Intrusion of beach-disposed dredger spoil into the Mhlathuze Estuary, South Africa, and its impact on Zostera capensis

Abstract

Port expansion at Richards Bay on the east coast of South Africa between 1996 and 1999 required the disposal of some 5,998,429 m³ of dredge spoil. This was deposited via a pipeline onto the adjacent beach and spilled into the swash zone. Current reversals in the near-shore environment resulted in transportation of some of the sediment to the south, rather than the north — as had been predicted by sediment-disposal models — and this has resulted in fine sediment intruding into the nearby Mhlathuze Estuary, a nationally important sanctuary area. Sediment intrusion and its impact on the beds of the seagrass Zostera capensis in the estuary were monitored from 1996 to 1999. Fine sediment did enter the estuary from the marine environment and was deposited mainly in its lower reaches, where its settling on the leaves of Zostera resulted in a major die-back. However, monitoring indicated that the Zostera recovered from this impact in under two years, following the washing out of sediment from the estuary. The bulk of the recovery emanated from plant material already in the system, but there were clear indications that recolonisation also took place.