A note on the effects of a flood of medium intensity on macrobenthos of soft substrata in the Swartkops Estuary,South Africa

The intensity and effects of a flood in September–October 1975 on certain macrobenthic species of the soft substrata of the Swartkops Estuary were assessed. Significant decreases were recorded in populations of the bivalve Solen cylindraceus and the prawns Upogebia africana and Callianassa kraussi in the middle and upper reaches. The reductions were respectively some 93, 49 and 28 percent of the pre-flood values. No decrease was noted in populations of the bivalve Dosinia hepatica in the middle reaches or of U. africana in the lower reaches. Both large and small S. cylindraceus and C. kraussi were apparently affected by the flood, but in the case of U. africana it was primarily small individuals that suffered. Possible causes of these decreases are discussed.