Lake St Lucia, on the East Coast of southern Africa, is a major nursery area for juvenile marine fish and prawns. It comprises 80% of the estuarine area of the Province of KwaZulu-Natal and more than 50% of that of South Africa.

In June 2002 drought resulted in mouth closure and hypersaline conditions followed reaching the highest on record (>200‰) while the lake level dropped to <10% of the system’s 325km².

Anthropogenic activities relating to the separation of the Mfolozi River from St Lucia over 50 years ago appear to be the root cause of the extremes reached. With the drought over and the mouth still closed 14 years later a semi-permanent connection between the two was established in July 2012. This resulted in a significant volume of freshwater entering St Lucia causing the lower part of the system to become fresh. It also caused a reverse salinity gradient to establish which ranged from 1 at the mouth to 15‰ in the northern part of the lake. A switch in ecosystem functioning, from the hypersaline state that typically ranges between 65 and 120‰ (and higher during the most recent event) to a freshwater state with salinities ranging from 0 to 12‰, was also initiated.

This paper reviews ecosystem state cycling in St Lucia and the changes that have occurred over the past 14 years as well as the impacts that the hypersaline period has had on the fish fauna.