Initial characterisation of the mesozooplankton community of the Mfolozi– Msunduzi estuarine system, KwaZulu-Natal, South Africa, during a low-flow period

Abstract

The mesozooplankton of the Mfolozi and Msunduzi estuaries, which share a combined mouth, was sampled twice per year during a two-year period of relatively low river flow. Samples were collected during March, after the rainy season when the estuary mouth was open, and during August, after the low-rainfall winter months when the mouth was closed. The estuarine calanoid copepods Pseudodiaptomus stuhlmanni and Acartia natalensis were numerically dominant, making up 75% of the total number of mesozooplankton recorded. Relatively high abundances were recorded for these copepods, with P. stuhlmanni reaching peak densities of 79 000 individuals m⁻³. In the low-salinity regions of the estuaries, especially the upper regions of the Msunduzi, the cladoceran Moina sp. also attained high densities, reaching 50 000 individuals m⁻³. During sampling sessions when the mouth of the estuaries was closed the systems became relatively fresh, in contrast to open-mouth conditions when seawater dominated the lower reaches. Peak densities of estuarine copepods occurred during times of mouth closure. When the mouth was open these copepods generally declined in number, and an influx of coastal marine species such as paracalanids, Corycaeidae and chaetognaths was evident. Although higher zooplankton densities were mostly recorded in the Msunduzi Estuary, the species compositions in the two systems were relatively similar, with no significant differences between them. The Mfolozi Estuary is normally classified as a river mouth type, but during the present study this estuary effectively functioned as a temporarily open/ closed type estuary. The low-flow conditions did not adversely affect the estuarine mesozooplankton, largely because the mouth never remained closed for any extended period.

Keywords:

• drought
• estuary
• salinity
• zooplankton