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Exploring nitrogen dynamics in stream networks is critical for understanding how these systems attenuate nutrient pollution while maintaining ecological productivity. We investigated Oak Creek, a dryland watershed in central Arizona, USA, to elucidate the relationship between terrestrial nitrate (NO3−) loading and stream NO3− uptake, highlighting the influence of land cover and hydrologic connectivity. We conducted four seasonal synoptic sampling campaigns along the 167‐km network combined with stream NO3− uptake experiments (in 370–710‐m reaches) and integrated the data in a mass‐balance model to scale in‐stream uptake and estimate NO3− loading from landscape to the stream network. Stream NO3− concentrations were low throughout the watershed (<5–236 μg N/L) and stream NO3− vertical uptake velocity was high (5.5–18.0 mm/min). During the summer dry (June), summer wet (September), and winter dry (November) seasons, the lower mainstem exhibited higher lateral NO3− loading (10–51 kg N km−2 d−1) than the headwaters and tributaries (<0.001–0.086 kg N km−2 d−1), likely owing to differences in irrigation infrastructure and near‐stream land cover. In contrast, during the winter wet season (February) lateral NO3− loads were higher in the intermittent headwaters and tributaries (0.008–0.479 kg N km−2 d−1), which had flowing surface water only in this season. Despite high lateral NO3− loading in some locations, in‐stream uptake removed >81% of NO3− before reaching the watershed outlet. Our findings highlight that high rates of in‐stream uptake maintain low nitrogen export at the network scale, even with high fluxes from the landscape and seasonal variation in hydrologic connectivity. Plain Language Summary: Exploring nitrogen dynamics in desert streams is critical for understanding how these systems can reduce pollution while maintaining healthy ecosystems. We examined how seasons and human activities, like farming and development, affect nitrogen pollution in a desert stream in Arizona, USA. We found that in seasons with little rain, nitrogen delivery to the stream was high in areas where irrigation is common. However, in‐stream nitrogen remained low because of the high capacity of the stream algal and microbial community to remove nitrogen from the water. This reveals that plants and microbes play a vital role in regulating nitrogen in deserts. This study has broad implications beyond this particular desert stream, emphasizing the importance of understanding complex interactions between human activities, water, and stream microbes. By studying these interactions, we can better manage and preserve desert streams with changing climate and human pressures. Key Points: Nitrate loading to a dryland stream network varies spatially and seasonally due to hydrologic connection to landscape nutrient sourcesHigh in‐stream nitrate uptake maintained low stream nitrate concentration and watershed export even when landscape inputs were highIn drylands, hydrologic disconnections in space and time between nitrate sources and the stream requires a flexible modeling approach [ABSTRACT FROM AUTHOR]

Leaf litter dominates particulate organic carbon inputs to forest streams. Using data‐informed simulations, we explored how litter type (slow‐ vs. fast‐decomposing species), pulsed autumn litter inputs, groundwater‐mediated temperature regimes, and climate warming affect litter breakdown in a 3rd‐order stream network. We found that the time‐dependent interactions of these variables govern network‐scale litter breakdown phenology, with greater thermal sensitivity of slow‐decomposing litter for both current and future scenarios. Groundwater thermal inputs modified litter breakdown phenology by reducing spring and summer and elevating winter litter breakdown fluxes. Under future warming scenarios, the source depth of contributing groundwater influenced summer detrital resources; shallow groundwater‐fed streams had reduced summer resources compared to deep groundwater‐fed streams. Our results demonstrate that predicting in‐stream carbon cycling requires explicit consideration of the phenology of resource inputs and the seasonal timing of environmental factors, notably stream thermal regimes. [ABSTRACT FROM AUTHOR]

Increased temperatures are altering rates of organic matter (OM) breakdown in stream ecosystems with implications for carbon (C) cycling in the face of global change. The metabolic theory of ecology (MTE) provides a framework for predicting temperature effects on OM breakdown, but differences in the temperature dependence of breakdown driven by different organismal groups (i.e., microorganisms vs. invertebrate detritivores) and litter species remain unresolved. Over two years, we conducted 12 60‐day leaf litterbag incubations in 20 headwater streams in the southern Appalachian Mountains (USA). We compared temperature dependence (as activation energy, Ea) between microbial and detritivore‐mediated breakdown, and between a highly recalcitrant (Rhododendron maximum) and a relatively labile (Acer rubrum) leaf species. Detritivore‐mediated breakdown had a higher Ea than microbial breakdown for both leaf species (Rhododendron: 1.48 > 0.56 eV; Acer: 0.97 > 0.29 eV), and Rhododendron breakdown had a higher Ea than Acer breakdown for both organismal groups. Similarly, the Ea of total (coarse‐mesh) Rhododendron breakdown was higher than the Ea of total Acer breakdown (0.89 > 0.52 eV). These effects for total breakdown were large, implying that the number of days to 95% mass loss would decline by 40% for Rhododendron and 26% for Acer between 12°C (our mean temperature value) and 16°C (+4°C, reflecting projected increases in global surface temperature due to climate change). Despite patterns in Ea, overall breakdown rates were higher for microbes than detritivores, and for Acer than Rhododendron over most of our temperature gradient. Additionally, the Ea for a subset of the microbial breakdown data declined from 0.40 to 0.22 eV when fungal biomass was included as a model predictor, highlighting the key role of fungi in determining the temperature dependence of litter breakdown. Our results imply that, as streams warm, routing of leaf litter C to detritivore‐mediated fates will increase faster than predicted by previous studies and MTE, especially for labile litter. As temperatures rise, earlier depletion of autumn‐shed, labile leaf litter combined with rapid breakdown rates of recalcitrant litter could exacerbate seasonal resource limitation and alter carbon storage and transport dynamics in temperate headwater stream networks. [ABSTRACT FROM AUTHOR]

Restorations of former cranberry farms ("bogs") aim to re-establish native wetland vegetation, promote cold water habitat, and attenuate nitrogen (N) delivery to coastal waters. It is unclear, though, how elements of restoration design such as microtopography, groundwater interception, and plant communities affect N removal via denitrification. In a recently restored riparian cranberry bog with created microtopography, we compared denitrification potential, nitrous oxide (N2O) yield of denitrification (ratio of N2O:N2O + N2 gases), in situ N2O fluxes, soil chemistry, and plant communities at the highest and lowest elevations within 20 plots and at four side-channel groundwater seeps. Denitrification potential was > 2 × greater at low elevations, which had plant communities distinct from high elevations, and was positively correlated with plant species richness (Spearman's rho = 0.43). Despite detecting high N2O yield (0.86 ± 0.16) from low elevation soils, we observed small N2O emissions in situ, suggesting minimal incomplete denitrification even in saturated depressions. Groundwater seeps had an order of magnitude higher denitrification potentials and 100–300 × greater soil NO3− concentrations than the typically saturated low elevation soils. Groundwater seeps also had high N2O yield (1.05 ± 0.15) and higher, but spatially variable, in situ N2O emissions. Our results indicate that N removal is concentrated where soils interact with NO3–rich groundwater, but other factors such as low soil carbon (C) also limit denitrification. Designing restoration features to increase groundwater residence time, particularly in low lying, species rich areas, may promote more N attenuation in restored cranberry bogs and other herbaceous riparian wetlands. [ABSTRACT FROM AUTHOR]

