The potential for zoobenthic fauna dispersal into the St Lucia Estuary from the Mfolozi-Msunduzi estuarine system, KwaZulu-Natal, South Africa

The onset of a prolonged drought in the St Lucia catchment in 2002 and subsequent mouth closure raised concern for the biota of the estuary. Ezemvelo KZN Wildlife proposed a scheme whereby the Mfolozi-Msunduzi mouth would be closed, causing fresh water to enter the St Lucia Estuary to prevent excessively hypersaline conditions (>65) from developing in the lower and middle reaches of the St Lucia system. Although this proposal caused concern about the impact of mouth closure on the ecology of the Mfolozi-Msunduzi estuarine system, it raised its potential as a source of benthic fauna dispersal into, and the restocking of, the St Lucia system. Quantitative benthic sampling in the Mfolozi-Msunduzi system in 2007 and 2008 revealed a benthic community generally dominated by polychaetes which, although low in terms of total species richness compared with St Lucia, included elements of a core benthic group present in the St Lucia system throughout the current drought. The presence of the gravimetrically important crab Paratylodiplax blephariskios in the Mfolozi-Msunduzi system was considered important as a source from which the once-abundant population in the St Lucia Narrows could be restocked via dispersal through the channel linking the Mfolozi-Msunduzi Estuary to St Lucia. It was concluded that, while the input of fresh water from the Mfolozi River is necessary to ameliorate hypersaline conditions and conserve the important core benthic group in the Narrows and South Lake, consideration must be given to allowing sufficient tidal exchange in the Mfolozi-Msunduzi system to maintain its stocks of benthos with an obligate marine phase.