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Wildfires in the western United States (US) have been demonstrated to affect water quality, including dissolved organic carbon (DOC), in streams. Elevated post‐wildfire DOC concentration poses a potential risk to drinking water treatment systems. In‐stream measurements of fluorescent dissolved organic matter (fDOM), a proxy for DOC, have shown potential to detect dynamic changes in DOC. High frequency monitoring of water temperature, turbidity, and fDOM was used in conjunction with discrete sampling during targeted storm events and at fixed intervals to estimate DOC in five western US streams following wildfires in 2020 and 2021 with the objective to characterise and compare responses to wildfire among sites. The elevated turbidity conditions typical after wildfire presented a challenge to fDOM measurements and there was a need to identify appropriate turbidity corrections at burned sites. A combination of established and novel methods corrected fDOM concentrations for turbidity effects up to 800 Formazin nephelometric units (FNU). Pre‐wildfire high frequency water quality data in adjacent burned and unburned watersheds allowed for separation of climate effects on DOC at one of the sites. Hydrology, climate and landcover were more important drivers of post‐wildfire DOC yield than wildfire characteristics. Seasonal patterns of DOC were unchanged by wildfire in snowmelt‐driven watersheds. Large, transient spikes in DOC concentration following frontal and convective storms were observed post‐wildfire at all burned sites, but not at the unburned site. These spikes often exceeded operational thresholds for drinking water treatment. This study highlights the ability to develop high frequency DOC estimates in surface waters up to 800 FNU using fDOM sensors and targeted storm sampling and emphasises the value of high frequency pre‐wildfire data in adjacent burned and unburned watersheds for separating climate and wildfire effects. [ABSTRACT FROM AUTHOR]

Non‐perennial headwaters experience extremes in flow conditions that likely influence carbon fate. As surface waters contract through dry periods, reconnect during storms, and re‐expand or dry again, there is a great deal of variability in carbon emissions and export. We measured discharge, dissolved oxygen, carbon dioxide (CO2), and dissolved organic carbon (DOC) continuously in a persistent pool at the base of a non‐perennial, forested headwater stream in the southeastern United States to characterize how flow changes affect carbon emissions and export as the stream expands and shrinks. We also compared carbon concentrations and export during different stream flow categories before and after fall wet‐up. CO2 concentrations were high when discharge was lowest (median = 10.2 mg L−1) and low during high flows (3.2 mg L−1) and storms (1.1 mg L−1). High CO2 concentrations led to high emissions on a per area basis during low flow times, but whole‐channel stream CO2 emissions were limited by the small surface area of the stream during periods of surface water disconnection. DOC concentration varied by season (range = 0.1–16.2 mg L−1) with large pulses during smaller summer storms. We found that CO2 and DOC concentrations differed among binned stages of stream flow. As non‐perennial streams become more prevalent across the southeastern United States due to shifts in climate, the relationships between flow and carbon movement into and out of stream networks will become increasingly critical to understanding stream carbon biogeochemistry. Plain Language Summary: Non‐perennial streams lack continuously flowing surface water. We studied the movement of carbon through a forested non‐perennial headwater stream in the mountains of the southeastern United States. The stream channel was nearly empty at certain times in summer and early fall, when surface water was reduced to small pools, but storms could briefly cause the stream to flow again. These cycles of drying and rewetting influence concentrations of carbon dioxide gas and organic carbon dissolved in the stream water. As the stream dried up, oxygen decreased and carbon dioxide increased. When a storm occurred, much of the carbon dioxide in the stream was emitted into the atmosphere as water turbulence increased. Dissolved organic carbon was almost always very low, but storms sent large amounts of carbon downstream very quickly. Categorizing different levels of stream flow also helped explain how carbon cycles through a non‐perennial stream because distinct low flow phases varied in carbon concentration and transfer to downstream ecosystems. Non‐perennial streams are predicted to become more common as climate change affects precipitation patterns, so tracking how carbon moves into and out of streams that dry up and rewet will improve our understanding of freshwater contributions to the global carbon cycle. Key Points: Surface water disconnection led to high CO2 concentrations and emissionsDissolved organic carbon was low, except when storm pulses first reconnected surface water fragments with C sources within and beyond the stream channelInterim flows generated distinct C dynamics; focusing on only flowing or fragmented states likely misses key biogeochemical transitions [ABSTRACT FROM AUTHOR]

