Recovery dynamics of zooplankton following mouth-breaching in the temporarily open/closed Mdloti Estuary, South Africa

Mouth-breaching events have major impacts on biological processes in temporarily open/closed estuaries. The aim of this investigation was to monitor zooplankton recovery dynamics following artificial breaching of the Mdloti Estuary in February 2004. Spatial and temporal patterns in zooplankton distribution in the lower, middle and upper reaches of the estuary before, during and after breaching were compared over three months from 27 January to 26 April 2004. Thirty-five taxa were identified, with the dominant copepod Pseudodiaptomus hessei accounting for 42% and 58% of the total abundance and biomass, respectively. Breaching resulted in a 98% loss of zooplankton biomass. While a temporary recovery was recorded during open-mouth conditions, it was not sustained due to continual sediment scouring and flushing. Pre-breaching zooplankton abundance and biomass levels were only attained nine days after mouth closure, with peaks being recorded after 19, 28 and 35 days in the lower, upper and middle reaches, respectively. Recovery was, therefore, faster in the lower reaches. Mouth state was primarily responsible for regulating zooplankton stock. However, the zooplankton in the various reaches did not recover in synchrony after mouth closure, because abiotic factors and food availability were different in the three estuarine reaches.