Investigation into the mortality of mangroves at St. Lucia Estuary

Abstract

Lake St. Lucia is the largest estuarine lake in South Africa and is a UNESCO World Heritage Site. The St. Lucia Estuary resides in a poor state due to drought and human impacts which have resulted in the closure of the mouth to the sea since 2002 and changes in freshwater inflow particularly from the Mfolozi River. This provides a unique opportunity to study the responses of the mangroves that usually grow in an intertidal environment. High water levels and inundation caused mangroves to die in The Narrows in 2013/2014 which caused concern. The aim of this study was to investigate the response of the mangroves to fluctuating environmental conditions. This was done by measuring the population structure and sediment characteristics at four sites in 2010, 2013 and 2014. The reconnection of the Mfolozi River to St. Lucia in 2013 via the Beach Canal and Back Channel as well as good rains led to an increase in water level which caused permanent inundation of pneumatophores (aerial roots of the white mangrove Avicennia marina) and anoxic conditions resulting in the mortality of mangroves fringing the main estuary channel (Site 3 on The Narrows). Self-thinning is a natural process that has occurred at Sites 1 and 2 while Site 4, which was previously considered a dying population, had a recruitment of a new seedling cohort in 2014. The dieback of mangroves along the water's edge is part of the natural dynamics because this would have occurred under the natural state for this system when the mouth closed to the sea, water level was high and the estuary was fresher. This study has shown that site-specific differences will allow for recruitment and overall survival of mangroves at St. Lucia Estuary.