Microalgal biomass in the St Lucia Estuary during the 2004 to 2007 drought period

Abstract

Phytoplankton and microphytobenthic biomass were investigated over a 4 yr period in the drought-stricken St Lucia Estuary, KwaZulu-Natal, South Africa. This is the largest estuarine lake in Africa and part of the iSimangaliso Wetlands Park, South Africa's first World Heritage Site and a Ramsar Wetland of International Importance. The present study aimed at identifying the factors con- trolling the extreme variability in its ecological functioning. Water-column chlorophyll a (chl a) con- centrations averaged (±SD) 32.3 ± 53.4 mg m -3 , with the highest value reaching 413 mg m -3 . Within the sediment, average (±SD) chl a concentration was 201 ± 377 mg m -2 , with a maximum of 2576 mg m -2 . Both are among the highest values so far reported in the literature for any estuarine ecosystem and by far the highest values reported for any South African estuary. Areal phytoplankton biomass was highest in the Mouth and Narrows region, declining in the South and North lakes, while micro- phytobenthos showed the reverse trend. Ordinations and cluster analysis revealed that phytoplankton biomass differed between the southern and northern halves of the system, while differences in micro- phytobenthic biomass were found between the Narrows and the rest of the system. Dissolved inor- ganic nitrogen to phosphorus ratios were greater and lower than the Redfield optimum (16) 60% and 27% of the time, respectively, indicating that microalgal production may be limited by availability of either P or N for part of the year, with a peak in growth during summer. Water level and mouth state are key factors controlling microalgal biomass within the estuary. During the closed phase, the shal- low waters and vast surface area of the lakes region led to the development of hypersaline conditions and temperatures occasionally approaching 50°C. The negative effects of such extreme conditions on grazers, as well as negative correlations between grazer densities and microalgal biomass, have been documented in previous work on the system. This would suggest that the interaction between the physical environment and grazers may be important in regulating microalgal biomass in the estuary.