Diversity of bivalve molluscs within the St Lucia estuarine system, with emphasis on the ecophysiology of Solen cylindraceus and Brachidontes virgiliae.

Abstract

The St Lucia estuarine system, Africa’s largest estuarine lake, is characterised by cyclic changes from hypersaline to oligo/mesohaline conditions in response to alternations between drought and wetter than average years. In addition, St Lucia also experiences stochastic disturbances, such as flooding events that rapidly decrease salinity levels. Due to their sessile and slow moving nature, bivalves are particularly vulnerable to rapid or prolonged changes in the physico-chemical environment. The recent freshwater deprivation crisis that prevailed for the last decade resulted in a significant loss in bivalve species richness within the system. An annotated and illustrated bivalve census revealed the occurrence of twenty-four species within St Lucia between the years 1925 and 2011. However, only six species were recorded during the most recent survey in March 2011. The infaunal razor clam, Solen cylindraceus, and the epifaunal brackwater mussel, Brachidontes virgiliae, are currently the dominant bivalve species within St Lucia. This study, therefore, aimed to record the species richness of bivalves found in Lake St Lucia and to investigate key biological aspects of the two dominant bivalve taxa within the system, under different salinity regimes. Experiments revealed that S. cylindraceus can tolerate salinities between 15 and 65, while B. virgiliae prefers salinity levels ranging from freshwater to 20. The varying tolerance limits, therefore, dictate the distribution of these species during different climatic conditions within the estuarine lake. During wet periods, S. cylindraceus is restricted to the northern reaches, unable to tolerate the oligohaline conditions present in the rest of the system. Conversely, B. virgiliae, often restricted to the Narrows, becomes ubiquitous throughout the system under such conditions. Solen cylindraceus can reach a maximum length of 95 mm. However, in the St Lucia estuarine system, specimens seldom exceed a length of 55 mm, probably because prevailing/re-occurring harsh conditions prevent them from reaching maximum size. In situ measurements of this species also revealed less growth during the first year of life than for the same species in different systems. While B. virgiliae is substantially smaller than S. cylindraceus, the high densities that this species is able to attain makes it an important grazer with the potential to have significant feeding impacts on the local phytoplankton biomass. Results showed that in localised areas, B. virgiliae populations may consume up to eight times the available phytoplankton biomass. These key bivalve species are strongly influenced by the fluctuation in climatic conditions from wet to dry phases. Thus, understanding the effects that climatic shifts have on key estuarine species is essential, as flood and drought events are predicted to increase in frequency, intensity and duration as a result of global climate change.