Using synoptic classification techniques, synoptic‐scale weather types associated with large exports of dissolved organic carbon and nitrate‐nitrogen in three forested watersheds in the northeastern United States are analyzed. In contrast to Siegert et al. (2021, https://doi.org/10.1029/2020jd033413; Part 1), which details the general synoptic conditions associated with stream chemistry variations, this study focuses on the long‐term frequency, trends, and global‐scale forcing mechanisms of individual, chemistry‐relevant, synoptic types. Nine individual types are identified as the most important to stream chemistry exports in northeastern watersheds during a 2.5‐year period of high‐frequency stream chemistry observation (2014–2016). Streamflow chemistry was the most influenced by Northwest Flow synoptic weather types in Vermont, cold front passages associated with Weak Westerly Flow types in Rhode Island, and Southwest Flow (SWF) types in Maryland. Each type provides unique atmospheric conditions that result in variations in precipitation, and thus chemical signals. From 1948 to 2017, the Arctic and North Atlantic Oscillations greatly influenced the interannual frequency of the analyzed synoptic types. When the indices were positively phased, SWF types were more frequent, while Northwest Flow and Weak Westerly Flow types were less frequent. Long‐term trends in synoptic type frequency are, inpart, due to documented changes in these teleconnection indices toward more positively phased configurations. As the climate continues to change into the 21st century, such associations may continue to drive large changes in synoptic type frequency, and thus watershed hydrology and biogeochemistry in the Northeast region. Key Points: Individual synoptic weather types uniquely influence the export of dissolved organic carbon (DOC) and NO3 solutes in watersheds of the Northeast United StatesThe interannual frequency and precipitation signals of multiple chemistry‐relevant synoptic types exhibit long‐term linear trendsVariations in synoptic type frequency, and thus stream chemistry, are partially forced by teleconnection indices, such as the North Atlantic Oscillation (NAO) [ABSTRACT FROM AUTHOR]

Wildfires cause direct and indirect CO2 emissions due to combustion and post-fire decomposition. Approximately half of temperate forest carbon (C) is stored in soil, so post-fire soil C cycling likely impacts forest C sink strength. Soil C sink strength is partly determined by soil microbial anabolism versus catabolism, which dictates the amount of C respired versus stored in microbial biomass. Fires affect soil C availability and composition, changes that could alter carbon use efficiency (CUE) and microbial biomass production, potentially influencing C sink recovery. Wildfire intensity is forecast to increase in forests of the western United States, and understanding the impacts of fire intensity on microbial anabolism is necessary for predicting fire-climate feedbacks. Our objective was to determine the influence of soil heating intensity and pyrogenic organic matter (PyOM) on microbial anabolism. We simulated the effects of fire intensity by heating soils to 100 or 200 °C for 30 min in a muffle furnace, and we amended the soils with charred or uncharred organic matter. Higher intensity soil heating (200 °C) led to lower microbial biomass carbon (MBC) accumulation, greater C respiration, and lower CUE proxies compared to unheated soils. Conversely, lower intensity heating (100 °C) yielded MBC accumulation and estimated CUE that was similar to unheated soils. Soils amended with PyOM exhibited similar MBC accumulation compared to uncharred organic matter, but lower CO2 emissions. These results indicate that high intensity soil heating decreases soil C-sink strength over the short-term by decreasing the amount of microbial anabolism relative to catabolism. [ABSTRACT FROM AUTHOR]

