The zooplankton community of the Mhlathuze (Richards Bay) Estuary: Two decades after construction of the harbour

The Mhlathuze (Richards Bay) Estuary on the KwaZulu-Natal coast, South Africa, was substantially altered during the early 1970s, when it was divided into two separate systems. The northern section was developed into a deep-water harbour, whereas the southern part was envisaged to function as the new estuary. This study investigates the composition of the zooplankton community of the new estuary 20 years after its construction and compares it with historical data. Compared with the original estuary, a much-reduced body of water is currently available to estuarine plankton. During the sampling period between February 1996 and February 1997, the surface area of the new estuary was about 75% smaller than that of the original waterbody, originally as a result of a section being lost to harbour development and later to mangrove encroachment. Salinity levels throughout the new estuary were close to that of seawater, except for a few occasions during particularly strong freshwater inflows. Multivariate analyses indicate that patterns of zooplankton distribution of abundance during the present study were mostly determined by a combination of salinity and seasonal temperature variation. Zooplankton density had a seasonal pattern. Overall abundance of the estuarine zooplankton community of the system was low compared with the original system and other South African estuaries. The original estuarine zooplankton community, characterized by the typical estuarine copepods Pseudodiaptomus stuhlmanni and Acartia natalensis, had been replaced by a predominantly marine community, dominated by small paracalanid copepods.