The ecology of two degraded estuaries on the north coast of Natal, South Africa

Abstract

The biota of the Tongati and Mdloti estuaries were studied in relation to the environment and human interference. Results are compared with those obtained from a relatively undamaged estuary. The Tongati receives treated sewage effluent, is rarely closed from the sea, has low salinities and low oxygen tensions, but is rich in phosphorus and nitrogen. Energy values of benthic floc from Tongati were high and large quantities of water hyacinth (Eichornis crassipes) occurred. The zooplankton and zoobenthos were impoverished and dominated by freshwater species. The roots of Elchornis provided a habitat for many invertebrates, chiefly insect larvae and the crab Varuna litterata. The fish fauna was poor and most species were confined to the lagoon near the mouth. lliophagous species, mainly Mugilidae, were dominant. The Mdloti, frequently closed from the sea but often artificially opened, exhibited typical estuarine salinity pattems, was well oxygenated but relatively poor in phosphorus and nitrogen. Primary production and energy values of benthic floc were low. Zooplankton and zoobenthos were impoverished. The fish fauna, similar to that of Tongati, was dominated by Mugilidae. The food chain from benthic floc to iliophagous fish remains viable in these degraded estuaries.