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Carbon Dynamics in the Hyporheic Zone of a Headwater Mountain Stream in the Cascade Mountains, Oregon, Facilities Were Provided by the H.J

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Carbon Dynamics in the Hyporheic Zone of a Headwater Mountain Stream in the Cascade Mountains, Oregon, Facilities Were Provided by the H.J PUBLICATIONS Water Resources Research RESEARCH ARTICLE Carbon dynamics in the hyporheic zone of a headwater 10.1002/2016WR019303 mountain stream in the Cascade Mountains, Oregon Key Points: Hayley A. Corson-Rikert1, Steven M. Wondzell2, Roy Haggerty1, and Mary V. Santelmann1 The hyporheic zone (HZ) is a source of DIC to the stream 1 2 During base flow periods, neither College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA, U.S. Forest Service, stream water nor groundwater nor Pacific Northwest Research Station, Corvallis, Oregon, USA hillslope soil water are significant sources of DOC or DIC to the HZ Most hyporheic DIC exported to the Abstract We investigated carbon dynamics in the hyporheic zone of a steep, forested, headwater stream is generated by microbial respiration of buried particulate catchment western Oregon, USA. Water samples were collected monthly from the stream and a well organic matter 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 Correspondence to: groundwater or lateral inputs of shallow soil water influenced carbon dynamics. Rather, carbon dynamics H. A. Corson-Rikert, appeared to be controlled by local processes in the hyporheic zone and overlying riparian soils. DOC [email protected] 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 Citation: Corson-Rikert, H. A., S. M. Wondzell, much greater than DOC (0.35–0.7 mM) and increased with nominal travel time through the hyporheic zone R. Haggerty, and M. V. Santelmann (0.2–0.4 mM gained over 100 h). DOC in stream water could only account for 10% of the observed increase (2016), Carbon dynamics in the in DIC. In situ metabolic processing of buried particulate organic matter as well as advection of CO from hyporheic zone of a headwater 2 mountain stream in the Cascade the vadose zone likely accounted for the remaining 90% of the increase in DIC. Overall, the hyporheic zone Mountains, Oregon, Water Resour. Res., was a source of DIC to the stream. We suggest that, in mountain stream networks, hyporheic exchange 52, 7556–7576, doi:10.1002/ facilitates the transformation of particulate organic carbon buried in floodplains and transports the DIC that 2016WR019303. is produced back to the stream where it can be evaded to the atmosphere. Received 6 JUN 2016 Accepted 11 SEP 2016 Accepted article online 16 SEP 2016 Published online 1 OCT 2016 1. Introduction Aquatic ecosystems process carbon on timescales that are short in comparison to timescales of carbon pro- duction, cycling, and storage in terrestrial ecosystems [Battin et al., 2008]. As a result, streams and rivers play an active role in global carbon cycling [Cole and Caraco, 2001; Regnier et al., 2013]. Terrestrial carbon enters streams through direct inputs such as leaf-fall and throughfall, and through subsurface flow. Once in streams, terrestrial carbon is not only advected to downstream reaches, but is also transformed, evaded, and stored. Understanding the physical and biogeochemical controls on carbon transformation in aquatic ecosystems is critical because rates of carbon transformation influence the movement of carbon through a system and determine the form in which it is exported. Headwater streams are particularly important locations for the transformation of terrestrially derived organ- ic carbon because these streams make up 75% of global stream length [Downing et al., 2012], receive large amounts of carbon from adjacent terrestrial ecosystems, and continuously exchange water with streambed sediments and adjacent floodplains [Kasahara and Wondzell, 2003; Battin et al., 2008; Wondzell, 2011]. This exchange, defined as hyporheic exchange flow, increases the residence time of organic molecules that are transported by the stream water and simultaneously increases their exposure to biofilms and aggregates that dominate microbial communities in the subsurface. It also provides these communities with a steady supply of oxygen and nutrients [Battin et al., 2008]. As a result, organic matter is actively processed and metabolized in hyporheic zones, and large quantities of CO2 are generated via respiration [Grimm and Fish- er, 1984; Findlay et al., 1993; Pusch and Schwoerbel, 1994; Pusch, 1996; Mulholland et al., 1997; Baker et al., 1999; Battin, 1999; Sobczak and Findlay, 2002]. Several studies have examined the transformation of DOC along hyporheic flow paths through gravel bars located within active stream channels where hyporheic water does not mix with water from other sources. VC 2016. American Geophysical Union. These studies showed that the hyporheic zone was a sink for DOC and a source of DIC, with consumption of All Rights Reserved. DOC and O2 and production of DIC occurring