Preferential groundwater discharge features along stream corridors are ecologically important at local and stream network scales, yet we lack quantification of the multiscale controls on the spatial patterning of groundwater discharge. Here we identify physical attributes that best explain variation in the presence and lateral extent of preferential groundwater discharges along two 5th order streams, the Housatonic and Farmington Rivers, and 32 1st to 4th order reaches across the Farmington River network. We mapped locations of preferential groundwater discharge exposed along streambanks using handheld thermal infrared cameras paired with high‐resolution topographic and land use land cover datasets, surficial soil characteristic maps, and depth‐to‐bedrock geophysical measurements. The unconfined Housatonic River, MA, USA (12 km) had fewer discharge locations and less lateral extent (41 discharge locations with 38 m of active discharge/km of river) compared to the partially confined Farmington River, CT, USA (26 km; 169 discharge locations with 129 m of active discharge/km of river). Using a moving window analysis, we found along both rivers that discharge was more likely to occur where bank slopes were steeper, floodplain extent was narrower, and degree of confinement was higher. Along the Farmington River, groundwater discharge was more likely to occur where saturated hydraulic conductivity was higher and depth‐to‐bedrock was shallower. Among the 32 stream reaches surveyed (33.2 km of total stream length) within the Farmington River watershed, preferential discharge was observed in all but two stream reaches, varied from 0 to 25% of lateral extent along stream banks (mean = 6%), and was more likely to occur where stream reach slopes were steep, saturated hydraulic conductivity was high, and watershed urbanization was low. Our results show that, though both surface (e.g., topographic, land use land cover) and subsurface (e.g., soil characteristics, bedrock depth) factors control the prevalence of streambank preferential groundwater discharge, the dominant controls vary across valley settings and stream sizes. [ABSTRACT FROM AUTHOR]

Tidal wetlands are important blue carbon reservoirs, but it is unclear how sea-level rise (SLR) may affect carbon cycling and soil microbial communities either by increased inundation frequency or via shifting plant species dominance. We used an in-situ marsh organ experiment to test how SLR-scenarios (0, + 7.5, + 15 cm) and vegetation treatments (Spartina alterniflora, Spartina patens, Phragmites australis, unvegetated controls) altered CO2 fluxes (net ecosystem exchange, ecosystem respiration), soil carbon mineralization rates, potential denitrification rates, and microbial community composition. Increasing inundation frequency with SLR treatments decreased the carbon sink strength and promoted carbon emissions with + 15-cm SLR. However, SLR treatments did not alter soil chemistry, microbial process rates, or bacterial community structure. In contrast, our vegetation treatments affected all carbon flux measurements; S. alterniflora and S. patens had greater CO2 uptake and ecosystem respiration compared to P. australis. Soils associated with Spartina spp. had higher carbon mineralization rates than P. australis or unvegetated controls. Soil bacterial assemblages differed among vegetation treatments but shifted more dramatically over the three-month experiment. As marshes flood more frequently with projected SLR, marsh vegetation composition is predicted to shift towards more flood-tolerant S. alterniflora, which may lead to increased CO2 uptake, though tidal marsh carbon sink strength will likely be offset by increased abundance of unvegetated tidal flats and open water. Our findings suggest that plant species play a central role in ecosystem carbon dynamics in vegetated tidal marshes undergoing rapid SLR. [ABSTRACT FROM AUTHOR]

Deicing salts are a major contributor to freshwater salinization in the Northeastern United States, disrupting ecosystem function, aquatic life, and infrastructure integrity. It is often presumed that surface runoff is the dominant mode of chloride transport from impervious surfaces to surface waters despite empirical evidence for soil and groundwater transport. Our project aimed to (1) estimate the hydrologic pathways for chloride transport in a small, urbanized catchment in the Northeastern United States; and (2) provide a critical evaluation of simulation routines commonly used to guide the management of deicing salts. We investigated possible variations in urban chloride pathways by monitoring stormwater discharge and chloride across a winter season with deicing salt spreading and snow accumulation and melt. We analyzed these time series with US EPA SWMM, which presumes chloride transport occurs exclusively via surface runoff into stormwater systems. We develop two parallel models: one using the base empirical temperature-index snow routines, and a second substituting a process-based snowmelt model. Each model was calibrated via the generalized likelihood uncertainty estimation algorithm to both stormwater volumes and chloride mass. Both models adequately simulated water volumes but failed to reproduce observed chloride dynamics, suggesting substantial deicing salt movement through soils and groundwater as a major urban hydrologic pathway. The results of this study will inform the development of urban hydrologic fate and transport models and chloride best management practices. [ABSTRACT FROM AUTHOR]

Nutrient uptake, storage, and release are critical ecosystem functions that affect carbon processing and food web dynamics. Yet, mechanisms controlling when ecosystems are net sinks or sources of nutrients are uncertain. Specifically, how nutrient supply ratios alter rates and ratios of net nutrient uptake and release is unclear. To assess whether net nitrogen (N) and phosphorus (P) uptake and release are linked to supply N:P, we experimentally enriched five forest streams at different N:P (target molar N:P range 2:1–128:1) for 2 years. We quantified net nutrient exchange (NNE) as the difference between the expected N and P fluxes assuming conservative transport (background concentrations plus experimental inputs) and the observed nutrient fluxes at the downstream end of each experimental stream reach. Supply N:P did not affect the magnitude of NNE for either N or P, but the likelihood of net N and P uptake was greatest at intermediate N:P supply (N:P = 99:1 and 55:1, respectively). Streams appeared to be highly flexible in their N:P uptake and release; the slopes between NNEN and NNEP within each stream increased with supply N:P. Furthermore, slopes comparing supply N:P to uptake and release N:P were near one (0.98 ± 0.06 SE and 0.82 ± 0.13 SE, respectively), indicating a high degree of flexibility. Overall, we found greater stoichiometric flexibility than has been shown in short-term nutrient-addition experiments. We suggest that this flexibility results from changes in nutrient recycling within biofilms or changes in community structure, which may take longer to manifest than the duration of shorter-term experiments. [ABSTRACT FROM AUTHOR]

