Is freshwater quality adequately defined by physico-chemical components? Results from two drought-affected estuaries on the east coast of South Africa

Abstract

From 1992 to 1994 the physico-chemical conditions of two systems on the east coast of southern Africa shifted from estuarine to freshwater as a result of mouth closure during the prevailing drought. Although there was gradual colonization by freshwater benthic invertebrates, both systems were dominated by estuarine benthic infauna. The upper reaches of the Siyaya Estuary were entirely characterized by freshwater taxa (95%) by 1994, whereas Nhlabane Estuary showed a smaller increase in the number of these fauna. Non-metric multidimensional scaling plotted benthic populations as separate groups each year. These groups were not the result of a gradual shift in benthic fauna from an estuarine towards a freshwater bias. Rather, the upper estuarine reaches were dominated by taxa characteristic of freshwater environments, despite similar water quality conditions along all reaches. Three quarters of the zoobenthic communities (expressed as density m^–2) were still estuarine in nature. The two populations were comparable to those occurring in nearby freshwater coastal lakes, which have been subject to marine influence in geological history. It was concluded that assessment of the biological component in addition to the physico-chemical properties should be considered when defining water quality in such systems.