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1. Biotic controls on solute distribution and transport in headwater catchments

   https://doi.org/10.5194/hessd-12-213-2015  

   Herndon, E. M. ; Dere, A. L. ; Sullivan, P. L. ; Norris, D. ; Reynolds, B. ; Brantley, S. L. ( January 2015 , Hydrology and Earth System Sciences Discussions)

   Solute concentrations in stream water vary with discharge in patterns that record complex feedbacks between hydrologic and biogeochemical processes. In a comparison of headwater catchments underlain by shale in Pennsylvania, USA (Shale Hills) and Wales, UK (Plynlimon), dissimilar concentration-discharge behaviors are best explained by contrasting landscape distributions of soil solution chemistry – especially dissolved organic carbon (DOC) – that have been established by patterns of vegetation. Specifically, elements that are concentrated in organic-rich soils due to biotic cycling (Mn, Ca, K) or that form strong complexes with DOC (Fe, Al) are spatially heterogeneous in pore waters because organic matter is heterogeneously distributed across the catchments. These solutes exhibit non-chemostatic "bioactive" behavior in the streams, and solute concentrations either decrease (Shale Hills) or increase (Plynlimon) with increasing discharge. In contrast, solutes that are concentrated in soil minerals and form only weak complexes with DOC (Na, Mg, Si) are spatially homogeneous in pore waters across each catchment. These solutes are chemostatic in that their stream concentrations vary little with stream discharge, likely because these solutes are released quickly from exchange sites in the soils during rainfall events. Differences in the hydrologic connectivity of organic-rich soils to the stream drive differences in concentrationmore »behavior between catchments. As such, in catchments where soil organic matter (SOM) is dominantly in lowlands (e.g., Shale Hills), bioactive elements are released to the stream early during rainfall events, whereas in catchments where SOM is dominantly in uplands (e.g., Plynlimon), bioactive elements are released later during rainfall events. The distribution of vegetation and SOM across the landscape is thus a key component for predictive models of solute transport in headwater catchments.« less
2. Drivers of Dissolved Organic Carbon Mobilization From Forested Headwater Catchments: A Multi Scaled Approach

   https://doi.org/10.3389/frwa.2021.578608  

   Adler, Thomas ; Underwood, Kristen L. ; Rizzo, Donna M. ; Harpold, Adrian ; Sterle, Gary ; Li, Li ; Wen, Hang ; Stinson, Lindsey ; Bristol, Caitlin ; Stewart, Bryn ; et al ( July 2021 , Frontiers in Water)

   Understanding and predicting catchment responses to a regional disturbance is difficult because catchments are spatially heterogeneous systems that exhibit unique moderating characteristics. Changes in precipitation composition in the Northeastern U.S. is one prominent example, where reduction in wet and dry deposition is hypothesized to have caused increased dissolved organic carbon (DOC) export from many northern hemisphere forested catchments; however, findings from different locations contradict each other. Using shifts in acid deposition as a test case, we illustrate an iterative “process and pattern” approach to investigate the role of catchment characteristics in modulating the steam DOC response. We use a novel dataset that integrates regional and catchment-scale atmospheric deposition data, catchment characteristics and co-located stream Q and stream chemistry data. We use these data to investigate opportunities and limitations of a pattern-to-process approach where we explore regional patterns of reduced acid deposition, catchment characteristics and stream DOC response and specific soil processes at select locations. For pattern investigation, we quantify long-term trends of flow-adjusted DOC concentrations in stream water, along with wet deposition trends in sulfate, for USGS headwater catchments using Seasonal Kendall tests and then compare trend results to catchment attributes. Our investigation of climatic, topographic, and hydrologic catchment attributesmore »vs. directionality of DOC trends suggests soil depth and catchment connectivity as possible modulating factors for DOC concentrations. This informed our process-to-pattern investigation, in which we experimentally simulated increased and decreased acid deposition on soil cores from catchments of contrasting long-term DOC response [Sleepers River Research Watershed (SRRW) for long-term increases in DOC and the Susquehanna Shale Hills Critical Zone Observatory (SSHCZO) for long-term decreases in DOC]. SRRW soils generally released more DOC than SSHCZO soils and losses into recovery solutions were higher. Scanning electron microscope imaging indicates a significant DOC contribution from destabilizing soil aggregates mostly from hydrologically disconnected landscape positions. Results from this work illustrate the value of an iterative process and pattern approach to understand catchment-scale response to regional disturbance and suggest opportunities for further investigations.« less
3. Increased {DOC} concentrations and earlier stream flow generation in response to warming in a high elevation mountain watershed in Colorado

   Kerins, Devon ; Zhi, Wei ; Sullivan, Pamela ; Williams, Kenneth ; Brown, Wendy ; Dong, Wenming ; Carroll, Rosemary ; Kirchner, James ; Li, Li ( December 2021 , American Geophysical Union annual meeting)