Sunlight can oxidize dissolved organic carbon (DOC) to dissolved inorganic carbon (DIC) in freshwaters. The importance of complete photooxidation, or photomineralization, as a sink for DOC remains unclear in temperate rivers, as most estimates are restricted to lakes, high latitude rivers, and coastal river plumes. In this study, we construct a model representing over 75,000 river reaches in the Connecticut River Watershed (CRW), USA, to calculate spectrally resolved photomineralization. We test the hypothesis that photomineralization is a negligible DOC sink across all reaches and flow conditions relative to DOC fluxes. Our model quantifies reaction rates and transport drivers within the river reaches for the ranges of flow conditions, incoming solar irradiance, and canopy cover shading observed throughout the year. Our model predicts average daily areal photomineralization rates ranging from 1.16 mg‐C m−2 day−1 in low flow river reaches in the winter, to 18.33 mg‐C m−2 day−1 in high flow river reaches during the summer. Even for high photomineralization fluxes, corresponding photomineralization uptake velocities are typically at least an order of magnitude smaller than those reported for other instream processes. We calculate DOC elimination by photomineralization relative to DOC fluxes through individual stream reaches as well as the entire riverine portion of the CRW. We find that relative photomineralization fluxes are highest in summer drought conditions in low order streams. In median flows and mean light intensities, for an average watershed travel distance, 3%–5% of the DOC fluxes are eliminated, indicating that photomineralization is a minor DOC sink in temperate rivers. Plain Language Summary: Rivers are an important part of the carbon cycle, moving carbon compounds from land to the ocean. Within rivers, dissolved organic carbon molecules can be broken down into inorganic carbon molecules, including the greenhouse gas carbon dioxide. Sunlight shining into rivers can cause these organic molecules to break down in a process called photomineralization, but it is not clear if this process is important compared to the total amount of organic carbon that travels through rivers every day. In this paper, we build a model for the river sections of temperate Connecticut River Watershed, which calculates photomineralization for possible river flow conditions, dissolved organic carbon concentrations, and seasons, and compares the size of the sunlight‐driven breakdown of dissolved organic carbon to the amount of dissolved organic carbon in the river. This is the first model that puts photomineralization rates in the context of a flowing temperate river network. We show that compared with the dissolved organic carbon amounts present in the river at any time of the year or any flow conditions, photomineralization is essentially an unimportant process, removing on average 3%–5% of the dissolved organic carbon through an average watershed river route. Key Points: We build a model calculating reaction and transport drivers for photomineralization in 75,000 temperate river reachesPhotomineralization is a negligible dissolved organic carbon (DOC) sink relative to DOC fluxesDOC elimination by photomineralization in temperate rivers is limited by short water residence times and canopy cover [ABSTRACT FROM AUTHOR]

In boreal regions, increased concentrations of dissolved organic carbon (DOC) have been linked to extreme wet years; however, less is known about the extent to which precipitation events are altering DOC concentration and quality. We assessed the effects of rain events on a suite of six lakes in Maine, U.S.A., to better understand how events alter DOC quantity and quality. DOC concentrations and DOC quality (measured as DOC-specific absorption coefficients (Specific Ultraviolet Absorbance (SUVA 254 (also a * 254), a * 320 , and a * 380)) were quantified 24 h before, and at three time points (24–48 h, 5–7 days, and 3 weeks) after five different precipitation events. Our results revealed three types of responses across the lakes: (1) an initial spike in DOC concentrations of 30–133% and in the three quality metrics of 20–86% compared to pre-storm levels, followed by return to pre-storm concentrations; (2) a sustained increase in DOC concentrations (by 4–23%) and an increase in the three DOC quality metrics (by 1–43%) through the second post-storm sampling, with concentrations falling by the third post-storm sampling compared to pre-storm levels; and (3) no change during all sampling periods. Lake residence time was a key driver of changes in DOC concentration and DOC quality in response to storm events. Our research provides evidence that precipitation events contribute to short-term abrupt changes in DOC quantity and quality that are largely driven by key landscape and lake characteristics. These changes in DOC may have important implications for management of water utilities, including alteration or implementation of treatment strategies. Image 1 • Precipitation events result in short-term abrupt change in DOC quantity and quality. • DOC spiked, was sustained, or did not change in response to precipitation events. • Response of SUVA 254 , a * 320 , and a * 380 to storm events was more variable than DOC. • Residence time was a key driver of changes in DOC, SUVA 254 , a * 320 , and a * 380. [ABSTRACT FROM AUTHOR]