Riverbank groundwater discharge faces are spatially extensive areas of preferential seepage that are exposed to air at low river flow. Some conceptual hydrologic models indicate discharge faces represent the spatial convergence of highly variable age and length groundwater flowpaths, while others indicate greater consistency in source groundwater characteristics. Our detailed field investigation of preferential discharge points nested across mainstem riverbank discharge faces was accomplished by: (1) leveraging new temperature‐based recursive estimation (extended Kalman Filter) modelling methodology to evaluate seasonal, diurnal, and event‐driven groundwater flux patterns, (2) developing a multi‐parameter toolkit based on readily measured attributes to classify the general source groundwater flowpath depth and flowpath length scale, and, (3) assessing whether preferential flow points across discharge faces tend to represent common or convergent groundwater sources. Five major groundwater discharge faces were mapped along the Farmington River, CT, United States using thermal infrared imagery. We then installed vertical temperature profilers directly into 39 preferential discharge points for 4.5 months to track vertical discharge flux patterns. Monthly water chemistry was also collected at the discharge points along with one spatial synoptic of stable isotopes of water and dissolved radon gas. We found pervasive evidence of shallow groundwater sources at the upstream discharge faces along a wide valley section with deep bedrock, as primarily evidenced by pronounced diurnal discharge flux patterns. Discharge flux seasonal trends and bank storage transitions during large river flow events provided further indication of shallow, local sources. In contrast, downstream discharge faces associated with near surface cross cutting bedrock exhibited deep and regional source flowpath characteristics such as more stable discharge patterns and temperatures. However, many neighbouring points across discharge faces had similar discharge flux patterns that differed in chloride and radon concentrations, indicating the additional effects of localized flowpath heterogeneity overprinting on larger scale flowpath characteristics. [ABSTRACT FROM AUTHOR]

Hypoxia in coastal waters and lakes is widely recognized as a detrimental environmental issue, yet we lack a comparable understanding of hypoxia in rivers. We investigated controls on hypoxia using 118 million paired observations of dissolved oxygen (DO) concentration and water temperature in over 125,000 locations in rivers from 93 countries. We found hypoxia (DO < 2 mg L-1 ) in 12.6% of all river sites across 53 countries, but no consistent trend in prevalence since 1950. High-frequency data reveal a 3-h median duration of hypoxic events which are most likely to initiate at night. River attributes were better predictors of riverine hypoxia occurrence than watershed land cover, topography, and climate characteristics. Hypoxia was more likely to occur in warmer, smaller, and lower-gradient rivers, particularly those draining urban or wetland land cover. Our findings suggest that riverine hypoxia and the resulting impacts on ecosystems may be more pervasive than previously assumed. [ABSTRACT FROM AUTHOR]

Groundwater is critical for maintaining stream baseflow and thermal stability; however, the influence of groundwater on streamflow has been difficult to evaluate at broad spatial scales. Techniques such as baseflow separation necessitate streamflow records and do not directly indicate whether groundwater inflow may be sourced from more dynamic shallow flowpaths. We present a web tool application PASTA (Paired Air and Stream Temperature Analysis; https://cuahsi.shinyapps.io/pasta/) that capitalizes on increased public stream temperature data availability and large‐scale, gridded climate observations to provide new and efficient insights regarding relative groundwater influence on streams. PASTA analyzes paired air and stream water temperature signals to evaluate spatiotemporal patterns in stream thermal sensitivity and relative groundwater influence, including inference regarding the dominant source groundwater depth (shallow or deep (i.e., approximately >6 m depth)). The tool is linked to publicly available stream temperature datasets and accepts user‐uploaded datasets. As local air temperature is not often monitored, PASTA pulls daily air temperature data from the comprehensive Daymet products when directly measured data are unavailable, allowing the repurposing of existing stream temperature data. After data are selected or uploaded, the tool (a) fits sinusoidal curves of daily stream and air temperatures by year (water or calendar) to indicate groundwater influence characteristics and (b) performs linear regressions for stream versus air temperatures to indicate stream thermal sensitivity. Results are exported in ASCII file format, creating an efficient and approachable analysis tool for the adoption of newly developed heat tracing analysis from stream reach to landscape scales. Plain Language Summary: Comparing stream temperature to air temperature records can identify streamflow sources, or examine stream vulnerability to land use (e.g., impervious cover), climate change, or river use practices (e.g., dams). We have created a website that allows users to conduct two methods of stream and air temperature comparison using their stream temperature data or use publicly available stream temperature resources in a consistent and accessible format. These analyses generate metrics that indicate similarities and differences in air and stream temperature records over time that can provide insight into hydrologic connectivity for a single site or can be compared across time or space to inform changes in stream process within an area or period of interest. Key Points: PASTA is a web application for analyzing paired stream water and air temperature from public and user‐owned datasets, even without air dataThe output metrics can evaluate stream thermal sensitivity, relative groundwater contributions, and effective source depthApplication design allows method consistency and accessibility for various flowing water communties [ABSTRACT FROM AUTHOR]

Salt marsh vegetation zones shift in response to large‐scale environmental changes such as sea‐level rise (SLR) and restoration activities, but it is unclear if they are good indicators of soil nitrogen removal. Our goal was to characterize the relationship between denitrification potential and salt marsh vegetation zones in tidally restored and tidally unrestricted coastal marshes, and to use vegetation zones to extrapolate how SLR may influence high marsh denitrification at the landscape scale. We conducted denitrification enzyme activity assays on sediment collected from three vegetation zones expected to shift in distribution due to SLR and tidal flow restoration across 20 salt marshes in Connecticut, USA (n = 60 sampling plots) during the summer of 2017. We found lower denitrification potential in short‐form Spartina alterniflora zones (mean, 95% CI: 4, 3–6 mg N h−1 m−2) than in S. patens (25, 15–36 mg N h−1 m−2) and Phragmites australis (56, 16–96 mg N h−1 m−2) zones. Vegetation zone was the single best predictor and explained 52% of the variation in denitrification potential; incorporating restoration status and soil characteristics (soil salinity, moisture, and ammonium) did not improve model fit. Because denitrification potential did not differ between tidally restored and unrestricted marshes, we suggest landscape‐scale changes in denitrification after tidal restoration are likely to be associated with shifts in vegetation, rather than differences driven by restoration status. Sea‐level‐rise‐induced hydrologic changes are widely observed to shift high marsh dominated by S. patens to short‐form S. alterniflora. To explore the implications of this shift in dominant high marsh vegetation, we paired our measured mean denitrification potential rates with projections of high marsh loss from SLR. We found that, under low and medium SLR scenarios, predicted losses of denitrification potential due to replacement of S. patens by short‐form S. alterniflora were substantially larger than losses due to reduced high marsh land area alone. Our results suggest that changes in vegetation zones can serve as landscape‐scale predictors of the response of denitrification rates to rapid changes occurring in salt marshes. [ABSTRACT FROM AUTHOR]

