The response of the diatom flora of St Lucia Lake and estuary, South Africa, to a severe drought

Abstract

Lake St. Lucia, South Africa's largest estuarine system, was isolated from the sea by a beach berm throughout a severe drought from 2002 to 2007, with the lake water level being extremely low over much of its total area. A reverse salinity gradient resulted, with the lowest salinity in the south near the sea and the highest in the northern lakes. Cholnoky (1968), the first to examine the diatoms of St Lucia and the streams flowing into it, identified 159 diatoms, of which 79 taxa were from the freshwater streams and 80 from the saline lakes. A further study between 1973 and 1975 by Johnson (1976) showed the presence of neritic diatoms as the dominant group in the phytoplankton. During the present survey carried out during 2004 and 2006 only a few taxa from the 1965 survey, and no taxa from the 1976 survey, were recorded. Raphid diatoms were found to survive for months in dry lake bed sediments, in salinity up to five times that of seawater. Species richness increased with depth into the dry sediment, leading to the hypothesis that, when the rains return, the diatom flora would regain species richness and diversity within weeks. Species composition was different at the same sites at different times, indicating that water circulation is responsible for distributing species around the lake, especially during shallow-water, hypersaline drought conditions. No centric diatom taxa were found in the water column, suggesting that there was no true phytoplankton community, only resuspended benthic taxa, in St Lucia at the time of the study.

Keywords: Benthic organisms, diatoms, phytoplankton, drought, salinity, St. Lucia, survival

African Journal of Aquatic Science 2008, 33(1): 1–15