Responses in a temporarily open/closed estuary to natural and artificial mouth breaching

Abstract

The Great Brak Estuary is a temporarily open/closed estuary located on the south coast of South Africa. The construction of the Wolwedans Dam in the catchment has reduced flow to the estuary by 56%; reducing the intensity of flushing events and causing the mouth to breach less often. The estuary experiences dense growth of the macroalga Cladophora glomerata particularly during the closed mouth condition. This study compared the condition of the estuary following artificial and natural mouth breaching events. The estuary was artificially breached on 1 February 2011 but closed within 16days. When open, a well-developed halocline was present with hypoxic water (<2mgl−1) at depth suggesting little flushing had occurred. A month later, C. glomerata attained its highest area cover (60000m2). In June 2011, the estuary was breached by a 1-in-100year flood, which resulted in a strong tidal exchange of oxygenated water (>4mgl−1). Consequently, this prevented the re-establishment of C. glomerata and supported the growth of submerged macrophytes. This research highlights the direction that small microtidal estuaries around the world are heading; as the number of dams and the amount of water being abstracted increases, the amount of water needed for ecological processes is decreasing and the mouths of estuaries are likely to close more frequently and for longer.