Oral Presentation Australian Society for Limnology Congress 2013

Forget pH – Puissance d’Sédiment! - the power of sediment to restructure your wetland. (#91)

Jane Chambers 1 , James Tweedley 1 , Robyn Paice 1
  1. Environmental and Conservation Sciences, Murdoch University, Murdoch, WA, Australia

This study highlights the integral nature of sediments in key processes that define nutrient status and the dominance of particular ecological regimes in the Ramsar-listed Vasse Wonnerup Wetlands in south-west Western Australia. Coarse sand (>50%) supported an aquatic macrophyte dominated regime in the upper Vasse, while the upper Wonnerup sediments were comprised of fine sand and silt, and supported macroalgae. Fine particle size correlated with high values and distribution of total organic carbon, total nitrogen, total phosphorus, available phosphorus (Fe-bound) favouring the growth of macroalgae and phytoplankton. Measuring sedimentation rates was highly problematic due to highly variable water depths, but estimates for gross sedimentation were high (up to 600g m-2 d-1). The ratio of plate (net sedimentation) and pipe traps (total sedimentation rates) in winter were about a 0.3-0.5 in the Wonnerup but 0.1-0.2 in the Vasse, suggesting a predominance of resuspension in the Wonnerup but due to newly added sediments from the catchment in the Vasse. Seasonal measurement of turbidity indicated very clear water (<5NTU). This suggests that sedimentation was event-based (during storm periods) rather than a continual process. However, sediments caught in sediment traps had up to 16x higher nitrogen and phosphorus than benthic sediments. This was particularly alarming in the upper Vasse, where macrophytes Ruppia megacarpa and Lepilaena australis currently dominate a low nutrient environment. However an obvious ecological impact has not yet been detected. Seasonal drying of the upper Vasse may be a key factor in maintaining low sediment nutrients.