Zooplankton of the St. Lucia Estuary during the current drought cycle: a comparison between open- and closed-mouth conditions

The St. Lucia Estuary is currently experiencing a drought-induced crisis, resulting in the system having been closed off from the sea for approximately 8 yr. This closure was interrupted by a brief open-mouth phase, induced by a unique combination of extreme climatic events. The primary aim of the present study was to compare zooplankton dynamics during open- and closed-mouth con- ditions. Sampling was undertaken during quarterly surveys from February 2006 to November 2008. During the closed-mouth phase, up to 70% of the lake bed was dry and salinities in the northern lakes often exceeded 90, making these areas largely uninhabitable for zooplankton. However, in the lower regions where drought effects were less harsh, zooplankton were characterized by high densi- ties and biomass of typical estuarine taxa such as the copepods Pseudodiaptomus stuhlmanni, Acar- tia natalensis and the mysid Mesopodopsis africana. Of the 69 taxa recorded during the study period, only 27 were present during the closed-mouth phase. Under open-mouth conditions, previously excluded marine taxa (e.g. the prawn Penaeus indicus and fish larvae), once again re-entered the system, increasing its diversity significantly. A unique occurrence after mouth re-closure was the colonisation of the mouth area by swarms of the tunicate Oikopleura dioica (>10 3 ind. m -3 ), while pre- viously dominant zooplankton grazers virtually disappeared. These findings emphasize the complex- ity of the system and stress the need for further research into the potential impacts of environmental and climate changes on this key African estuarine lake.