Abstract Rivers and lakes have been traditionally studied as separate entities for carbon transport. However, there is a gap in our knowledge of the connectivity of dissolved carbon in a river-lake continuum. In this study, we analyzed dissolved carbon along the Little River-Catahoula Lake in subtropical Louisiana, United States to assess carbon biogeochemistry in such a river-lake continuum. Monthly in-situ measurements and water sample collections were made at four locations during April 2015 to February 2016 to determine riverine carbon transport into and out of the lake. Results show that much of the dissolved inorganic carbon (DIC) in the river-lake continuum originated from 13C depleted sources with an average δ13C DIC of −18.5‰. Significant decreases in DIC were found after the river passed through the lake (from 482 to 399 μmol L−1), which was most prevalent when the lake was not affected by backwater flow from the downstream river. CO 2 outgassing could be mainly responsible for the sink behavior of the lake for DIC. Dissolved organic carbon (DOC) in the studied watershed were mostly terrigenous with low δ13C DOC averaged at −29.2‰. Significant, consistent decreases in DOC concentrations were found from the river to the lake inflow and then to the lake outflow. During the majority of the year, the lake reduced DOC concentrations from the river inflow water, but switched to functioning as a source of DOC during warmer, dryer conditions in September and October due to increased water residence time. Therefore, the lake functioned both as a sink and as a source for DOC. Graphical abstract Unlabelled Image Highlights • In 9 out of 11 months the lake was a sink for DOC sustained by terrigenous sources. • The lake changed to a DOC source during months with extended water residence time. • DIC in the river-lake continuum originated mainly from 13C depleted sources. • CO 2 outgassing made the lake a sink for the down-network DIC transport. • Backwater containing high-level DIC could offset the sink function of the lake. [ABSTRACT FROM AUTHOR]

Two intense rainfalls [Hurricane Joaquin (2015) and Hurricane Matthew (2016)], one year apart, provided a unique opportunity to examine changes in dissolved organic matter (DOM) dynamics in coastal blackwater rivers under extreme flooding conditions in the southeastern United States. Two sites along Waccamaw River (a coastal blackwater river) and the outflow of 18 sub-basins of Yadkin-Pee Dee Basin were sampled during and after the flooding events. The peaks of dissolved organic carbon (DOC) and nitrogen (DON) concentrations were observed 18 and 23 days after peak discharge in 2015 and 2016, respectively. Moreover, DOM aromaticity and abundance of humic substances significantly increased during the same period. Separation of discharge hydrograph into surface runoff and subsurface flow suggested that temporal changes were mainly due to contributions from subsurface flow flushing organic matter from wetlands and organic-rich riparian zones. The spatial analysis highlighted the key role of the forested wetlands as the only land use that significantly correlated with both DOM quantity (DOC and DON load) and DOM composition (i.e., aromaticity). The Yadkin-Pee Dee River basin alone exported more than 474 million kg DOC into the ocean during high-flow conditions from the 2016 event, indicating that such extreme short-term events mobilized enormous amounts of organic carbon and nitrogen to the ocean. Considering the predicted increase in frequency and intensity of extreme rainfall events in the eastern U.S., the results of this study can shed light on changes in DOM dynamics that may occur under such conditions in the near future. [ABSTRACT FROM AUTHOR]

