Coupling of Flow and Biogeochemical Processes Controlling the Environmental Conditions in the Hyporheic Zone: Implications for the Streambed Habitat

The hyporheic zone of streams not only connects groundwater and surface water, but is essential for nutrient and carbon cycling and provides crucial habitat for organisms (termed hyporheos). Here we demonstrate how flow (groundwater discharge and hyporheic exchange) and biogeochemical processes interact to form environmental conditions and habitat for hyporheos. This provides the understanding to assess how hyporheic conditions may be altered by changing flow paths caused by flow perturbations such as groundwater pumping. The studied reaches in the Maules Creek Catchment in New South Wales, Australia, comprise intermittent losing, perennial gaining and perennial losing sections, and are therefore well-suited to study how different flow paths affect water quality. Surface water, hyporheic zone pore water (at depths between 0.4 and 0.8 m), and groundwater from monitoring bores, was sampled and analysed for water quality and hyporheos. For each hyporheic site the hydraulic potential for upwelling or down-welling was measured by the vertical hydraulic head difference. Upwelling regional groundwater was generally oxic with detectable nitrate and low DOC (dissolved organic carbon). On the other hand, hyporheic water in down-welling zones became anoxic at shallow depths (< 1 m), with dissolved reduced species such as Fe2+, Mn2+ and NH4+ and no O2, forming a steep vertical redox gradient from the streambed into the sediment. Upwelling hyporheic water (originating from the stream) was found to have a similar hydrochemical signature. These zones did not support habitat for hyporheic invertebrates because metazoan organisms cannot permanently inhabit anoxic environments. No invertebrates were found for Fe2+ concentrations above 2 mg/L. The hyporheos in these zones appeared to be dominated by anaerobic microbes including Fe-reducing bacteria. Our results show that flow conditions affect water quality, which in turn regulates the habitat of hyporheic invertebrates as they will not exist under anoxic conditions. The results have implications for understanding the ecological responses of the hyporheic zone to perturbations which affect flow and biogeochemical processes.