Biogeochemical processes in the hyporheic zone: the role of flow regime in controlling habitat.

Flow of water is essential to provide food (organic matter - OM), nutrients (e.g. N and P) and electron acceptors (e.g. oxygen, nitrate, etc.) for groundwater-dependent ecosystems. In the hyporheic zone, subsurface flows may have multiple sources, i.e. regional groundwater or surface water via hyporheic exchange. These water sources may have very different chemical composition in terms of nutrients, electron acceptors and OM and its quality (i.e. reactivity). Regional groundwater tends to be geochemically evolved and has low concentrations of labile OM. In contrast, surface water has higher concentrations of labile OM. Hence down-welling surface water leads to steep redox-gradients along hyporheic flow paths caused by rapid degradation of OM. As a consequence contrasting subsurface environmental conditions can be formed depending on the source of water, with a range of redox-conditions from oxic to anoxic. Up-welling oxic groundwater will support ecosystems with higher trophic levels (i.e. invertebrates) whereas anoxic conditions will lead to an anaerobic microbe-dominated ecosystem. Altering the flow regime in hyporheic systems will change the hydrogeochemical conditions, OM degradation and microbial processes, and the habitat for invertebrates. For instance depriving streams of their groundwater contribution (e.g. due to groundwater abstraction) could decrease amount and duration of surface flow, enhance down-welling of surface water and alter water quality towards more anoxic conditions. This has significant implications for ecohydrological processes and ecosystem functioning in the hyporheic zone. This presentation will explore these processes by theoretical considerations and field studies undertaken at Maules Creek, NSW