Ecology of southern African estuaries: Part XIII. The Palmiet River estuary in the south-western Cape

Abstract

The Palmiet estuary is only 1,67 km in length, but has a relatively large catchment of 539 km2. Rain falls largely during winter when it scours the estuary and widens the mouth, but by late summer the mouth narrows and, in some years, may close. The estuary never closes for more than a few months, and salinities are normal. The entire estuary has clear bottom water and high bottom salinities, but the surface waters are usually fresh and darkly stained with humic acid. This permanent stratification allows many marine species to penetrate the estuary.Owing to the scouring in winter, the sediments are coarse, >98% consisting of sand and gravel, with a low organic content. In situ primary production is low, contributing only about one fifth of the organic material in the system, the rest being imported from the river and the sea. Species richness is low, only 28 species of invertebrates being recorded, but many of these occur in enormous numbers. Callianassa kraussi plays a crucial role, trapping fine particles around its burrows and locally enhancing the organic content of the sediments. Its faeces are an important vehicle for the concentration and turnover of organic matter.Four of the 19 species of fish recorded breed in the estuary, the rest being represented largely by juveniles or small adults. Only three species occurred in the estuary year-round, the other species being absent during the period that the estuary is in flood.Floods and a relatively high flow of river water are critical to keeping the estuary open. If, as proposed, a dam is built on the river, steps will have to be taken to ensure that the mouth is kept open. A related problem will be the reduction of organic material entering the estuary from the river and the sea.