Development of a Halotolerant Community in the St. Lucia Estuary (South Africa) during a Hypersaline Phase

Background: The St. Lucia Estuary, Africa’s largest estuarine lake, is currently experiencing unprecedented freshwater deprivation which has resulted in a northward gradient of drought effects, with hypersaline conditions in its northern lakes. Methodology/Principal Findings: This study documents the changes that occurred in the biotic communities at False Bay from May 2010 to June 2011, in order to better understand ecosystem functioning in hypersaline habitats. Few zooplankton taxa were able to withstand the harsh environmental conditions during 2010. These were the flatworm Macrostomum sp., the harpacticoid copepod Cletocamptus confluens, the cyclopoid copepod Apocyclops cf. dengizicus and the ciliate Fabrea cf. salina. In addition to their exceptional salinity tolerance, they were involved in a remarkably simple food web. In June 2009, a bloom of an orange-pigmented cyanobacterium (Cyanothece sp.) was recorded in False Bay and persisted uninterruptedly for 18 months. Stable isotope analysis suggests that this cyanobacterium was the main prey item of F. cf. salina. This ciliate was then consumed by A. cf. dengizicus, which in turn was presumably consumed by flamingos as they flocked in the area when the copepods attained swarming densities. On the shore, cyanobacteria mats contributed to a population explosion of the staphylinid beetle Bledius pilicollis. Although zooplankton disappeared once salinities exceeded 130, many taxa are capable of producing spores or resting cysts to bridge harsh periods. The hypersaline community was disrupted by heavy summer rains in 2011, which alleviated drought conditions and resulted in a sharp increase in zooplankton stock an

The zooplankton community of Richards Bay Harbour and adjacent Mhlathuze Estuary, South Africa

During construction of the current Richards Bay Harbour in the early 1970s, the original shallow Mhlathuze Estuary was divided into two separate systems: a new estuary and a deep-water harbour. This study compares the zooplankton communities of the current systems with reference to that of the original, before harbour construction. Zooplankton were sampled during 1996 and 1997, and in 2003. Both systems are predominantly marine in terms of salinity, which was reflected in the presence of marine zooplankton throughout. Zooplankton densities were generally higher in the harbour. Densities of the estuarine calanoids Pseudodiaptomus stuhlmanni and Acartia natalensis, which dominated the system before harbour construction, were low in both current systems and were predominantly restricted to areas that still received some freshwater input. Oithona spp. and paracalanid copepods dominated the zooplankton numerically in both systems. Spearman rank correlation analysis indicated that temperature and salinity, in combination, were the two environmental factors that best match the distribution of zooplankton abundance in the two systems. The calanoid Acartia spinicauda, an introduced species, was recorded in the harbour and further zooplankton samples collected in 2003 showed an increase in its abundance. The species was probably introduced through ballast water discharged in the harbour. Because of the dysfunctional tide-gate between the current estuary and harbour, alien species introduced into the harbour could also eventually colonise the estuary.Keywords: alien species; harbour construction; Mhlathuze Estuary; Richards Bay Harbour; zooplanktonAfrican Journal of Marine Science 2008, 30(1): 55–6