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Summary
The St Lucia estuary and lake comprise some 80 % of the estuarine area of KwaZulu-Natal. The system is subject to floods and hypersaline conditions, the latter twice exceeding 100 since 1970. The lake is connected to the sea by a meandering tidal channel called the Narrows, which historically has been largely overlooked in terms of benthic research in favour of the lake. Changes in the macrobenthic infauna of the Narrows were investigated over an eleven year period during which the Narrows was exposed to unprecedented cyclonic flooding in February 1984, floods in October 1987, a net freshwater outflow following heavy seasonal rainfall in November 1989, and a period of hypersalinity following mouth closure between December 1992 and September 1993. Although the benthos was generally numerically and gravimetrically dominated by the crab Paratylodiplax blephariskios, significant changes in species composition and densities were recorded after the cyclonic flooding in 1984, during the net fresh water outflow in 1989/90 and again during the hypersaline period in 1992/93. These changes indicated an overall change in benthic community structure between 1983 and 1994 characterised by the decline of the polychaete Marphysa macintoshi after the cyclones; the appearance of the amphipod Victoriopsia chilkensis in 1987 during a period of relative stability between 1987 and 1990; the appearance and subsequent decline of the bivalve Solen cylindraceus after the cyclones and its reappearance in hypersaline conditions in 1993; the appearance of the bivalves Dosinia hepatica and Eumarcia paupercula during the hypersaline period; and the proliferation of the tanaid Apseudes digitalis after the cyclones, the 1987 floods and the hypersaline period. The generally higher and more variable diversity of the upper Narrows reflected reduced numbers of Paratylodiplax blephariskios and increased numbers of typical lake species such as the bivalves and the polychaete Prionospio sexoculata. This indicated that the upper Narrows forms a transitional benthic community between the Narrows and the lake.
