Variability in estuarine water temperature gradients and influence on the distribution of zooplankton: a biogeographical perspective

Abstract

Structure and variability of water temperature gradients and potential influence on distribution of two tropical zooplankters (the mysid Mesopodopsis africana and the copepod Acartia natalensis) and their temperate congenerics (M. wooldridgei and A. longipatella) was investigated over a 10-year period in the Mgazi Estuary, South Africa. Intra- and inter-monthly water temperatures were highly variable, driven primarily by the interactive effects of river runoff, tidal exchange and mouth state. Variability was greatest in summer, temperatures ranging between <13 °C (linked to nearshore upwelling and a wide open mouth) and >30 °C when the mouth was near-closed. A strong horizontal temperature gradient characterised the estuary, the upper reaches remaining consistently warmer compared to the lower estuary. In winter, abundance of the tropical zooplankters showed a positive and significant correlation with increasing distance upstream. No pattern was evident in summer. The temperate species reflected a negative trend in abundance moving upstream during both seasons, but correlations were not significant. In terms of climate change, future population responses remain uncertain. This uncertainty is primarily linked to ongoing changes in the relative dominance of key drivers influencing estuarine water temperatures. Estuary mouth state will also respond to changes in nearshore wind regimes and morphodynamics of the adjacent beach.