Estuaries of Contrasting Trophic Status in KwaZulu-Natal, South Africa

Abstract

The aim of the present investigation was to assess the feasibility of using sediment diatom communities to monitor pollution levels in the estuaries of KwaZulu-Natal. To this end, benthic diatom communities were examined along the length of two morphometrically similar estuaries of contrasting trophic status; the eutrophic Manzimtoti and oligotrophic Mhlabatshane. It was observed that: (1) the total number of diatoms was significantly higher in the Manzimtoti estuary than in the Mhlabatshane; and (2) species composition, rather than community structure, correlated well with nutrient levels of the water. In this regard, the benthos of the Manzimtoti was dominated byNitzschiaspp. andNaviculaspp. whileDiploneisspp. andAchnanthidiumspp. were common in the Mhlabatshane estuary. Furthermore, variations in the diatom communities were recorded along the length of the estuaries, in that: (1) species richness, diversity, equitability of distribution and number of individuals tended to decrease downstream; and (2) the brackish and freshwater assemblages were similar within each estuary but differed markedly from saline sites located at the estuary mouths. As a result, these observations indicated that any attempt to use benthic diatoms to monitor the nutrient loading of estuaries in KwaZulu-Natal must make use of species composition data from sediments sampled at comparable sites along the length of the estuary.