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• 1. [Article] Spatiotemporal dynamics and drivers of stream pCO₂ in a headwater mountain catchment in the Cascade Mountains, Oregon

  We examined the spatial and temporal variability of stream carbon dioxide (CO₂) and the drivers of these variations in a headwater catchment. To examine temporal variation and drivers, we measured stream ...

  Citation × Citation Citation Abstract Copy Title: Spatiotemporal dynamics and drivers of stream pCO₂ in a headwater mountain catchment in the Cascade Mountains, Oregon Author: Dosch, Nicholas T. We examined the spatial and temporal variability of stream carbon dioxide (CO₂) and the drivers of these variations in a headwater catchment. To examine temporal variation and drivers, we measured stream and hyporheic pCO₂ at high temporal resolution over 11 months in a 95.9-ha forested headwater catchment in the Western Cascades of Central Oregon, USA. Stream and hyporheic pCO₂ showed high seasonal and event-scale variability with distinct stream and hyporheic dynamics during storm discharge events. Hyporheic exchange flow exported 37.5 kg-C yr⁻¹ per watershed hectare (confidence interval 4.0-122.3 kg-C ha⁻¹ yr⁻¹) from the riparian zone to the stream. Summing CO₂ evasion and downstream advection suggests that one third of inorganic carbon export originated in the hyporheic zone. Hyporheic exchange flow had greatest influence over stream pCO₂ during low and high baseflow, while CO₂ evasion had greatest influence during storm discharge events. These findings suggest that the hyporheic zone actively participates in carbon cycling in this headwater stream and continuously replenishes stream CO₂. To examine spatial variation and drivers, we measured stream CO₂ at monthly intervals from July 2013 through July 2014 at 38 locations across the 6400-ha HJ Andrews Experimental Forest. Stream pCO₂ was consistently supersaturated with respect to atmospheric concentrations. Stream pCO₂ ranged from atmospheric (~400 μatm) to 20 times atmospheric concentrations (8150 μatm) and exhibited strong spatial and temporal variability. The distribution of pCO₂ over the study period was different in small and large streams within the drainage network. At the watershed scale, pCO₂ decreased with distance downstream. At the reach scale, we did not detect clear patterns in the downstream direction. However, individual transects displayed persistent profile shape, with consistent high and low pCO₂ locations. We found negative relationships between stream pCO₂ and stream discharge, mean velocity and the carbon dioxide gas transfer velocity. Stream pCO₂ exhibited changes over short distances, with large changes in pCO₂ over less than 50 m. Longitudinal variability indicates spatial variability of in-stream controls on pCO₂ at this scale. Stream pCO₂ shows generally higher concentrations during the summer and lower concentrations in the winter, with considerable intrannual variability. Title: Spatiotemporal dynamics and drivers of stream pCO₂ in a headwater mountain catchment in the Cascade Mountains, Oregon Author: Dosch, Nicholas T. We examined the spatial and temporal variability of stream carbon dioxide (CO₂) and the drivers of these variations in a headwater catchment. To examine temporal variation and drivers, we measured stream and hyporheic pCO₂ at high temporal resolution over 11 months in a 95.9-ha forested headwater catchment in the Western Cascades of Central Oregon, USA. Stream and hyporheic pCO₂ showed high seasonal and event-scale variability with distinct stream and hyporheic dynamics during storm discharge events. Hyporheic exchange flow exported 37.5 kg-C yr⁻¹ per watershed hectare (confidence interval 4.0-122.3 kg-C ha⁻¹ yr⁻¹) from the riparian zone to the stream. Summing CO₂ evasion and downstream advection suggests that one third of inorganic carbon export originated in the hyporheic zone. Hyporheic exchange flow had greatest influence over stream pCO₂ during low and high baseflow, while CO₂ evasion had greatest influence during storm discharge events. These findings suggest that the hyporheic zone actively participates in carbon cycling in this headwater stream and continuously replenishes stream CO₂. To examine spatial variation and drivers, we measured stream CO₂ at monthly intervals from July 2013 through July 2014 at 38 locations across the 6400-ha HJ Andrews Experimental Forest. Stream pCO₂ was consistently supersaturated with respect to atmospheric concentrations. Stream pCO₂ ranged from atmospheric (~400 μatm) to 20 times atmospheric concentrations (8150 μatm) and exhibited strong spatial and temporal variability. The distribution of pCO₂ over the study period was different in small and large streams within the drainage network. At the watershed scale, pCO₂ decreased with distance downstream. At the reach scale, we did not detect clear patterns in the downstream direction. However, individual transects displayed persistent profile shape, with consistent high and low pCO₂ locations. We found negative relationships between stream pCO₂ and stream discharge, mean velocity and the carbon dioxide gas transfer velocity. Stream pCO₂ exhibited changes over short distances, with large changes in pCO₂ over less than 50 m. Longitudinal variability indicates spatial variability of in-stream controls on pCO₂ at this scale. Stream pCO₂ shows generally higher concentrations during the summer and lower concentrations in the winter, with considerable intrannual variability. Close

