Riparian Zones and the Role of Hyporheic Exchange in the Carbon Budget of a Small, Forested, Headwater Stream, Western Oregon, USA.

Abstract:

Recent estimates have identified streams as important conduits in the global carbon budget. Stream waters are typically super-saturated with CO₂. This CO₂ is assumed to come from carbon fixed in the upland terrestrial environment and then transported to the stream via soil water or groundwater. Evasion of CO₂ occurs at the stream surface, which usually comprises less than 2% of the watershed area, yet this flux might account for as much as 30% of the net ecosystem exchange in a watershed. This view does not consider the role of hyporheic exchange, despite the fact that hyporheic exchange fluxes can be very large in headwater streams, which drain the majority of the landscape. Using continuously recording probes, we show that pCO₂ averages 890 ppmv in stream water and 7,680 ppmv in hyporheic water in a 96-ha watershed. Independent estimates show that stream water turn-over lengths through the hyporheic zone are less than 100 m at baseflow, which suggests that stream water is continuously recharged with CO₂ every time it is cycled through the hyporheic zone. We monitored DIC and DOC in a co-located well network and show that DOC decreases, and DIC increases, with travel time through the hyporheic zone. However, respiration of stream-source DOC can only account for approximately 10% of the increase in DIC. Previous hydrologic studies suggest that lateral inputs of soil water or groundwater are limited within this study reach, so the large increases in DIC must come from particulate organic matter buried in the hyporheic zone and from the overlying soil. These measurements suggest that riparian zones supply, via hyporheic exchange, a disproportionately large fraction of carbon to headwater streams and may therefore play an outsized role in the global carbon cycle.