along a gradient of travel distances, or travel times, through CORSON-RIKERT ET AL. HZ CARBON DYNAMICS IN HEADWATER STREAM 7556 Water Resources Research 10.1002/2016WR019303 the hyporheic zone. Stream-source DOC was a critical input to the hyporheic zone and accounted for 50%– 85% of the heterotrophic respiration that occurred along hyporheic flow paths, with the remainder accounted for by POC buried in gravel bar sediment [Findlay et al., 1993; Jones et al., 1995; Sobczak and Find- lay, 2002; Zarnetske et al., 2011]. However, the simplified setting of an unvegetated gravel bar and the domi- nance of stream-source water in hyporheic flow paths within this setting limits the transferability of these results to other stream reaches. Patterns of change in concentrations of DOC along hyporheic flow paths through forested riparian zones in mountain streams likely differ from those in midchannel gravel bars. Particulate organic matter buried in the hyporheic sediment may be a source of DOC for both the hyporheic zone and the stream [Schindler and Krabbenhoft, 1998]. Also, lateral inputs of shallow soil water and vertical leaching from the overlying soil organic horizons could serve as sources of DOC and deeper, long-residence time ground water might serve as a source of DIC. These additional inputs of DOC or DIC would influence subsurface carbon cycling and could potentially distort or obscure simple patterns of carbon metabolism previously observed along hypo- rheic flow paths [Sobczak and Findlay, 2002; Zarnetske et al., 2011]. This study focuses on the seasonal dynamics of DOC and DIC within the floodplain of a small mountain stream during base flow periods. Our starting conceptual model builds on the work of Findlay et al. [1993], Jones et al. [1995], and Zarnetske et al. [2011], postulating that stream-source DOC will be the primary source of organic carbon for heterotrophic respiration in the hyporheic zone and that residence time will be the major factor determining the amount of DOC consumed and DIC produced along hyporheic flow paths. Pre- vious research at this site lends substantial support for our conceptual model. First, ground water flow mod- els calibrated to the site suggested that the median hyporheic residence time was 17 h, and that residence times along some hyporheic flow paths stretched to several weeks [Kasahara and Wondzell, 2003]. These simulation results also suggested that, at base flow, the turnover length of stream water through the hypo- rheic zone was less than 100 m and that lateral inputs of shallow soil water or deeper groundwater did not substantially influence hyporheic flow paths through the study site. These model results were well sup- ported by tracer tests which were conducted across a range of discharges and suggested that the residence times and magnitudes of hyporheic exchange fluxes change little with changes in stream discharge over a wide range of base flow conditions [Wondzell, 2006; Ward et al., 2012; Voltz et al., 2013; Ward et al., 2016]. Nonetheless, because the chemistry of stream water at our study site is dilute, relatively small volumes of DOC or DIC-rich water from lateral inflow could have a significant influence on concentrations of dissolved carbon at different locations within the hyporheic zone. As a result, we also examine the degree to which different source waters might influence DOC and DIC dynamics in the hyporheic zone during base flow periods, and how this influence might change between the summer dry season and the fall and early- winter wet season. The mixing of different source waters can be deciphered using end-member mixing analyses (EMMA). These analyses are used to discern the relative proportion of water contributed by each source, as well as how that proportion varies in response to changing catchment wetness. The EMMA approach is typically applied to whole catchments and has been used to examine how mixing controls changes in the quantity [McGlynn and McDonnell, 2003] and quality [Inamdar et al., 2011] of DOC. Here, we apply EMMA to examine the potential influence of mixing of different source waters on the concentrations of DOC and DIC observed during base flow periods within a well network located in the floodplain of a mountainous, headwater stream. The focus of our study was to investigate carbon dynamics in the hyporheic zone of a headwater catchment in the Western Cascade Mountains of Oregon. We compare the influence of hyporheic biogeochemical pro- cesses to the influence of mixing of different source waters, and examine how the relative importance of these processes changed from the summer dry season to the winter wet season. 2. Study Site This study was conducted in the lower portion of Watershed 1 (WS1), a study watershed in the H.J. Andrews Experimental Forest, located in the Western Cascades of Oregon, USA (44 12’ 28.00N, 122 15’ 30.00W; Fig- ure 1). WS1 is a steep, forested catchment that is 95.5 ha in size, and ranges in elevation from 450 to CORSON-RIKERT ET AL.
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