Zooplankton community changes in Nhlabane estuary, South Africa, induced by man-made structures and drought

Abstract

The Nhlabane estuary and coastal lake system has suffered a number of man-made impacts since 1977, when the lake and estuary were separated by the construction of a barrage. More recent man-made events included the construction of temporary sand walls, mid-way along the estuary, to allow crossing of a mining dredger during 1993. These walls remained in place until 1996, coinciding with a local drought period between 1991 and late 1995, during which time the estuary mouth remained closed. The present study investigates changes in the zooplankton community of Nhlabane estuary between February 1992 and October 1996. Due to continuous freshwater inputs from groundwater seepage, the then closed estuary soon became fresh. This led to significant changes in the zooplankton community, including the appearance of freshwater taxa such as rotifers, cyclopoids (Mesocyclops sp. and Thermocyclops sp.), freshwater cladocerans and insect larvae, reflecting the freshwater conditions. Most estuarine species became less abundant or were absent two years after mouth closure. The estuarine copepod Pseudodiaptomus hessei was least affected and remained dominant. The copepod Acartia natalensis was absent after the first two sampling sessions and only reappeared after the mouth was breached in August 1995. Other taxa that were adversely affected included the mysid Mesopodopsis africana, and larval stages of polychaetes, decapods and fish. Some meroplankton taxa recovered after the mouth reopened.

African Journal of Aquatic Science 2005, 30(1): 29–35