Wetland restoration is often conducted in Eastern U.S. coastal plain watersheds alongside agricultural lands that frequently export significant amounts of nitrogen in drainage water. Restoration plans that incorporate the addition of agricultural drainage water can simultaneously increase the success of achieving a target hydroperiod and reduce discharge of nitrogen to nearby surface water. The potential nitrogen removal effectiveness of two wetland restoration sites with such a restoration plan was evaluated in a two-year mesocosm study. Six large wetland mesocosms (3.5 m long × 0.9 m wide × 0.75 m deep) along with unplanted controls were used in this experiment. Three replicates of two soils that differed in organic matter and pH were planted with soft-stem bulrush (Schoenoplectus tabernaemontani) and allowed to develop in the two growing seasons prior to the study. Simulated drainage water was loaded into the mesocosms over eighteen batch studies across seasons with target nitrate-N levels between 2.5 to 10 mg L −1 . Grab samples were collected from the water column and analyzed for nitrate-N, dissolved organic carbon, and chloride, along with other environmental parameters such as pH, water temperature, and soil redox. Seasonally, nitrogen and carbon within the wetland plants and soil were also measured. Multivariate statistical analyses were utilized to determine differences in nitrate-N reductions between treatments. Variables included carbon availability, temperature, antecedent moisture condition, nitrogen loading, and water pH. Contrary to the hypothesis that higher nitrate-N removal rates would be observed in the wetlands with higher organic matter, overall removal rates were higher in the wetland mesocosms containing Deloss soils (WET-Min) (maximum of 726 mg m −2 d −1 ) than those containing Scuppernong soil (WET-Org) (maximum of 496 mg m −2 d −1 ) and were dependent on daily NO 3 -N concentrations and season. Significant differences in NO 3 -N removal were found between seasons and soil types (α = 0.05), which helped to provide insight to the expected magnitude of nitrogen removal within these systems throughout the year, and potential mechanisms (i.e. denitrification vs. plant uptake) that will govern these removals. [ABSTRACT FROM AUTHOR]

The decay of mountain pine beetle (MPB) ( Dendroctonus ponderosae) affected ponderosa pine ( Pinus ponderosa) forests has increased organic carbon (OC) loading within the Rapid Creek watershed in the Black Hills of South Dakota, with total OC (TOC) concentrations up to 6.2 mg L−1 observed in 2015. The impacts of large-scale tree mortality on water quality were delayed. OC concentrations in the watershed correlated with recent MPB impact stage progression. Regional ponderosa pine forests that were 3 and 6 years after MPB infection (grey and snagfall phases) exhibited increased OC loading within the Rapid Creek watershed. A better understanding of regional watershed response patterns for MPB tree mortality may assist with mitigating ancillary water quality impacts. [ABSTRACT FROM AUTHOR]

Bacteria in reservoir systems are controlled by a variety of factors. In this study, a lacustrine system with high primary production fed by a river with lower algal biomass was used to examine the relationship between bacterial community composition (BCC) and function under varying environmental conditions. Bacterial abundance, respiration, production, and growth efficiency were measured in the Cuyahoga River and Lake Rockwell reservoir in Ohio (USA). BCC was determined via terminal restriction length polymorphisms of 16S rRNA genes. Also, in laboratory experiments, bacterial communities were exposed to algal exudates and leaf leachates. Concurrently with physicochemical differences between lacustrine and riverine sites, there were differences in BCC that seemed unrelated to environmental conditions and DOC sources (color: chlorophyll a ratio). Functional attributes varied among dates but not sites, except for bacterial heterotrophic productivity. Overall, the source of DOC differed between the river and reservoir as did BCC. In contrast, bacterial functions, with the exception of productivity, were similar between the river and reservoir sites. Environmental variables (e.g., temperature, nutrient, and chlorophyll a) were correlated with differences in bacterial function. Differences in BCC did not coincide with functional differences in the field or controlled laboratory conditions suggesting that structure and function were uncoupled. [ABSTRACT FROM AUTHOR]

