Carbon dynamics in the hyporheic zone of a headwater mountain stream in the Cascade Mountains, Oregon

Abstract

We investigated carbon dynamics in the hyporheic zone of a steep, forested, headwater catchment western Oregon, USA. Water samples were collected monthly from the stream and a well network during base flow periods. We examined the potential for mixing of different source waters to explain concentrations of DOC and DIC. We did not find convincing evidence that either inputs of deep groundwater or lateral inputs of shallow soil water influenced carbon dynamics. Rather, carbon dynamics appeared to be controlled by local processes in the hyporheic zone and overlying riparian soils. DOC concentrations were low in stream water (0.04–0.09 mM), and decreased with nominal travel time through the hyporheic zone (0.02–0.04 mM lost over 100 h). Conversely, stream water DIC concentrations were much greater than DOC (0.35–0.7 mM) and increased with nominal travel time through the hyporheic zone (0.2–0.4 mM gained over 100 h). DOC in stream water could only account for 10% of the observed increase in DIC. In situ metabolic processing of buried particulate organic matter as well as advection of CO2 from the vadose zone likely accounted for the remaining 90% of the increase in DIC. Overall, the hyporheic zone was a source of DIC to the stream. We suggest that, in mountain stream networks, hyporheic exchange facilitates the transformation of particulate organic carbon buried in floodplains and transports the DIC that is produced back to the stream where it can be evaded to the atmosphere.

Effect of Hyporheic Exchange on Macroinvertebrate Community in the Weihe River Basin, China

“…This indicated that downwelling can affect the growth of invertebrate communities by removing organic matter from the river, which controls their metabolism. This finding is supported by the literature on carbon cycling [77,83]. Chironomidae were dependent on HCO 3 − (Figure 3c), hence, their demand for organic matter was more pronounced.…”

Solute Transport and Transformation in an Intermittent, Headwater Mountain Stream with Diurnal Discharge Fluctuations

“…Similar to other detrital-based, heterotrophic streams, low nitrogen and phosphorus concentrations at our study site could limit metabolic activity [120]. The bioavailability of these nutrients likely limit microbial metabolism in our system under all flow conditions [116,121,122]. However, we would expect these limitations to be minimized along shallow, intermittently inundated, and hydrologically youngest flowpaths that experience warmer temperatures, are nearer to light exposure, increased reactive surface area, and availability of fresh organic matter sources.…”

“…Lastly, metabolic activity may be energetically constrained on timescales relevant to the water residence time along these longer and hydrologically older flowpaths. Previous work has suggested that buried particulate organic matter from debris flows promotes hyporheic microbial respiration at the study site [122]. While this buried organic matter, likely western hemlock (Tsuga heterophylla) and Douglas fir (Pseudotsuga menziesii) wood and needles, may persistently increase metabolic activity of the oldest water at the site, it is also likely a low-quality organic matter source with less reaction potential, relative to the younger organic matter along the shallower hyporheic flowpaths, given its composition and old age within the stream corridor [123].…”

Hydrologic controls on hyporheic exchange in a headwater mountain stream

“…See Table and supporting information Figure S1 for uniform and spatially variable groundwater inflow velocities during the high, medium, and low stream discharge conditions. Stream discharge and UAA estimates are consistent with recent work done in the catchment [ Corson‐Rikert et al ., ].…”