Community structure and composition of meiofauna over a sea-induced mouth-breaching event in St Lucia Estuary, South Africa

Mouth state significantly affects the ecology of temporarily open-closed estuaries. Breaching typically occurs due to river flooding during high rainfall periods, but occasionally a breach may be initiated by the sea due to storm surges and high sea levels. There is little information on the effects of this type of breach on estuarine ecosystems. St Lucia Estuary is one of the most important estuaries in South Africa in terms of biodiversity; however, it has been severely affected by drought and prolonged mouth closure since 2002, except for a 6 mo breaching event in 2007. Meiofauna are poorly known from the estuary, with virtually no studies prior to the drought. This study therefore examines the effects of a sea-induced breach on meiofauna communities in the St Lucia Estuary, with particular reference to the influence of mouth state and spatial variations between sites in different regions of the estuary. Samples were collected quarterly from August 2006 to May 2008. Four study sites were selected to represent the mouth area, the narrow channel joining the lakes, South Lake and the northernmost lake at False Bay. The breach had a significant impact on community structure and composition, with variations among sites. The greatest impact was at the mouth. Density increased, peaking after reclosure; however, taxon richness and diversity were significantly reduced throughout the open phase, likely due to sediment disturbance and strong flows. In contrast, density and richness generally increased in the shallow lakes, where water depths and salinity increased after the breach. Communities in the deeper estuary channel appear to be structured mainly by temperature and food availability, whereas salinity is likely the most important environmental driver in the shallow lakes.