The effects of sediment disturbance on the macrobenthos of the St. Lucia Narrows, Natal.

Abstract

Estuarine studies worldwide have shown that sediment disturbance effects on the macrobenthos are reIated to the nature and scale of the disturbance. Decreased species densities, diversity and richness have been found where the substratum and current patterns have been altered either by direct removal or by the creation of channels . Sediment disturbance in the St. Lucia Narrows has occurred through dredging, beam trawling and episodic floods. The Narrows, a meandering tidal channel approximately 21 km long linking the st Lucia Lakes to the sea, were dredged between 1952 and 1971 to provide a greater flow of seawater to the lakes during periods of low lake levels. A canal was cut through land from the Mfolosi River to the Narrows in an attempt to ameliorate hypersaline conditions in the Lakes, but was never commissioned. Beam trawling has formed the basis of a prawn bait fishery since the 1930's. The bait boats trawl on the mudflats over the entire Narrows on a daily basis and often churn the substratum with their propellers. The objectives of this study were to determine the effects of dredging, a once-off large scale disturbance, and beam trawling, a frequent small scale disturbance, on the macrobenthos of the Narrows. Studies in 1983 and 1984 showed that the dredged channel was impoverished compared with the adjacent mudflat, and that the Link Canal was devoid of benthos. In 1988 species densities, especially of polychaetes, were found to be lower in areas open to beam trawling than in adjacent closed areas. The dredged channel during the present study was again impoverished compared with the adjacent mudflats. The three most abundant species occurring on the mudflats, the crab Tylodiplax blephariskios, the amphipod Victoriopsia chilkensis and capitellid polychaetes, were recorded at densities an order of magnitude lower in the channel than on the mudflats. The substratum in the channel was generally sandier than the mudflats, and this condition appeared to be maintained by the scouring action of tidal currents. It was calculated that the creation of the dredged channel had reduced the standing benthic biomass in the Narrows by a minimum of approximately 20%. The Link Canal was colonised by the three major mud flat species , but at densities an order of magnitude lower than the mudflats. Beam trawling of experimental sites at monthly and 6-monthly intervals on muddy and sandy substrata in the Narrows between July 1989 and July 1990 did not appear to have a negative effect on the benthos. The coverage of the bait boats was calculated to be comparable to the trawling effort in this study, and suggested that the bait fishery is not having a detrimental effect on the benthos. It was concluded that the macrobenthos in the Narrows represented a pioneering community characteristic of estuaries, either not affected by, or able to recover from small scale and episodic disturbances provided that there was no long term habitat modification.