Oral Presentation Australian Society for Limnology Congress 2013

Biogeochemical responses to hydrological cycles in streams of the Pilbara region, northwest Australia (#41)

Andre R Siebers 1 , Neil Pettit 2 , Grzegorz Skrzypek 1 , Jason Fellman 1 , Shawan Dogramaci 3 , Pauline Grierson 1
  1. Ecosystems Research Group, School of Plant Biology, University of Western Australia, Crawley, WA, Australia
  2. Centre for Excellence in Natural Resource Management, University of Western Australia, Albany, WA, Australia
  3. Rio Tinto Iron Ore, Perth, WA, Australia
Intermittent dryland streams are characterised by cycles of intense but short floods followed by long periods of drought and evaporative contraction, which in turn results in cycles of activity in microbial processes and biogeochemical cycling. We used stable isotopes (δ18O and δ2H) to determine the origin of surface water and evaporative changes in pools from four creek systems of the semi-arid Pilbara between May 2011 and October 2012. Change in dissolved organic matter (DOM) composition was used to examine relationships between pool biogeochemistry and hydrologic regime. Pools with groundwater inputs or shallow alluvial water throughflow showed low evaporative losses, while pools isolated from groundwater were more highly evaporated. Pool DOM composition was usually dominated by humic-like compounds derived from terrestrial organic matter. However, highly evaporated pools had large contributions of protein-like compounds, most likely derived from microbial turnover of organic matter. Concentrations of both humic and protein-like compounds, as well as the humic:protein-like ratio, were positively correlated to δ18O and δ2H values at 2-3 months since the last flood. However, in the absence of substantial rainfall events this relationship became increasingly decoupled over time, as the influence of groundwater inflow shifted from direct effects on concentration and DOM inputs to alleviation of contraction pressures. We suggest that other factors, including differences in UV exposure and establishment of aquatic vegetation, are important factors contributing to differences in DOM among pools with increasing time since flood.