Characterisation of the mesozooplankton community in response to contrasting estuarine salinity gradients in the Eastern Cape, South Africa

Abstract

Structure and pattern of mesozooplankton in three tidal estuaries experiencing contrasting salinity gradients were investigated. In the Kariega Estuary, where freshwater inflow was persistently low with little variation in flow, salinity values exceeded 28–30 for long periods (years) throughout the estuary. In the Keiskamma and Great Fish estuaries intermittent floods, coupled to strong and variable baseflows, ensured spatial and temporal variability in salinity distribution. Copepods numerically dominated the zooplankton (95–97%) in all three estuaries. Species richness in the freshwater-starved Kariega Estuary exceeded 35, coupled with reduced variance in abundance and low species dominance at most sites along the estuary. By contrast, the Keiskamma and Great Fish estuaries had relatively few species (21 and 20, respectively), high population variance and high species dominance at most sites. Two mesozooplankton species emerged as possible indicators of freshwater-rich and freshwater-starved estuaries. The copepod Pseudodiaptomus hessei was particularly abundant in the Keiskamma and Great Fish, following the pattern recorded for other estuaries with strong horizontal salinity gradients (>28). By contrast, population density of P. hessei was orders of magnitude lower in the marine-dominated Kariega. The copepod Tortanus capensis was present only in the Kariega Estuary. Characteristically, T. capensis favours upper estuarine reaches or sheltered marine embayments where salinity values consistently remain above c. 33–34 for long periods (months to years). The zooplankton of freshwater-rich estuaries is essentially a subset of the community found in freshwater-starved estuaries, and a reducing supply of fresh water to estuaries leads to estuarine mesozooplankton communities moving along a trajectory of increasing species richness, but decreasing variance and dominance within assemblages.

Keywords:

• copepods
• estuary
• freshwater
• mysids
• South Africa
• zooplankton