River networks regulate carbon and nutrient exchange between continents, atmosphere, and oceans. However, contributions of riverine processing are poorly constrained at continental scales. Scaling relationships of cumulative biogeochemical function with watershed size (allometric scaling) provide an approach for quantifying the contributions of fluvial networks in the Earth system. Here we show that allometric scaling of cumulative riverine function with watershed area ranges from linear to superlinear, with scaling exponents constrained by network shape, hydrological conditions, and biogeochemical process rates. Allometric scaling is superlinear for processes that are largely independent of substrate concentration (e.g., gross primary production) due to superlinear scaling of river network surface area with watershed area. Allometric scaling for typically substrate-limited processes (e.g., denitrification) is linear in river networks with high biogeochemical activity or low river discharge but becomes increasingly superlinear under lower biogeochemical activity or high discharge, conditions that are widely prevalent in river networks. The frequent occurrence of superlinear scaling indicates that biogeochemical activity in large rivers contributes disproportionately to the function of river networks in the Earth system. River networks play an important role in biogeochemical processes of the earth system. Here the authors show that cumulative river network function increases faster than watershed size for many biogeochemical processes, particularly at higher river flow, indicating large rivers contribute disproportionately to network function in the Earth System. [ABSTRACT FROM AUTHOR]

Mean annual temperature and mean annual precipitation drive much of the variation in productivity across Earth's terrestrial ecosystems but do not explain variation in gross primary productivity (GPP) or ecosystem respiration (ER) in flowing waters. We document substantial variation in the magnitude and seasonality of GPP and ER across 222 US rivers. In contrast to their terrestrial counterparts, most river ecosystems respire far more carbon than they fix and have less pronounced and consistent seasonality in their metabolic rates. We find that variation in annual solar energy inputs and stability of flows are the primary drivers of GPP and ER across rivers. A classification schema based on these drivers advances river science and informs management. [ABSTRACT FROM AUTHOR]

Dissolved organic carbon (DOC) and nitrogen (DON) are important energy and nutrient sources for aquatic ecosystems. In many northern temperate, freshwater systems DOC has increased in the past 50 years. Less is known about how changes in DOC may vary across latitudes, and whether changes in DON track those of DOC. Here, we present long‐term DOC and DON data from 74 streams distributed across seven sites in biomes ranging from the tropics to northern boreal forests with varying histories of atmospheric acid deposition. For each stream, we examined the temporal trends of DOC and DON concentrations and DOC:DON molar ratios. While some sites displayed consistent positive or negative trends in stream DOC and DON concentrations, changes in direction or magnitude were inconsistent at regional or local scales. DON trends did not always track those of DOC, though DOC:DON ratios increased over time for ~30% of streams. Our results indicate that the dissolved organic matter (DOM) pool is experiencing fundamental changes due to the recovery from atmospheric acid deposition. Changes in DOC:DON stoichiometry point to a shifting energy‐nutrient balance in many aquatic ecosystems. Sustained changes in the character of DOM can have major implications for stream metabolism, biogeochemical processes, food webs, and drinking water quality (including disinfection by‐products). Understanding regional and global variation in DOC and DON concentrations is important for developing realistic models and watershed management protocols to effectively target mitigation efforts aimed at bringing DOM flux and nutrient enrichment under control. [ABSTRACT FROM AUTHOR]

Forested wetlands of the temperate north are increasingly exposed to deicing salts, but it is unclear how this may alter wetland biogeochemistry and plant community composition. To investigate potential effects of deicing salts on exurban forested wetlands in southern New England, we employed a multi‐site field study to describe spatiotemporal patterns of soil physiochemical, water quality, and vegetation characteristics with distance from road deicing salt source. We surveyed nine road‐adjacent, red maple‐dominated wetlands to quantify a suite of soil parameters (Na+, K+, Mg2+, Ca2+, pH, electrical conductivity (EC), heavy metals, N, P, and soil moisture), as well as surface and groundwater salinity, and vegetation communities. With increasing distance from roads along 165‐m transects penetrating into each wetland, soil salinity (EC, Na+) decreased, while soil base cation concentrations (Mg2+, Ca2+) increased, likely due to cation exchange (Na+ displacing other base cations). We also measured foliar chemistry and observed elevated Na+ and reduced Mg2+ of dominant species leaf tissue near roads, suggesting plant nutrient uptake responds to road salt‐related changes in soil physicochemical variables. Despite this, we did not detect differences in plant community composition (ground, shrub layers) along road salt‐induced soil chemistry gradients in the field, likely because surface and groundwater salinities were relatively low (maximum: 0.64 ppt). To determine at which field salinities we could potentially expect changes in wetland plant communities, we conducted a full‐factorial, manipulative seed bank experiment to examine how NaCl concentrations (0, 0.5, 1, 2, 4, and 8 parts per thousand (ppt)), frequency of salt exposure (pulse, constant), and water level (surface, 2 cm below surface) affected soil seed bank responses. Seedling richness was reduced at salinities exceeding 1 ppt, and seedling density was reduced above 2 ppt, but pulsing tended to alleviate salt‐induced reductions in seed bank responses. As salinization of freshwater ecosystems continues to increase, our results suggest that field salinity levels of exurban New England forested wetlands are nearing yet still typically below the threshold for which we expect to see strong plant community responses. [ABSTRACT FROM AUTHOR]

