The influence of environmental gradients and benthic macrofauna on the erodibility of intertidal sediments

Abstract

Fine sediment inputs into estuaries and coastal zones can be significantly increased by human activities, which result in the modifications to habitats and negative effects on ecosystem function. Thus, the study of the whether the sediment will resuspend is critical in predicting the fate of both sediments, habitats and ecosystems in the long-term. This study measured the variation of erosion potential of the sediment in Whangateau Harbour, New Zealand. 57 samples within 19 stations are taken along a sediment mud content gradient and different variables defining both physical and biological sediment properties and biota composition were measured to help explain variation in the erosion potential of the sediment. The erodibility of sediment was described using 3 parameters: erosion threshold, erosion rate and erosion constant. The measurement of sediment erodibility was carried out by means of portable erosion instrument EROMES. Sediment samples were collected to measure sediment grain size, porosity, organic content, and chlorophyll-a concentration. Large macrofauna (>1mm) were also sampled. The two largest and most common animals, the bivalves Austrovenus stutchburyi and Macomona liliana were separated into adults (≥1cm shell length) and juvenile (<1cm shell length). These physical and biological variables were used to explain variation in the erosion potential. The statistical models developed by this data were able to explain 28% of the variation in erosion threshold, 74.5% of the variation in erosion rate and 21.5% of the variation in erosion constant. This study showed that physical variables, especially mean grain size, contributed some of the variation in the early stages of erosion. Many of the physical sediment variables were reasonably strongly correlated. Macrofaunal variables were most important in explaining variation in erosion parameters. The quantified variables highlighted the interaction between physical and biological factors. Thus, emphasising the importance of both physical and biological factors on the erodibility of estuarine sediments. This implies important changes in sediment resuspension as both physical habitat and ecological characteristics respond to environmental change and influence the fate of the ecosystem.