Buried particulate organic C fuels heterotrophic metabolism in the hyporheic zone of a montane headwater stream

We examined the interactions between stream water and subsurface sediment to quantify how these interactions influenced organic C respiration and dissolved inorganic C (DIC) production in the hyporheic zone of a high-gradient headwater mountain stream draining a forested catchment at the H. J. Andrews Experimental Forest, Oregon, USA. We compared measurements from a well network with those from six 2-m-long hyporheic mesocosms. The patterns in hyporheic metabolism were similar in wells and mesocosms: O2 declined and DIC increased with travel time. However, the dissolved organic C (DOC) showed little net change in concentration. The mesocosms showed that net losses of DOC could account for 7% of O2 consumed during summer and autumn and 24% of O2 consumed in the winter and spring. Previous research at our study site suggested that large volumes of hyporheic exchange are likely to result in continual processing of streamwater DOC through the hyporheic zone, which would limit the accumulation of bioavailable DOC. Consequently, hyporheic respiration in this forested headwater stream appears to rely primarily on organic C ultimately derived from sediment-bound or buried particulate organic matter (POC). We modeled the consumption of O2 (kO2) and the production of DIC (kDIC) as zero-order kinetic reactions. In the mesocosms, kO2 and kDIC were correlated to temperature and were 2x higher in the summer and autumn than in the winter and spring. In the well network, however, neither kO2 nor kDIC showed seasonal differences. The kDIC was also correlated to initial DOC concentrations in both the mesocosms and the wells. Further, the kDIC was correlated to the time since the mesocosms were packed, which suggests that sediment disturbance or incorporation of relatively fresh organic matter from packing the mesocosms increased the bioavailability of the POC. Further, changes in concentrations of O2, DOC, and DIC were approximately linear when plotted against travel time for both the mesocosms and the well field. These data suggest that the processes that release bioavailable DOC from POC are relatively constant along hyporheic flow paths and govern the rate of hyporheic metabolism in the hyporheic zone of this forested headwater stream.