A comprehensive cross‐biome assessment of major nitrogen (N) species that includes dissolved organic N (DON) is central to understanding interactions between inorganic nutrients and organic matter in running waters. Here, we synthesize stream water N chemistry across biomes and find that the composition of the dissolved N pool shifts from highly heterogeneous to primarily comprised of inorganic N, in tandem with dissolved organic matter (DOM) becoming more N‐rich, in response to nutrient enrichment from human disturbances. We identify two critical thresholds of total dissolved N (TDN) concentrations where the proportions of organic and inorganic N shift. With low TDN concentrations (0–1.3 mg/L N), the dominant form of N is highly variable, and DON ranges from 0% to 100% of TDN. At TDN concentrations above 2.8 mg/L, inorganic N dominates the N pool and DON rarely exceeds 25% of TDN. This transition to inorganic N dominance coincides with a shift in the stoichiometry of the DOM pool, where DOM becomes progressively enriched in N and DON concentrations are less tightly associated with concentrations of dissolved organic carbon (DOC). This shift in DOM stoichiometry (defined as DOC:DON ratios) suggests that fundamental changes in the biogeochemical cycles of C and N in freshwater ecosystems are occurring across the globe as human activity alters inorganic N and DOM sources and availability. Alterations to DOM stoichiometry are likely to have important implications for both the fate of DOM and its role as a source of N as it is transported downstream to the coastal ocean. Plain Language Summary: Ammonium and nitrate in freshwaters have received considerable attention due to their clear ecological and health effects. A comprehensive assessment of N in freshwaters that includes DON is lacking. Including DON in studies of surface water chemistry is important because it can cause eutrophication and certain forms can be rapidly removed by microbial communities. Here, we document how elevated levels of TDN impact the concentrations and relative proportions of all three forms of dissolved N and the stoichiometry of DOM. Our results suggest that human activities fundamentally alter the composition of the dissolved nitrogen pool and the stoichiometry of DOM. Results also highlight feedbacks between the C and N cycles in freshwater ecosystems that are poorly studied. Key Points: We synthesize a global data set of stream chemistry to examine how the composition of dissolved N and DOC:DON ratios respond to N enrichmentUnder low total dissolved N concentrations, the dominate form of N is highly variable but switches to primarily inorganic forms at high TDNWith N enrichment, DOM becomes more N‐rich (lower DOC:DON ratios) while concentrations of DON are less associated with concentrations of DOC [ABSTRACT FROM AUTHOR]

Thermal refuges are thermally distinct riverscape features used by aquatic organisms during unfavourable thermal events, facilitating resilience in marginal environments. However, the thermal refuge concept is nebulous, and the often interchangeable use of the term 'thermal refugia' creates additional ambiguity. We argue that lexical differences resulting from divergent scholarly trainings hinder holistic understanding of thermal refuges; thus, existing studies would benefit from a structured framework for thermal refuge conceptualization. Herein, we articulate an ecohydrological typology for defining and characterizing thermal refuges in streams and rivers by identifying key hydrological and thermal characteristics and variations in ecological function described in the literature. We use concepts that are easily definable, measurable and transferable across disciplines, riverscapes and species to discriminate among thermal refuge types. Future work can use our typology as a basis for more informed interdisciplinary discussion and interpretation of thermal refuges' role in riverscapes through more hypothesis‐driven research and conservation‐focused management. [ABSTRACT FROM AUTHOR]

Groundwater discharge generates streamflow and influences stream thermal regimes. However, the water quality and thermal buffering capacity of groundwater depends on the aquifer source-depth. Here, we pair multi-year air and stream temperature signals to categorize 1729 sites across the continental United States as having major dam influence, shallow or deep groundwater signatures, or lack of pronounced groundwater (atmospheric) signatures. Approximately 40% of non-dam stream sites have substantial groundwater contributions as indicated by characteristic paired air and stream temperature signal metrics. Streams with shallow groundwater signatures account for half of all groundwater signature sites and show reduced baseflow and a higher proportion of warming trends compared to sites with deep groundwater signatures. These findings align with theory that shallow groundwater is more vulnerable to temperature increase and depletion. Streams with atmospheric signatures tend to drain watersheds with low slope and greater human disturbance, indicating reduced stream-groundwater connectivity in populated valley settings. Groundwater discharge generates streamflow and influences stream thermal regimes. Classifying more than 1700 streams across the US by using an empirically-based approach the study shows that the vulnerability of streams to stressors depends on the aquifer source-depth of groundwater discharge [ABSTRACT FROM AUTHOR]

Denitrification removes reactive nitrogen (N) from ecosystems by transforming nitrate (NO3−) to dinitrogen (N2) gas. Incomplete denitrification produces nitrous oxide (N2O), a potent greenhouse gas. In salt marshes, denitrification, N2O production, and N2O yield (the fraction of denitrification that produces N2O) have implications for N load reduction and greenhouse gas emissions. We collected soil cores from three salt marsh zones (low marsh, high marsh, and invasive Phragmites australis) on five sampling dates that spanned the growing season (May through October) to quantify seasonal patterns of potential denitrification, N2O production, and N2O yield. Potential denitrification peaked at the beginning and end of the growing season, with rates several times higher in May and October than in June, but we found no significant differences among salt marsh zones. In contrast, seasonal patterns of potential N2O production depended on marsh zone; N2O production was aseasonal in high marsh and P. australis zones, but in the low marsh, N2O production was lower in July compared to all other months sampled. In this salt marsh, seasonal variation of potential denitrification and N2O production was greater than spatial variation over the marsh zones, highlighting the importance of understanding temporal patterns of salt marsh N cycling. [ABSTRACT FROM AUTHOR]

Exurban development is the fastest growing form of land use in the United States and already predominates southern New England. These housing developments alter stream ecosystems in a predictable manner described as urban stream syndrome. Yet, many stream salamanders are considered ubiquitous in the region. To determine what stream and watershed features enhance long-term population persistence in an exurban landscape, we compared the occupancy and abundance of two stream salamanders, Eurycea bislineata (northern two-lined salamander) and Desmognathus fuscus (northern dusky salamander), in watersheds that differed in housing density and time since construction. We estimated E. bislineata occupancy at 100% and found strong support for an interaction between the quadratic of average housing development age and housing density in watersheds influencing abundance. Abundance was sensitive to new and high-density housing developments and increased in watersheds that were more than 20 years post-construction when housing density was low. In comparison, D. fuscus occupancy was estimated at 18% and best explained by fine-scale stream features including soil temperature, water conductivity, dissolved oxygen, discharge, and sediment distribution. Notably, estimates of D. fuscus relative abundance within watersheds, including watersheds with low housing densities, were much lower than comparable estimates in the literature. We suggest an adaptive management approach to monitor and manage D. fuscus populations in the region. Future wildlife management decisions in exurban landscapes should consider interactions between housing age and density as well as development legacy effects. [ABSTRACT FROM AUTHOR]

High‐resolution data are improving our ability to resolve temporal patterns and controls on river productivity, but we still know little about the emergent patterns of primary production at river‐network scales. Here, we estimate daily and annual river‐network gross primary production (GPP) by applying characteristic temporal patterns of GPP (i.e., regimes) representing distinct river functional types to simulated river networks. A defined envelope of possible productivity regimes emerges at the network‐scale, but the amount and timing of network GPP can vary widely within this range depending on watershed size, productivity in larger rivers, and reach‐scale variation in light within headwater streams. Larger rivers become more influential on network‐scale GPP as watershed size increases, but small streams with relatively low productivity disproportionately influence network GPP due to their large collective surface area. Our initial predictions of network‐scale productivity provide mechanistic understanding of the factors that shape aquatic ecosystem function at broad scales. [ABSTRACT FROM AUTHOR]

