A Stream-Side Mesocosm Study of Organic Matter Processing in the Hyporheic Zone
Abstract
We designed and built hyporheic mesocosms (HZMs) to control hydrologic flowpaths and provide an experimental platform for studying biogeochemical processes. The HZMs are modular, comprised of 12 individual segments measuring 1-m in length and 20-cm in diameter, and are packed with native streambed materials. The HZMs are installed at the gage house in Watershed 1 in the H.J. Andrews Experimental Forest and stream water is continually pumped through the HZMs at a flow rates matching travel times observed in an adjacent well field. For the experiments reported here, we connected pairs of segments to make six individual HZMs with 2-m long flow paths and 10- to 12-hour median travel times. We examined organic carbon processes under background conditions when the HZMs received only DOC from stream water and during tracer additions of different types and concentrations of DOC (Na-acetate, leachates from alder leaves and the forest floor).
Stream-source DOC did not drive hyporheic (HZ) respiration under background conditions. Respiration rates were low and the HZMs always remained oxic. Losses of O2 and production of dissolved inorganic carbon (DIC) were usually substantially larger than the loss of DOC. Buried particulate organic carbon (POCb) was the likely source of carbon for hyporheic respiration. Further, POCb comprised 4% of the sediment weight, and at the observed rates, the POCb would be sufficient to fuel HZ respiration for more than 250 yr. In the DOC amendment experiments, the different DOC sources appeared equally labile and nearly all of the added DOC was consumed in the first meter of the HZMs. Further, DOC addition stimulated a priming effect of the stream water DOC where the DOC concentrations at the outlets of the treated HZMs were consistently lower than the control HZMs. Loss of injected DOC was usually much greater than the consumption of O2 or production of DIC, suggesting that either growth or luxury uptake were occurring. Finally, we observed a net loss of DIC early in several injection experiments. These results support prior observations that POCb is the primary driver of HZ respiration in the absence of pulses of highly labile DOC. However, our observations also suggest that such DOC pulses may stimulate priming of otherwise recalcitrant stream DOC.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.H22D..02W
- Keywords:
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- 1813 Eco-hydrology;
- HYDROLOGYDE: 1830 Groundwater/surface water interaction;
- HYDROLOGYDE: 1871 Surface water quality;
- HYDROLOGYDE: 1879 Watershed;
- HYDROLOGY