Swartvlei, which has been rated as the seventh most important estuarine system in South Africa, consists of a large (9 km²) and fairly deep lake, which is linked to the sea by a 7 km long estuary. The estuary is open to the sea approximately 50 percent of the time, and is often closed during the winter months. The estuary has large sandflats which provide an important habitat for burrowing, invertebrate bait species, which in turn are a food source for fishes and birds frequenting the estuary.
Field surveys done by SANParks in 2008 noted that the intertidal sand-flats were mostly bare of Cape eelgrass and supported large populations of the common sandprawn, but relatively few burrowing shellfish. However, since 2009 there has been a progressive increase in size of intertidal eelgrass beds, and in the abundance of burrowing shellfish, such as beaked clam, lesser heart-clam and littoral tellin, a concomitant decrease in the size of the sandprawn beds.
This was especially prominent on the sandflats of the lower and middle reaches of the estuary, and by late 2012 the extent of the sandprawn beds had decreased by approximately a quarter of its original size. Researchers from the University of Cape Town noted that the densities of the common sandprawn colonising the sandflats of Langebaan Lagoon were negatively correlated to the cover of Cape eelgrass, and at sites where eelgrass was absent sandprawns extended farther up the shore and reached a larger maximum size than in its presence.
The common sandprawn is itself an important structuring agent of faunal assemblages of sandflats. Through its burrowing and feeding activities, the sandprawn continually turn over the substratum and reduce its micro-algae contents. Experiments have shown that this process exerts a strongly negative influence on the feeding of the beaked clam and the surface grazing tick shell (a tiny whelk), and appears to substantially reduce the densities of these species on the sandflats.
Therefore, the current abundance of clams in the Swartvlei estuary is apparently closely linked to the extent of eelgrass cover. The distribution of eelgrass in temporary open and closed estuaries, like Swartvlei, is influenced mainly by the substratum and the height and turbidity of the water. A stable substratum and clear water generally promotes the growth of Cape eelgrass. Thus, the current situation in Swartvlei is likely to persist until the next large flood smoothers the eelgrass beds with fine silt. SANParks will continue to monitor the situation…