The recovery of wetland function after tidal flow restoration may be influenced by water pollution and sea level rise. Our objective was to examine the effects of tidal flow restoration on denitrification potential and carbon (C) mineralization across an urban coastal landscape. Soil cores were collected from 32 tidal wetlands in Connecticut, U.S.A., spanning a wide range of salinity (0.3–29 ppt) and watershed development (<1–79%). In brackish wetlands, denitrification potential increased with time since restoration, while C mineralization showed no significant relationship. Soil chemistry was also a strong predictor of process rates; best fit multiple linear regression models for denitrification included both soil chemistry variables and time since restoration. Although principal components analysis revealed soil chemistry overlapped by wetland type (freshwater, saline, or brackish), process rates in freshwater versus brackish wetlands had different relationships with soil chemistry. In freshwater wetlands, denitrification potential and C mineralization increased with soil metal content. In brackish wetlands, denitrification potential decreased with increasing salinity and C mineralization increased with increasing organic matter, soil moisture, and ammonium. Our results highlight the potential for biogeochemical processes to recover after wetland restoration, along with complex interactions between these processes and chemicals in developed coastal landscapes. [ABSTRACT FROM AUTHOR]

Changes in sea-level rise and precipitation are altering patterns of coastal wetland hydrology and salinization. We conducted paired laboratory (20 weeks) and field (15 weeks) marine salt addition experiments to disentangle the effects of hydrology (permanent versus intermittent flooding) and elevated marine salts (sulfate versus other salt ions) on greenhouse gas (GHG) emissions from freshwater forested wetland soils. Marine salt additions strongly affected GHG emissions in both experiments, but the magnitude, and even the direction, of GHG responses depended on the hydrologic context in which marine salt exposure occurred. Under permanent flooding, carbon dioxide (CO2) fluxes were unaffected by marine salts, whereas methane (CH4) fluxes were significantly suppressed by the addition of sulfate (as K2SO4) both with and without marine salts. In contrast, in intermittently flooded field and laboratory soils elevated salinity reduced carbon mineralization and CO2 fluxes, but enhanced CH4 fluxes relative to both controls and treatments with elevated sulfate. Thus, elevated salinity or alkalinity (and not sulfate) controlled both gaseous carbon fluxes under intermittent flooding. Nitrous oxide (N2O) fluxes had contrasting responses in the field and laboratory. In the laboratory, N2O fluxes were not significantly related to chemical treatment but increased with porewater ammonium concentrations, which increased in salinity treatments via cation exchange. In intermittently flooded field conditions, elevated salinity strongly suppressed N2O fluxes because ammonium did not accumulate in porewater; it was likely lost through advection, dispersion, or plant uptake. Understanding dynamic hydrologic and vegetation patterns across wetland landscapes will be critical for predicting both the magnitude and direction of wetland GHG responses to increasing marine salt across broad spatial scales. [ABSTRACT FROM AUTHOR]

Coastal forested wetlands provide important ecosystem services such as carbon sequestration, nutrient retention, and flood protection, but they are also important sources of greenhouse gas emissions. Human appropriation of surface water and extensive ditching and draining of coastal plain landscapes are interacting with rising sea levels to increase the frequency and magnitude of saltwater incursion into formerly freshwater coastal wetlands. Both hydrologic change and saltwater incursion are expected to alter carbon and nutrient cycling in coastal forested wetlands. We performed a full factorial experiment in which we exposed intact soil cores from a coastal forested wetland to experimental marine salt treatments and two hydrologic treatments. We measured the resulting treatment effects on the emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) over 112 days. Salinity effects were compared across four treatments to isolate the effects of increases in ionic strength from the impact of adding a terminal electron acceptor (SO42−). We compared control treatments (DI addition), to artificial saltwater (ASW, target salinity of 5 parts per thousand) and to two treatments that added sulfate alone (SO42−, at the concentration found in 5 ppt saltwater) and saltwater with the sulfate removed (ASW-SO42−, with the 5 ppt target salinity maintained by adding additional NaCl). We found that all salt treatments suppressed CO2 production, in both drought and flooded treatments. Contrary to our expectations, CH4 fluxes from our flooded cores increased between 300 and 1200% relative to controls in the ASW and ASW-SO42− treatments respectively. In the drought treatments, we saw virtually no CH4 release from any core, while artificial seawater with sulfate increased N2O fluxes by 160% above DI control. In contrast, salt and sulfate decreased N2O fluxes by 72% in our flooded treatments. Our results indicate that salinization of forested wetlands of the coastal plain may have important climate feedbacks resulting from enhanced greenhouse gas emissions and that the magnitude and direction of these emissions are contingent upon wetland hydrology. [ABSTRACT FROM AUTHOR]

In vegetated wetland ecosystems, plants can be a dominant pathway in the atmospheric flux of methane, a potent greenhouse gas. Although the roles of herbaceous vegetation and live woody vegetation in this flux have been established, the role of dead woody vegetation is not yet known. In a restored wetland of North Carolina’s coastal plain, static flux chambers were deployed at two heights on standing dead trees to determine if these structures acted as a conduit for methane emissions. Methane fluxes to the atmosphere were measured in five of the chambers, with a mean flux of 0.4 ± 0.1 mg m−2 h−1. Methane consumption was also measured in three of the chambers, with a mean flux of −0.6 ± 0.3 mg m−2 h−1. Standing dead trees were also a source of the flux of CO2 (114.6 ± 23.8 mg m−2 h−1) to the atmosphere. Results confirm that standing dead trees represent a conduit for the atmospheric flux of carbon gases from wetlands. However, several questions remain regarding the ultimate source of these carbon gases, the controls on the magnitude and direction of this flux, the mechanisms that induce this flux, and the importance of this pathway relative to other sources at the landscape level. [ABSTRACT FROM AUTHOR]

