Zooplankton composition following an artificial mouth breaching event in Lake St Lucia, South Africa

Abstract

Lake St Lucia is one of the most important estuarine systems in South Africa, but the long history of anthropogenic intervention has  threatened its integrity. With the exception of sporadic overtopping events and periodic marine connectivity via the uMfolozi River, the St  Lucia Estuary remained closed off from the Indian Ocean since 2007. Due to the build-up of sediment and prolonged mouth closure, the  beach berm was artificially breached in January 2021, fully reconnecting St Lucia to the Indian Ocean for the first time in 13 years. This  study aimed to determine the effect of this breaching event on St Lucia’s zooplankton community by analysing samples from March, May  and August 2021, at five representative sites throughout the lake system. The mouth remained open for only five months, and the system  was still largely oligo- to meso-haline due to high freshwater input. The patterns in the zooplankton were related more to the  prevailing environmental conditions than to the breaching event itself. The composition of zooplankton in the system did change;  however, the presence of marine taxa was not far-reaching nor long-lasting, most likely due to the limited tidal intrusion, strong outflow  and predominant salinity regime.