Seasonal shifts in dissolved oxygen, carbon dynamics, and resazurin transformation along a 12-m long artificial hyporheic flowpath

Abstract:

Hyporheic exchange facilitates many biogeochemical reactions, but it is difficult to predict exactly how water quality will change along hyporheic flowpaths. Water quality is often modeled as a function of hyporheic residence time with the assumption that reaction rates are uniform along hyporheic flowpaths. However, testing this assumption in the field would require sampling multiple locations along a flowpath while also accounting for non-steady boundary conditions. To test the uniformity of reaction rates along simplified hyporheic flowpaths, we monitored spatial patterns in water quality along a 12-m long artificial hyporheic mesocosm system at the HJ Andrews Experimental Forest (Oregon, USA). The mesocosm consists of twelve 1-m aluminum columns packed with local streambed sediment and connected in series, through which fresh streamwater is continually pumped. This system reduces the complexity of hyporheic monitoring by imposing a fixed flowpath geometry compared to spatially-variable flowpaths at many field sites. We collected a series of ambient water quality (DO, DOC, DIC, and various anions and cations) profiles at multiple points along the mesocosm. We also conducted conservative and reactive tracer injections (NaCl, uranine, and resazurin) to test whether reaction rates differed between the beginning, middle, and end of the mesocosm. We monitored the mesocosm over multiple seasons, with the bulk of measurements occurring during Winter 2019 and Summer 2020. Although the influent stream water was always well oxygenated (>9 mg/L), the end of the flowpath fluctuated between anoxic conditions ([DO] < 0.50 mg/L) and oxic conditions ([DO] ~5 mg/L) throughout the year, with oxic conditions prevailing during low temperatures, and anoxia developing above approximately 15°C. During Winter 2019 sampling the concentration dynamics for DOC, DIC, nitrate, and resazurin were relatively consistent along the mesocosm flowpath. During Summer 2020, however, reaction dynamics became more complex for dissolved compounds. This experiment advances our understanding of the controls on hyporheic reactivity 𐆑 that is, the potential for spatially and temporally-variable reaction rates even along a fixed hyporheic flowpath 𐆑 which can help constrain models and field systems with more complex hydraulics.