• 2. [Article] Carbon dynamics in the hyporheic zone of a headwater mountain stream in the Cascade Mountains, Oregon

  This study investigated carbon dynamics in the hyporheic zone of a steep, forested catchment in the Cascade Mountains of western Oregon, USA. Water samples were collected monthly from a headwater stream ...

  Citation × Citation Citation Abstract Copy Title: Carbon dynamics in the hyporheic zone of a headwater mountain stream in the Cascade Mountains, Oregon Author: Corson-Rikert, Hayley A. This study investigated carbon dynamics in the hyporheic zone of a steep, forested catchment in the Cascade Mountains of western Oregon, USA. Water samples were collected monthly from a headwater stream and well network during baseflow conditions from July to December 2013 and again in March 2014. We also sampled during one fall storm event, collecting pre-storm, rising leg, and extended high flow samples. The well network is located at the base of Watershed 1 (WS1) of the H.J. Andrews Experimental Forest and spans the full width of the floodplain (~14 m) along a 29 m reach of stream. We measured pH, temperature, water level, major anions, major cations, DOC, DIC, and total alkalinity. Flow paths, travel time to wells and hydraulic conductivity were available from previous studies. During baseflow periods, hyporheic DOC decreased with median travel time through the subsurface. DIC concentrations increased with travel time, but the magnitude of this increase in DIC was too large to be explained by metabolism of stream water DOC. This suggests that there are additional sources of DIC and/or DOC in the subsurface, and that hyporheic DIC concentrations are not well linked to stream-source DOC. The most likely supplemental sources of DIC to hyporheic water are soil CO₂ and microbial respiration of DOC leached from buried particulate organic matter and from overlying soils. Overall, the hyporheic zone appears to be a source of DIC to the stream. In summer, the hyporheic zone is likely isolated from vertical infiltration or lateral inflow of soil water, and particulate organic carbon is not present in stream water. Thus, spatial patterns in hyporheic zone biogeochemistry must result from underlying spatial patterns in hyporheic flowpaths, groundwater inputs, and buried particulate organic carbon. With the transition to the rainy season throughout the fall and early winter, vertical infiltration and leaching of accumulated solutes from the overlying soil appear to become important sources of carbon that help explain patterns in hyporheic zone biogeochemistry. During a small November storm event, DOC and nitrate concentrations in the stream displayed clockwise hysteresis. Travel time appeared to be associated with both nitrate and DOC response patterns in the hyporheic zone. In wells with long travel times, DOC and nitrate concentrations showed a clockwise hysteresis pattern that mimicked and even exceeded that observed in the stream. We hypothesize that these solutes were flushed from overlying soils into the hyporheic zone via vertically infiltrating rainwater. In wells with short travel times, we observed only a small peak in DOC and nitrate concentrations during the storm, potentially due to lateral infiltration of stream water later in the event. Overall, temporal patterns in hyporheic solute chemistry during the November storm differed from patterns we observed in the well network. This suggests that whole-watershed processes that controlled stream water chemistry during this storm event were different than those that controlled solute concentrations in the hyporheic zone. Nonetheless, the hyporheic zone must have been linked to the stream. That measurements in our well network reveal a very different response between the stream and the hyporheic zone suggests that: 1) Our hyporheic zone is not representative of stream-hyporheic riparian processes that occur within the larger watershed, or 2) Hillslope-stream or within-stream processes dominate during storms, and at these times the influence of the hyporheic zone on the stream is much weaker than during baseflow. During both baseflow and storm periods, the hydrology of the WS1 system is complex – hyporheic exchange flows follow extended, non-linear flow paths through a heterogeneous subsurface and may be augmented by lateral inflows of groundwater and, during storms, vertical infiltration of soil water. Our results from both