Zooplankton dynamics during a regime shift in the St Lucia Estuary, South Africa.

Abstract

The St Lucia Estuary is the largest estuarine lake in Africa and forms part of the iSimangaliso World Heritage Site however historical anthropogenic changes, including the separation of the Mfolozi River from the St Lucia system, have exacerbated periodic drought conditions. A Global Environment Facilitated (GEF) funded project was carried out (June 2016 to July 2017) to relink the Mfolozi River and St Lucia Estuary to allow unimpeded flow of freshwater into the St Lucia Estuary that would alleviate drought conditions and ideally restore natural mouth functioning. This reconnection, combined with increased precipitation from 2017 onwards, initiated a transition to a new wet phase in the estuary. Zooplankton form an essential link between primary producers and higher trophic levels, and they are important indicators of change in aquatic ecosystems. This study aimed to firstly record the changes in the zooplankton community before, during and after the restoration project in the mouth region from February 2015 to November 2018. The zooplankton community structure at the mouth differed significantly before and after the restoration project. Estuarine resident taxa including the mysid Mesopodopsis africana, the copepods Acartiella natalensis and Oithona brevicornis were absent post project completion as mainly freshwater taxa dominated. Both total suspended solids and salinity were identified as major determinants of the zooplankton community structure at the mouth region. Secondly, the study aimed to investigate the system-wide impacts by monitoring the zooplankton community at 5 representative sites within the system as it transitioned from a hypersaline state to a freshwater state from February 2015 to November 2017. Following the reconnection, water levels increased, and salinity levels decreased throughout the lake system, however the reversed salinity gradient persisted with a mean salinity of 23 recorded in the upper reaches of the lake system in 2017. There was a clear shift in the zooplankton community to freshwater taxa such as cyclopoids, rotifers and cladocerans. Resident taxa declined in abundance, as Oithona brevicornis and Acartiella natalensis were virtually absent in 2017, possibly due to low salinity, or increased competition from freshwater taxa. The calanoid copepod Pseudodiaptomus stuhlamnni was still present throughout the lake system (in lower abundance) and the mysid Mesopodopsis africana was restricted to the upper reaches which served as refuge from the low salinity areas. While the increased freshwater input is positive, it has not yet been substantial enough to initiate a mouth breaching event that would flush the system of the excessive silt build-up. Furthermore, continuation of this already extended wet phase could force the exclusion of the estuarine resident zooplankton.