Dynamics of pelagic and benthic microalgae during drought conditions in a shallow estuarine lake (Lake St. Lucia)

Microalgae are an important component of highly productive estuarine food webs worldwide. Due to their short generation times, they are able to respond quickly to changing environmental conditions and grazing pressures. We studied the dynamics of microalgae in Lake St. Lucia, a shallow estuarine lake system, and one of the largest estuaries in Africa. Recently, dry years in addition to the closure of the inlet in 2002 have left Lake St. Lucia with greatly reduced water levels and increased salinities. We sampled planktonic and benthic microalgal biomass (chl a) and physico-chemical parameters biweekly at three different stations located at the inlet, the southern, and the northern reaches of the system from October 2010 until September 2011. Microalgal biomass differed significantly between sites but did not show clear seasonal patterns. Stochastic climate phenomena had a larger influence than the seasonal cycle. However, the different sites were subject to different driving factors. Erratic rainfall led to rapid and large changes in salinity with most rain falling in summer 2010/2011. During long periods without rain, evaporation led to increasing salinities. In False Bay, in the northern reaches of Lake St. Lucia, this alternation of rainfall and evaporation additionally influenced algal biomass by dilution and concentration. This was not observed in the southern reaches and the inlet of the lake system where trophic interactions most likely played an important role in influencing microalgal dynamics. Although it was not possible to identify general patterns of microalgae dynamics for the entire system, the drivers at individual sites could be identified. Lake St. Lucia corresponded to the in general stochastic nature of microalgae dynamics found in systems worldwide, and in addition showed strong spatial heterogeneity.