1. Streams and rivers can be highly reactive sites for nitrogen (N) transformation and removal. Empirical and model-based research show how location in a stream network affects rates of N removal. Because the structure of stream networks can vary widely and N cycling in headwater streams may affect N cycling in downstream reaches, we hypothesised that network structure may affect whole stream network processing of N. 2. We generated three stream networks with the same catchment area but differing shapes, based on optimal channel network theory. We applied a model of nitrate (NO3- transport and denitrification, and implemented model scenarios to examine how network shape affects NO3- removal with (1) increased NO3- loading from the catchment, (2) altered spatial distributions of NO3- loading and (3) decreased drainage density (i.e. loss of headwater streams). 3. For all stream networks, the fraction of total NO3- removed decreased with increasing NO3- loading from the catchment. Stream networks in narrow catchments removed a higher fraction of NO3-, particularly at intermediate NO3- loading rates. Network shape also controlled the distribution of removal in small versus large streams, with larger streams removing a higher fraction of the total NO3- load in narrower networks. 4. The effects of network shape on NO3- removal when the spatial distribution of NO3- loading was altered varied with the magnitude of NO3- loading. At low loads, NO3- was entirely removed when added to distal parts of the stream network, and about 50% removed when added near the outlet; there was no effect of network shape. At intermediate and high loads, the fraction of total NO3- load removed by the narrow stream network was 1.59 higher than the rectangular and square networks when NO3- was added to distal parts of the networks. Network shape did not have an effect when NO3- load occurred near the outlet, regardless of the magnitude of the NO3- load. 5. The fraction of total NO3- removed by the stream network was up to 5% lower when drainage density was reduced from 1.0 to 0.74 km-1, with the least change for the narrow network. Reducing the drainage density also altered the role of small relative to large streams, with the net effect of moving the location of NO3- removal downstream. 6. Overall, effects of network shape contributed up to 20% of the variation in the fraction of NO3- removed by stream networks. Network shape was most important at intermediate to high NO3- loads and when NO3- was loaded to distal parts of the catchment. The narrow network removed more NO3- across model scenarios, with elevated removal in larger streams explaining most of the difference. We suggest the shape of the catchment may modulate the degree to which large streams contribute to whole network NO3- removal. [ABSTRACT FROM AUTHOR]

Wetlands serve as sinks for carbon and nutrients but they are also a large source of greenhouse gases. Our objective was to quantify emissions of carbon dioxide (CO) and methane (CH) from three free water surface-flow constructed wetlands in the presence and absence of emergent herbaceous vegetation ( Typha angustifolia L. and Typha latifolia L.) across a gradient of soil moisture. Measurements were collected on eight sampling dates during June and July, 2014. Similar to previous research, CO emissions were higher in vegetated plots, increasing from a median ± std. error of 242 ± 29 to 1612 ± 95 mg m h. Emissions of CH were also significantly higher in vegetated plots, but the relative magnitude of the effect of plants varied among wetlands. Emissions of CH were highest from vegetated plots in the wetland with the highest soil moisture (4.4 ± 1.0 mg m h). However, the largest effect of plants on methane emissions occurred in the wetland with intermediate soil moisture, with a 15-fold increase in CH emissions from 0.15 ± 0.90 to 2.4 ± 1.2 mg m h. Design and management that consider the interactive effects of soil moisture and plants on CH emissions may help reduce the greenhouse gas footprint of constructed wetlands. [ABSTRACT FROM AUTHOR]

The hydrologic transport of dissolved organic carbon (DOC) represents both a primary energetic loss from and a critical energetic link between ecosystems. Coastal freshwater wetlands serve as a primary source of DOC to estuaries; historically the magnitude and timing of DOC transfers has been driven by water movement. Extensive agricultural development throughout the coastal plain of the southeastern US has hydrologically connected much of the landscape via canals to facilitate drainage. The resulting large-scale loss of topographic relief and reduced mean elevation is interacting with increasingly frequent and severe droughts to facilitate the landward movement of seawater through the highly connected artificial drainage networks. The resulting changes in hydrologic regime and salinity are each expected to reduce DOC export from coastal freshwater wetlands, yet their individual and combined impacts are not well understood. Here we show that repeated saltwater incursion during late summer droughts substantially decreased DOC concentrations in surface water (from ~40 to ~18 mg/L) from a mature and a restored forested wetland in the coastal plain of North Carolina, USA. These declines in DOC concentration reduced annual export of DOC to the estuary by 70 % and dampened storm fluxes by 76 %. We used a long-term experiment with intact soil columns to measure the independent and combined effects of drought, salinity, and sulfate loading as potential drivers of the large changes in DOC concentration. We found that soil drying and salinization each reduced DOC similarly (20 % reduction by drought alone, 29 % by salinization) and their combined effect was additive (49 % reduction in salinization + drought treatments). Our results demonstrate that, well in advance of significant sea-level rise, drought and relatively low levels of saltwater incursion (<6 ppt) are already significantly altering the timing and magnitude of dissolved organic carbon flux between coastal forested wetlands and downstream estuaries. [ABSTRACT FROM AUTHOR]

Microbial decomposers and metazoan detritivores rely on and affect processing rates of detrital C in streams. Nutrient enrichment can affect the biomass of both groups, but we lack a quantitative understanding of how these groups affect breakdown rates of C under elevated nutrient conditions. We quantified the relative contribution of decomposers (fungi, bacteria) and detritivores (shredders) to breakdown rates in a reference vs a nutrient-enriched stream. We measured breakdown rates and litter-associated shredder, fungal, and bacterial biomass on Rhododendron maximum L. and Acer rubrum L. and modeled the contribution of these groups over the decay sequence for R. maximum and for a single time period for A. rubrum. We used published metabolic parameters for fungi and bacteria under high- and low-nutrient conditions and shredder feeding rates based on organisms and leaf litter from our study area. Breakdown rates of both litter types were higher (~4-5×) in the treatment than reference stream and were associated with large changes in biomass of fungi, bacteria, and shredders. Changes in microbial metabolic parameters (growth rate, growth efficiency) affected estimates of mass loss. Fungal and shredder contributions were roughly equivalent and greater than bacterial contributions under reference and treatment conditions, but nutrient enrichment accelerated the colonization sequence of fungi and shredders, and shredders had relatively greater effects on mass loss in later stages of breakdown. Our results suggest facilitation of detritivores by decomposers and large combined effects on breakdown with nutrient enrichment. Decomposer contributions could exceed those of detritivores with changes in microbial metabolic rates. Large increases in breakdown rates could be attributed to biotic response to nutrient enrichment, are sensitive to microbial metabolic rates, and have the potential to alter residence times of litter in streams. [ABSTRACT FROM AUTHOR]

Carbon and nitrogen cycles are coupled through both stoichiometric requirements for microbial biomass and dissimilatory metabolic processes in which microbes catalyse reduction-oxidation reactions. Here, we integrate stoichiometric theory and thermodynamic principles to explain the commonly observed trade-off between high nitrate and high organic carbon concentrations, and the even stronger trade-off between high nitrate and high ammonium concentrations, across a wide range of aquatic ecosystems. Our results suggest these relationships are the emergent properties of both microbial biomass stoichiometry and the availability of terminal electron acceptors. Because elements with multiple oxidation states (i.e. nitrogen, manganese, iron and sulphur) serve as both nutrients and sources of chemical energy in reduced environments, both assimilative demand and dissimilatory uses determine their concentrations across broad spatial gradients. Conceptual and quantitative models that integrate rather than independently examine thermodynamic, stoichiometric and evolutionary controls on biogeochemical cycling are essential for understanding local to global biogeochemical patterns. [ABSTRACT FROM AUTHOR]

