The influence of prolonged drought and anthropogenic impact reversal on fish community structure and ecosystem state cycling of Lake St Lucia, Africa’s largest estuarine lake

Abstract

Historically, the St Lucia Estuarine Lake System and uMfolozi River joined via an estuarine bay before exiting to the sea. In 1952, separate mouths were created, because of sediment problems originating from the uMfolozi River. During the 2004 drought, the mouth of the 525 km2 St Lucia System closed and has remained so to date, except for a single opening in 2007, lasting six months. From 2008, management actions have been implemented to 'reconnect' St Lucia to uMfolozi. This study assessed the effects of a new channel between the two on the St Lucia fish community and its effectiveness for recruitment of marine species. Twelve marine species previously absent and a 67% increase in abundance in the Lower Narrows were recorded after channel opening, which allowed for limited recruitment of marine species. This paper reviews the effects of a permanent connection between St Lucia and uMfolozi, via the uMfolozi Spillway, established in 2012, which has pushed the system into a prolonged
freshwater state severely reducing the estuarine fish fauna. This, the lack of a connection to the sea for 15 years and issues potentially associated with increased sedimentation in the lower reaches are reviewed in relation to management decisions that have been implemented.

Keywords: estuary, management actions, uMfolozi River, uMfolozi spillway, recruitment, river mouth state