Abstract: This study aims to understand the influences of climate change and land surface processes on the variation of in-stream DOC concentrations in coastal rivers crossing different climate zones. Monthly observations spanning multiple years in seven major rivers in four different climate zones within the U.S. were analyzed for correlations between dissolved organic carbon (DOC) concentration and surface air temperature, precipitation, land cover and discharge. The major watersheds were the Altamaha River (GA), the Apilachicola River (FL), the Columbia River (OR), the Delaware River (NJ), the Sacramento and San Joaquin Rivers (CA) and the Susquehanna River (MD). One minor watershed, the Neponset River (MA) was also analyzed. Results indicate that temperature is the most important variable for DOC export when the variation of annual mean temperatures is large (e.g., >5°C) with sufficient precipitation levels. Land-surface characteristics and discharges are better correlated to DOC concentrations when the variations of annual mean temperatures are small (e.g. <2°C). However, results from the small watershed (Neponset) showed that land surface processes can vary annual DOC concentrations about ±1.65mg/L from mean value. This study is the first to examine DOC relationships in watersheds in multiple climate zones, and it was determined that weak correlations between temperature and DOC found in previous studies may be attributable to the fact that those studies examined small watersheds contained within a single climate zone. DOC flux per square meter was calculated based on incremental temperature increases. The results indicate that an increase of 1°C would result in a 0.476mg/L increase of in-stream DOC in large watersheds. Climate warming would have a greater impact on riverine DOC yields in cooler climate zones (up to 26% per °C) than on those in warmer climate zones (up to 6% per °C). [Copyright &y& Elsevier]

Abstract: The purpose of this study was to determine the flux of dissolved organic carbon (DOC) from high-energy beach sand to the coastal ocean. The DOC concentration of seawater was measured before and after exposure to the beach sand in laboratory experiments. Increases were observed in all experiments (n=36) conducted on seawater and high-energy beach sand collected on a monthly basis for one year. Final dissolved organic carbon in seawater after exposure to surface sands were positively correlated with water column DOC concentrations measured in the surf zone at the same time of sand collection for laboratory experiments. Increases in DOC during laboratory experiments displayed marked seasonality with larger increases observed during summer with a corresponding low DOC carbon to nitrogen ratio (C:N) suggesting phytoplankton as the source of DOC during this time of year. Higher DOC C:N was observed during winter indicating a more aged organic material as the source of DOC during this time of low flux. Sandy beach sediments release over an order of magnitude more DOC per meter squared compared to coastal shelf sediments and rainwater suggesting that in locations where there are no direct river inputs these beach environments play a significant, previously unrecognized role as an external source of DOC to the immediate coastal ocean. It is likely that DOC released from these environments has a significant effect on surf zone biogeochemistry potentially affecting oxygen concentrations in coastal regions. [Copyright &y& Elsevier]

The present study examined the effect of dissolved organic carbon (DOC), both added and inherent, on Cu toxicity in glochidia, the larvae of freshwater mussels. Using incremental additions of natural DOC concentrate and reconstituted water, a series of acute copper toxicity tests were conducted. An increase in DOC from 0.7 to 4.4 mg C/L resulted in a fourfold increase (36-150 µg Cu/L) in the 24-h median effective concentration (EC50) and a significant linear relationship (r2 = 0.98, p = 0.0008) between the DOC concentration and the Cu EC50 of Lampsilis siliquoidea glochidia. The ameliorating effect of added DOC on Cu toxicity was confirmed using a second mussel species, the endangered (in Canada) Lampsilis fasciola. The effect of inherent (i.e., not added) DOC on Cu toxicity was also assessed in eight natural waters (DOC 5-15 mg C/L). These experiments revealed a significant relationship between the EC50 and the concentration of inherent DOC (r2 = 0.79, p = 0.0031) with EC50s ranging from 27 to 111 µg Cu/L. These laboratory tests have demonstrated that DOC provides glochidia with significant protection from acute Cu toxicity. The potential risk that Cu poses to mussel populations was assessed by comparing Cu and DOC concentrations from significant mussel habitats in Ontario to the EC50s. Although overall mean Cu concentration in the mussel's habitat was well below the acutely toxic level given the concentration of DOC, episodic Cu releases in low DOC waters may be a concern for the recovery of endangered freshwater mussels. The results are examined in the context of current Cu water quality regulations including the U.S. Environmental Protection Agency's (U.S. EPA) biotic ligand model. Environ. Toxicol. Chem. 2010;29:2519-2528. © 2010 SETAC [ABSTRACT FROM AUTHOR]

