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

Abstract

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; zooplankton

African Journal of Marine Science 2008, 30(1): 55–62