   High elevation mountain watersheds are undergoing rapid warming and declining snow fractions worldwide, causing earlier and quicker snowmelt. Understanding how this hydrologic shift affects subsurface flow paths, biogeochemical reactions, and solute export has been challenging due to the entanglement of hydrological and biogeochemical processes. Coal Creek, a high-elevation catchment (2,700 3,700 m, 53 km2) in Colorado, is experiencing a higher rate of warming than surrounding low-lying areas. This warming corresponds with dynamic and increased responses from biogenic solutes and dissolved organic carbon (DOC), whereas the behavior of geogenic solutes and dissolved inorganic carbon (DIC) has remained relatively unchanged. DOC has experienced the largest concentration increase (>3x), with annual average flow weighted concentrations positively correlated to average annual temperature. This suggests temperature is the main driver of increasing DOC levels. Although DOC and DIC response to warming is influenced by many drivers, the relative contribution of each remains unknown. DOC and DIC were analyzed to incorporate both carbon component products of soil respiration (DOC and CO2) and to represent high solute concentrations transported by shallow (DOC) versus deep (DIC) subsurface flow. The contrasting behavior of these carbon solutes indicates climate change and warming are driving changes in organic matter decomposition andmore »soil respiration. Modeling results from the process-based model HBV-BioRT show increased temperatures cause earlier snowmelt and streamflow generation and lower peak discharge. As stream flow generation occurs earlier, so do DOC flushing and DIC dilution events. Additionally, post-snowmelt periods show greater DOC production and concentrations under warming scenarios. Results indicated increased production of DOC in post-snowmelt periods. DOC is then flushed out by earlier snowmelt partitioned through the shallow soil zone. Most process-based studies lack a watershed-scale understanding of carbon transformation and flow path alterations. This work demonstrates complex hydrologic and biogeochemical coupling at the watershed scale to illustrate how water flow paths and chemistry are responding to a changing climate in highelevation mountain watersheds.« less
4. Multi-Scale Temporal Patterns in Stream Biogeochemistry Indicate Linked Permafrost and Ecological Dynamics of Boreal Catchments

   https://doi.org/10.1007/s10021-021-00709-6  

   Webster, Alex J. ; Douglas, Thomas A. ; Regier, Peter ; Scheuerell, Mark D. ; Harms, Tamara K. ( October 2021 , Ecosystems)

   Temporal patterns in stream chemistry provide integrated signals describing the hydrological and ecological state of whole catchments. However, stream chemistry integrates multi-scale signals of processes occurring in both the catchment and stream. Deconvoluting these signals could identify mechanisms of solute transport and transformation and provide a basis for monitoring ecosystem change. We applied trend analysis, wavelet decomposition, multivariate autoregressive state-space modeling, and analysis of concentration–discharge relationships to assess temporal patterns in high-frequency (15 min) stream chemistry from permafrost-influenced boreal catchments in Interior Alaska at diel, storm, and seasonal time scales. We compared catchments that varied in spatial extent of permafrost to identify characteristic biogeochemical signals. Catchments with higher spatial extents of permafrost were characterized by increasing nitrate concentration through the thaw season, an abrupt increase in nitrate and fluorescent dissolved organic matter (fDOM) and declining conductivity in late summer, and flushing of nitrate and fDOM during summer rainstorms. In contrast, these patterns were absent, of lower magnitude, or reversed in catchments with lower permafrost extent. Solute dynamics revealed a positive influence of permafrost on fDOM export and the role of shallow, seasonally dynamic flowpaths in delivering solutes from high-permafrost catchments to streams. Lower spatial extent of permafrost resulted in staticmore »delivery of nitrate and limited transport of fDOM to streams. Shifts in concentration–discharge relationships and seasonal trends in stream chemistry toward less temporally dynamic patterns might therefore indicate reorganized catchment hydrology and biogeochemistry due to permafrost thaw.« less
5. Sensitivity of headwater streamflow to thawing permafrost and vegetation change in a warming Arctic

   https://doi.org/10.1088/1748-9326/ac5f2d  

   Koch, J. C. ; Sjöberg, Y. ; O’Donnell, J. A. ; Carey, M. P. ; Sullivan, P. F. ; Terskaia, A. ( April 2022 , Environmental Research Letters)

   Climate change has the potential to impact headwater streams in the Arctic by thawing permafrost and subsequently altering hydrologic regimes and vegetation distribution, physiognomy and productivity. Permafrost thaw and increased subsurface flow have been inferred from the chemistry of large rivers, but there is limited empirical evidence of the impacts to headwater streams. Here we demonstrate how changing vegetation cover and soil thaw may alter headwater catchment hydrology using water budgets, stream discharge trends, and chemistry across a gradient of ground temperature in northwestern Alaska. Colder, tundra-dominated catchments shed precipitation through stream discharge, whereas in warmer catchments with greater forest extent, evapotranspiration (ET) and infiltration are substantial fluxes. Forest soils thaw earlier, remain thawed longer, and display seasonal water content declines, consistent with greater ET and infiltration. Streambed infiltration and water chemistry indicate that even minor warming can lead to increased infiltration and subsurface flow. Additional warming, permafrost loss, and vegetation shifts in the Arctic will deliver water back to the atmosphere and to subsurface aquifers in many regions, with the potential to substantially reduce discharge in headwater streams, if not compensated by increasing precipitation. Decreasing discharge in headwater streams will have important implications for aquatic and riparian ecosystems.