Abstract: Continental shelves are believed to play a major role in carbon cycling due to their high productivity. To improve our understanding of carbon dynamics on continental margins, a dissolved organic matter (DOM) model was developed and imbedded within an existing coupled ocean circulation-biogeochemical model of the U.S. East coast. A model simulation with the DOM module was compared with the reference model (without the DOM module) to illustrate the role of DOM dynamics in coastal ocean biogeochemical cycling. Model results reveal that the progressive release of dissolved organic nitrogen (DON) in the ocean’s upper layer during summer increases the regenerated primary production by 30–300%, which, in turn, enhances the dissolved organic carbon (DOC) production mainly from phytoplankton exudation in the upper layer and solubilization of particulate organic matter (POM) deeper in the water column. This analysis suggests that DOM is a necessary component for representing ecosystem functioning and organic fluxes in models because DOM (1) is a major organic pool directly related to primary production, (2) decouples partially the carbon and nitrogen cycles (through carbon excess uptake, POM solubilization and DOM mineralization) and (3) is intimately linked to the residence time of water masses for its distribution and export. The seasonally produced DOC on the shelf can be exported to the open ocean by horizontal transport at comparable rates (1–2 mol C m−2 yr−1) to particulate organic carbon burial in the southern U.S. Mid-Atlantic Bight (MAB). [Copyright &y& Elsevier]

1. Agricultural and urban land use may increase dissolved inorganic nitrogen (DIN) concentrations in streams and saturate biotic nutrient demand, but less is known about their impacts on the cycling of organic nutrients. To assess these impacts we compared the uptake of DIN (as ammonium, NH4+), dissolved organic carbon (DOC, as acetate), and dissolved organic nitrogen (DON, as glycine) in 18 low-gradient headwater streams in southwest Michigan draining forested, agricultural, or urban land-use types. Over 3 years, we quantified uptake in two streams in each of the three land-use types during three seasons (spring, summer and autumn). 2. We found significantly higher NH4+ demand (expressed as uptake velocity, Vf) in urban compared to forested streams and NH4+ Vf was greater in spring compared to summer and autumn. Acetate Vf was significantly higher than NH4+ and glycine Vf, but neither acetate nor glycine Vf were influenced by land-use type or season. 3. We examined the interaction between NH4+ and acetate demand by comparing simultaneous short-term releases of both solutes to releases of each solute individually. Acetate Vf did not change during the simultaneous release with NH4+, but NH4+ Vf was significantly higher with increased acetate. Thus, labile DOC Vf was not limited by the availability of NH4+, but NH4+ Vf was limited by the availability of labile DOC. In contrast, neither glycine nor NH4+ Vf changed when released simultaneously indicating either that overall N-uptake was saturated or that glycine and NH4+ uptake were controlled by different factors. 4. Our results suggest that labile DOC and DON uptake can be equivalent to, or even higher than NH4+ uptake, a solute known to be highly bioreactive, but unlike NH4+ uptake, may not differ among land-use types and seasons. Moreover, downstream export of nitrogen may be exacerbated by limitation of NH4+ uptake by the availability of labile DOC in headwater streams from the agricultural Midwestern United States. Further research is needed to identify the factors that influence cycling of DOC and DON in streams. [ABSTRACT FROM AUTHOR]

