Coast KZN

25 Jan 2016

St Lucia Estuary: life, but not as we knew it

Amanda Watson (The Citizen)

The desiccated remains of a Grunter, one of dozens, on the dry estuary bed of St Lucia. Photo Credit: Amanda Watson

Only half the species that were found in 1982 are still present in the St Lucia estuary – many examples are empty shells.

They lie in their millions on the St Lucia estuary bed. Dull spots of mottled brown bivalve shells break the unending glitter of salt crystals left by the evaporation of the water under a harsh South African sun and howling wind.

Is it the end of bivalve life in the St Lucia estuary as we know it? Possibly. The shells littering the estuary bed are an obvious sign estuarine animals have borne the brunt of an extended period of extremely low rainfall and hyper salinity exacerbated by a lack of fresh water from the Umfolozi – St Lucia’s main feeder river.

Many are still connected, the tendon joining the shell halves seemingly refusing to acknowledge the fight is over.

The mass death is the result of a 1952 decision to stop the Umfolozi River from emptying into the St Lucia estuarine system in the belief silt sediments carried by the river posed a threat to the lake.

“During the initial closed period, the lake reached its lowest level yet recorded in July 2006, with water only covering an estimated 10% of the lake’s 325 km2 surface area while salinities reached an all-time high of 200 parts per thousand towards the end of 2003,” Digby Cyrus et al wrote in the research article, Lake St Lucia, Africa’s largest estuarine lake in crisis.

It was the final nail in a perfect storm from which the bivalves and molluscs had no escape. With almost no watery habitat to support organisms, research on the current state of marine life in the estuary is scant.

“Ecosystems worldwide are rapidly losing taxonomic, phylogenetic, genetic, and functional diversity as a result of human appropriation of natural resources, modification of habitats and climate, and the spread of pathogenic, exotic, and domestic plants and animals,” Shahid Naeem et al noted in the paper The Functions of Biological Diversity in an Age of Extinction.

The rise and fall of the molluscs is demonstrated by a 2014 paper by University of KwaZulu-Natal professor Renzo Perissinotto et al on the Diversity of bivalve molluscs in the St Lucia Estuary.

This paper on the area noted that a total of 24 bivalve species were recorded in St Lucia between 1925 and 2011.

Perissinotto noted only 12 of the 24 species had been reported in published literature and 10 species identified during a 1982/1983 survey had not appeared in published literature for St Lucia.

However, only nine of these were considered true inhabitants of the estuary.

“During the survey of 1982/1983, 22 bivalve species were found, as empty shells, on the shorelines and islands of the St Lucia estuarine system.

“Only 11 species were recorded during the survey of 2005,” the paper found.

Perissinotto noted because “the majority of the bivalves were collected as empty shells, there is some uncertainty regarding the exact time at which the bivalves were alive and growing in the system”.

One such example is Forsskal’s oyster which was previously recorded as abundant in parts of St Lucia during 1982/1983, “but only as empty and old shells in 2011”.

Others include Sandwich Islands oyster, the saddle oyster, and the fragile cockle.

It’s too soon to tell which species will survive the current conditions; however it is unlikely marine life in the St Lucia estuary will ever be the same again.