Benthic macrofaunal changes in response to restoration efforts in the St Lucia estuarine lake, South Africa

Abstract

The St Lucia estuarine lake and Mfolozi River mouth on the Indian Ocean coast of South Africa were once a single, connected system but were artificially separated in 1952 to prevent siltation from the Mfolozi River catchment which risked shallowing the globally significant estuarine lake. This separation deprived the St Lucia estuarine lake of its primary freshwater source and threatened its integrity. A rehabilitation project financed by the Global Environment Facility was implemented in 2017 to re-link the system to restore natural mouth functioning and marine connectivity. This study investigated benthic macrofaunal changes associated with the Mfolozi River reconnection, over the period February 2015 to November 2018. A decrease in salinity, increased water level and silt buildup coincided with significant differences in the benthic macrofaunal assemblage. This shifted to the presence of taxa indicating the disturbance from siltier sediment and freshwater predominance, such as the invasive freshwater gastropod Tarebia granifera, the brackish-water tanaidacean crustacean Halmyrapseudes cooperi and chironomid larvae. The mouth reconnection has increased freshwater input into the St Lucia estuarine lake system, reviving it from a prolonged dry or hypersaline state; however, further intervention is required to prevent further silt input via the freshwater inflow as this risks changes to the benthic macrofaunal assemblage, increased sediment buildup and regular mouth closure.