Predicting the Effects of Restoring Tidal Connectivity on the Vegetation of Fresh and Oligohaline Wetlands: Clarence River Floodplain, Northern NSW

Abstract

Tidal wetlands are decreasing in number and extent worldwide due to the effects of drains and tidal barriers. These disrupt salinity gradients, reduce the depth, duration and frequency of inundation, prevent exchange of organic and inorganic materials, and interrupt movement of aquatic biota and propagules. Common effects include reductions in bird and fish populations, invasion by terrestrial and freshwater macrophytes, sediment subsidence caused by peat degradation, and activation of acid sulfate soils leading to land degradation and water quality problems. Active management of floodgates has been proposed to restore tidal exchange to waterways and wetlands of the Clarence River floodplain, on the north coast of New South Wales, Australia. Predicting the potential effects of tidal restoration on macrophyte communities is of high priority, particularly for wetlands in the fresher half of the estuarine salinity gradient. The vegetation at these sites provides important foraging and nesting habitat for rare waterbirds and a valuable pasture resource. Our ability to predict the effects of increased tidal connectivity on macrophyte communities in these wetlands is limited. Previous research in Australia has focused on saltmarsh species found in saline habitats and few data are available on the salinity and inundation tolerance ranges of macrophytes found further upstream. Existing models for predicting the effects of tidal restoration on macrophyte community composition are generally inapplicable to these communities because of the need for reference data, either from natural tidal wetlands located nearby or from surveys carried out at rehabilitation sites prior to drainage and tidal restriction. Neither of these are available for wetlands on the Clarence River floodplain. An extensive survey was used to determine distributions of macrophyte species in floodgate-affected wetlands along the Clarence River floodplain, and to relate these distributions to environmental variables, including salinity, relative elevation, acidity, water management and grazing intensity. Strong significant correlations were found between community composition and both site salinity and water depth, indicating the potential value of these variables as predictors of species occurrence.