baseflow and storm sampling suggest that a complex set of physical mechanisms and biogeochemical processes influence carbon transport and transformation within this hyporheic environment. Title: Carbon dynamics in the hyporheic zone of a headwater mountain stream in the Cascade Mountains, Oregon Author: Corson-Rikert, Hayley A. This study investigated carbon dynamics in the hyporheic zone of a steep, forested catchment in the Cascade Mountains of western Oregon, USA. Water samples were collected monthly from a headwater stream and well network during baseflow conditions from July to December 2013 and again in March 2014. We also sampled during one fall storm event, collecting pre-storm, rising leg, and extended high flow samples. The well network is located at the base of Watershed 1 (WS1) of the H.J. Andrews Experimental Forest and spans the full width of the floodplain (~14 m) along a 29 m reach of stream. We measured pH, temperature, water level, major anions, major cations, DOC, DIC, and total alkalinity. Flow paths, travel time to wells and hydraulic conductivity were available from previous studies. During baseflow periods, hyporheic DOC decreased with median travel time through the subsurface. DIC concentrations increased with travel time, but the magnitude of this increase in DIC was too large to be explained by metabolism of stream water DOC. This suggests that there are additional sources of DIC and/or DOC in the subsurface, and that hyporheic DIC concentrations are not well linked to stream-source DOC. The most likely supplemental sources of DIC to hyporheic water are soil CO₂ and microbial respiration of DOC leached from buried particulate organic matter and from overlying soils. Overall, the hyporheic zone appears to be a source of DIC to the stream. In summer, the hyporheic zone is likely isolated from vertical infiltration or lateral inflow of soil water, and particulate organic carbon is not present in stream water. Thus, spatial patterns in hyporheic zone biogeochemistry must result from underlying spatial patterns in hyporheic flowpaths, groundwater inputs, and buried particulate organic carbon. With the transition to the rainy season throughout the fall and early winter, vertical infiltration and leaching of accumulated solutes from the overlying soil appear to become important sources of carbon that help explain patterns in hyporheic zone biogeochemistry. During a small November storm event, DOC and nitrate concentrations in the stream displayed clockwise hysteresis. Travel time appeared to be associated with both nitrate and DOC response patterns in the hyporheic zone. In wells with long travel times, DOC and nitrate concentrations showed a clockwise hysteresis pattern that mimicked and even exceeded that observed in the stream. We hypothesize that these solutes were flushed from overlying soils into the hyporheic zone via vertically infiltrating rainwater. In wells with short travel times, we observed only a small peak in DOC and nitrate concentrations during the storm, potentially due to lateral infiltration of stream water later in the event. Overall, temporal patterns in hyporheic solute chemistry during the November storm differed from patterns we observed in the well network. This suggests that whole-watershed processes that controlled stream water chemistry during this storm event were different than those that controlled solute concentrations in the hyporheic zone. Nonetheless, the hyporheic zone must have been linked to the stream. That measurements in our well network reveal a very different response between the stream and the hyporheic zone suggests that: 1) Our hyporheic zone is not representative of stream-hyporheic riparian processes that occur within the larger watershed, or 2) Hillslope-stream or within-stream processes dominate during storms, and at these times the influence of the hyporheic zone on the stream is much weaker than during baseflow. During both baseflow and storm periods, the hydrology of the WS1 system is complex – hyporheic exchange flows follow extended, non-linear flow paths through a heterogeneous subsurface and may be augmented by lateral inflows of groundwater and, during storms, vertical infiltration of soil water. Our results from both baseflow and storm sampling suggest that a complex set of physical mechanisms and biogeochemical processes influence carbon transport and transformation within this hyporheic environment. Close