Many coastal plain wetlands receive nutrient pollution from agricultural fields and are particularly vulnerable to saltwater incursion. Although wetlands are a major source of the greenhouse gases methane (CH) and nitrous oxide (NO), the consequences of salinization for greenhouse gas emissions from wetlands with high agricultural pollution loads is rarely considered. Here, we asked how saltwater exposure alters greenhouse gas emissions from a restored freshwater wetland that receives nutrient loading from upstream farms. During March to November 2012, we measured greenhouse gases along a ~2 km inundated portion of the wetland. Sampling locations spanned a wide chemical gradient from sites receiving seasonal fertilizer nitrogen and sulfate (SO) loads to sites receiving seasonal increases in marine salts. Concentrations and fluxes of CH were low (<100 µg L and <10 mg m h) for all sites and sampling dates when SO was high (>10 mg L), regardless of whether the SO source was agriculture or saltwater. Elevated CH (as high as 1,500 µg L and 45 mg m h) was only observed on dates when air temperatures were >27 °C and SO was <10 mg L. Despite elevated ammonium (NH) for saltwater exposed sites, concentrations of NO remained low (<5 µg L and <10 µg m h), except when fertilizer derived nitrate (NO) concentrations were high and NO increased as high as 156 µg L. Our results suggest that although both saltwater and agriculture derived SO may suppress CH, increases in NO associated with fertilizer derived NO may offset that reduction in wetlands exposed to both agricultural runoff and saltwater incursion. [ABSTRACT FROM AUTHOR]

Saltwater incursion carries high concentrations of sea salts, including sulfate, which can alter anaerobic microbial processes and plant community composition of coastal freshwater marshes. We studied these phenomena in a recently restored wetland on the coastal plain of North Carolina. We measured water inundation patterns, porewater chemistry, microbial process rates, plant tissue chemistry and iron plaque on plant roots, and quantified plant community composition across a hydrologic and salinity gradient to understand the potential interactions between saltwater incursion and changes in microbial processes and plant communities. Plant communities showed no obvious response to incursion, but were structured by inundation patterns and plant growth form (for example, graminoid versus forb). Saltwater incursion increased chloride and sulfate concentrations in surface and porewater, and drove resulting spatial patterns in anaerobic microbial metabolism rates. Plots experiencing saltwater incursion had higher sulfate reduction rates and were dominated by graminoid plant species (for example, sedges, rushes, and grasses). Graminoid plant species' roots had greater iron plaque formation than forb and submerged species, indicative that graminoid plant species are supplying more oxygen to the rhizosphere, potentially influencing microbial metabolism. Future studies should focus on how plant and microbial communities may respond to saltwater incursion at different time scales, and on parsing out the influence that plants and microbes have on each other as freshwater wetlands experience sea level rise. [ABSTRACT FROM AUTHOR]

Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20–25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks. [ABSTRACT FROM AUTHOR]

Groundwater discharge to rivers takes many forms, including preferential groundwater discharge points (PDPs) along riverbanks that are exposed at low flows, with multi-scale impacts on aquatic habitat and water quality. The physical controls on the spatial distribution of PDPs along riverbanks are not well-defined, rendering their prediction and representation in models challenging. To investigate the local riverbank sediment controls on PDP occurrence, we tested drone-based and handheld thermal infrared to efficiently map PDP locations along two mainstem rivers. Early in the study, we found drone imaging was better suited to locating tributary and stormwater inflows, which created relatively large water surface thermal anomalies in winter, compared to PDPs that often occurred at the sub-meter scale and beneath riparian tree canopy. Therefore, we primarily used handheld thermal infrared imaging from watercraft to map PDPs and larger seepage faces along 12-km of the fifth-order Housatonic River in Massachusetts, USA and 26-km of the Farmington River in Connecticut, USA. Overall, we mapped 31 riverbank PDPs along the Housatonic reach that meanders through lower permeability soils, and 104 PDPs along the Farmington reach that cuts through sandier sediments. Riverbank soil parameters extracted at PDP locations from the Soil Survey Geographic (SSURGO) database did not differ substantially from average bank soils along either reach, although the Farmington riverbank soils were on average 5× more permeable than Housatonic riverbank soils, likely contributing to the higher observed prevalence of PDPs. Dissolved oxygen measured in discharge water at these same PDPs varied widely, but showed no relation to measured sand, clay, or organic matter content in surficial soils indicating a lack of substantial near-surface aerobic reaction. The PDP locations were investigated for the presence of secondary bank structures, and commonly co-occurred with riparian tree root masses indicating the importance of localized physical controls on the spatial distribution of riverbank PDPs. [ABSTRACT FROM AUTHOR]

Large-scale wetland restoration often focuses on repairing the hydrologic connections degraded by anthropogenic modifications. Of these hydrologic connections, groundwater discharge is an important target, as these surface water ecosystem control points are important for thermal stability, among other ecosystem services. However, evaluating the effectiveness of the restoration activities on establishing groundwater discharge connection is often difficult over large areas and inaccessible terrain. Unoccupied aircraft systems (UAS) are now routinely used for collecting aerial imagery and creating digital surface models (DSM). Lightweight thermal infrared (TIR) sensors provide another payload option for generation of sub-meter-resolution aerial TIR orthophotos. This technology allows for the rapid and safe survey of groundwater discharge areas. Aerial TIR water-surface data were collected in March 2019 at Tidmarsh Farms, a former commercial cranberry peatland located in coastal Massachusetts, USA (41°54′17″ N 70°34′17″ W), where stream and wetland restoration actions were completed in 2016. Here, we present a 0.4 km2 georeferenced, temperature-calibrated TIR orthophoto of the area. The image represents a mosaic of nearly 900 TIR images captured by UAS in a single morning with a total flight time of 36 min and is supported by a DSM derived from UAS-visible imagery. The survey was conducted in winter to maximize temperature contrast between relatively warm groundwater and colder ambient surface environment; lower-density groundwater rises above cool surface waters and thus can be imaged by a UAS. The resulting TIR orthomosaic shows fine detail of seepage distribution and downstream influence along the several restored channel forms, which was an objective of the ecological restoration design. The restored stream channel has increased connectivity to peatland groundwater discharge, reducing the ecosystem thermal stressors. Such aerial techniques can be used to guide ecological restoration design and assess post-restoration outcomes, especially in settings where ecosystem structure and function is governed by groundwater and surface water interaction. [ABSTRACT FROM AUTHOR]