Benthic macrofaunal responses to mouth breaching events in Lake St Lucia

Lake St Lucia forms part of the iSimangaliso Wetland Park, South Africa’s first UNESCO World Heritage Site. Freshwater deprivation over multiple decades and a closed mouth condition in Lake St Lucia led to the loss of ecosystem functioning and marine connectivity, with cascading negative effects on ecotourism and other stakeholders in the region. Between the years 2002 and 2021, Lake St Lucia experienced two breaching events. The first in 2007 occurred naturally as a result of high sea swells from Cyclone Gamede and the next in 2021 was an artificial breach as a management intervention to restore marine connectivity after 13 years of closed mouth conditions. This study aimed to assess the response of benthic macrofaunal assemblages to breaching events in Lake St Lucia. A total of six marine species that were recorded during the 2007 breaching event were not documented during the 2021 breach, most likely because of limited tidal exchange and the predominant oligo-to-mesohaline state in 2021. The prevailing environmental conditions were the dominant drivers of benthic macrofaunal assemblage composition during these breaching periods. As a result, the differences in the assemblages were primarily driven by hydrological phases (drought vs wet) rather than by breaching events (natural vs artificial). The predominantly oligohaline conditions, reduced oxygen and increased mud during 2021 indicate that the system was not flushed and reset. Importantly, our study demonstrates that the 2021 breach event was ineffective due to insufficient tidal exchange hindered by the system’s unnaturally high sediment load.