Predicting macrophyte states in a small temporarily open/closed estuary

Abstract

Temporarily open/closed estuaries (TOCEs) shift between abiotic states associated with mouth status. The aim of this study was to assess whether macrophyte states could be identified based on the dominant cover abundance of different species representative of specific habitats. A 5-year dataset of monthly macrophyte cover was assessed for the East Kleinemonde Estuary in South Africa. Three macrophyte states were identified: namely open and tidal (predominantly intertidal salt marsh); closed and low water level (predominantly salt marsh); and closed and high water level (with submerged macrophytes). The threshold water level for the change from salt marsh to submerged macrophytes was 1.6 m above mean sea level (amsl). A fourth state where macroalgae were dominant was identified for high salinity conditions (above 30 PSU). It was then possible to examine simulated water level data for different inflow scenarios to determine how often these macrophyte states occurred. Available macrophyte habitat was also calculated for different water levels using a spatial model written in Modelbuilder (ArcGIS 9.3.1). Both methods used to predict available macrophyte habitats are rapid, requiring only information on the elevation range of the main habitats, as well as present distribution and bathymetric maps. These predictive techniques are useful in the determination of the ecological water requirements of small estuaries.