Influence of mouth status and water level on the macrophytes in a small temporarily open/closed estuary

The monthly responses of macrophytes in the East Kleinemonde Estuary were examined in relation to changes in physical factors between March 2006 and March 2007. The East Kleinemonde is a small temporarily open/closed system where the mouth breaches in response to high water levels (>2 m amsl) or following high river inflow. On breaching there is a rapid drop in water level that causes the submerged macrophytes to be exposed and they die as a result of desiccation. Salt marsh plants then establish in the vacant habitat. Correlation analysis showed that water level and duration of inundation influenced macrophyte cover abundance. Inundation for 3 months caused die-back of intertidal salt marsh. Under open and tidal conditions, intertidal salt marsh increased at a maximum monthly expansion rate of 25% change in cover. Supratidal salt marsh expanded at maximum monthly rates of 33% change in cover. Because of its position at a relatively high elevation compared to other vegetation, supratidal salt marsh was only affected by water levels of >1.8 m amsl and only after being inundated for 1–2 months. Submerged macrophytes developed in inundated areas when stable water levels were present for longer than 2 months at a monthly maximum expansion rate of 23% cover change. In this study macrophytes responded quickly to water level fluctuations and indicate that monthly monitoring is needed to provide an understanding of macrophyte response. This is the first study that reports on rates of macrophyte habitat development in temporarily open/closed estuaries. These data can be used in mouth management plans and freshwater requirement studies to predict the growth and establishment of a diversity of macrophyte habitats.