse the variability and temporal stability of planktonic and macrobenthic invertebrate density and biomass in two KwaZulu-Natal estuaries over a dry/wet cycle. The results suggest that “stable” variability and species synchronization could be the mechanisms whereby the estuarine biota of these systems compensate for environmental changes and attain a degree of environmental homeostasis. The third major thrust involved an assessment of the spatio-temporal variations in the elemental composition and stoichiometry of suspended and sediment detritus, zooplankton and macrobenthic taxa from two estuaries over a dry/wet cycle. Significant seasonal variations in the elemental composition of detritus, zooplankton and macrobenthic species were found, with the variations in the elemental content of sediment and suspended detritus being related to the seasonal changes in river inflow, while the among-taxa variability was mainly explained by feeding mode. Finally, static seasonal carbon and nitrogen ecosystem network models were developed for the East Kleinemonde, Mlalazi and Mpenjati estuaries to investigate their nutrient dynamics and ecosystem functioning. The results indicated that the East Kleinemonde and Mpenjati estuaries were mainly dependent on primary producers during the dry season, especially the high standing stocks of phytoplankton and microphytobenthos. Similarly, the dependency on detritus was higher during the wet season due to the high riverine imports during this season. Consequently, higher detritivory was recorded in all three study systems during the wet season. Cycling of nitrogen was higher than of carbon on a seasonal basis, with higher recycling of nitrogen during the dry season implying a lower availability of this element due to reduced freshwater inflow and nutrient input during the low rainfall period. System indices indicated that the organization of these systems was higher during the dry season, while the overheads on imports and exports peaked during the wet season. The ecosystem models analysed here provide an initial insight into the overall carbon and nitrogen dynamics of estuaries on the east coast of South Africa.