Distribution and status of Zostera capensis in South African estuaries — A review

Abstract

This review collates the available knowledge on the distribution and area covered by eelgrass Zostera capensis and shows that it has a wide distribution occurring in 62 estuaries from the Olifants Estuary on the west coast to Kosi Bay on the east coast. It has protected meristems, a strong root system and flexible leaves which enable it to grow under strong tidal conditions as well as tolerate periods of exposure and desiccation characteristic of the intertidal zone. Estuaries with the largest areas of Z. capensis are Olifants, Langebaan, Berg, Knysna, Keurbooms, Swartkops and Bushmans. Only Langebaan and Knysna have some formal protection status. Large populations were lost from KwaZulu-Natal estuaries at the time of Durban Bay harbour development in the 1950s and more recently from St Lucia. There have been a number of studies in South Africa showing the utilisation of eelgrass beds by a variety of invertebrate and fish species for habitat, shelter, foraging sites and nurseries. Fewer studies have addressed nutrient recycling and other ecosystem services such as carbon sequestration. In all estuaries the distribution of Z. capensis is highly dynamic and therefore difficult to map, assess changes over time and use as an indicator of estuary health. Cover and biomass are removed after large floods and have been shown to fluctuate in response to bait digging and disturbance by boats. This review identifies eutrophication as an emerging threat that encourages macroalgal growth, which smothers Z. capensis. Despite its wide distribution range, Z. capensis is sensitive to human impacts which have led to the species being listed as vulnerable in the Red Data List. It is encouraging that approximately 46% of all estuaries with Z. capensis have estuary management plans as these strive to protect sensitive habitats using activity zoning. Studies tracking the changes over time of the Z. capensis beds in specific estuaries are needed to understand dynamic responses. In addition there is urgency for a national assessment of the distribution and health of this keystone species in South Africa.