Three categories of digital wetland maps widely available in the United States were used to develop models relating wetlands to DOC: (1) wetlands mapped by the U.S. National Wetlands Inventory (NWI) (2) wetland vegetation cover mapped by the U.S. National Land Cover Dataset (NLCD), and (3) maps of hydric soils. Data extracted from these maps for 27 headwater catchments of the Ontonagon River in northern Michigan, USA were used with DOC concentrations measured in catchment streams to develop stepwise multiple regressions based on wetland area and type. The catchments of the 27 tributaries ranged in area from 2 to 66 km2 and wetlands constituted 10 to 53% of their area. Although all three databases provided regressions that were highly significant (p <0.001), the variance explained was greater for NWI maps (R 2 =0.75) than for NLCD (R 2 =0.61) or soil maps (R 2 =0.60). Wetland–stream relationships were strongest during September 2002, but were significant for nine out of ten dates sampled during subsequent seasons. The individual wetland type most highly correlated (r >0.62) with stream DOC concentrations was conifer peatland, represented on the NWI maps as Palustrine Needle-leaved Forest, the NLCD maps as woody wetland, and the soil maps as organic soils. For the NWI dataset, DOC was negatively correlated with area of palustrine emergent wetlands (i.e., sedge meadows and graminoid fens) and bog shrubs, inferring that these wetland types may be sinks for DOC. Because of the different effects of wetland vegetation types on DOC, a GIS data source such as the NWI which depicts those wetland types is superior for predicting landscape contributions to stream DOC concentrations. [Copyright &y& Elsevier]

Abstract: It is widely recognized that wetlands, especially those rich in organic matter and receiving appreciable atmospheric mercury (Hg) inputs, are important sites of methylmercury (MeHg) production. Extensive wetlands in the southeastern United States have many ecosystem attributes ideal for promoting high MeHg production rates; however, relatively few mercury cycling studies have been conducted in these environments. We conducted a landscape scale study examining Hg cycling in coastal Louisiana (USA) including four field trips conducted between August 2003 and May 2005. Sites were chosen to represent different ecosystem types, including: a large shallow eutrophic estuarine lake (Lake Pontchartrain), three rivers draining into the lake, a cypress-tupelo dominated freshwater swamp, and six emergent marshes ranging from a freshwater marsh dominated by Panicum hemitomon to a Spartina alterniflora dominated salt marsh close to the Gulf of Mexico. We measured MeHg and total Hg (THg) concentrations, and ancillary chemical characteristics, in whole and filtered surface water, and filtered porewater. Overall, MeHg concentrations were greatest in surface water of freshwater wetlands and lowest in the profundal (non-vegetated) regions of the lake and river mainstems. Concentrations of THg and MeHg in filtered surface water were positively correlated with the highly reactive, aromatic (hydrophobic organic acid) fraction of dissolved organic carbon (DOC). These results suggest that DOC plays an important role in promoting the mobility, transport and bioavailability of inorganic Hg in these environments. Further, elevated porewater concentrations in marine and brackish wetlands suggest coastal wetlands along the Gulf Coast are key sites for MeHg production and may be a principal source of MeHg to foodwebs in the Gulf of Mexico. Examining the relationships among MeHg, THg, and DOC across these multiple landscape types is a first step in evaluating possible links between key zones for Hg(II)-methylation and the bioaccumulation of mercury in the biota inhabiting the Gulf of Mexico region. [Copyright &y& Elsevier]

The fate of terrestrially-derived dissolved organic carbon (DOC) is important to carbon (C) cycling in both terrestrial and aquatic environments, and recent evidence suggests that climate warming is influencing DOC dynamics in northern ecosystems. To understand what determines the fate of terrestrial DOC, it is essential to quantify the chemical nature and potential biodegradability of this DOC. We examined DOC chemical characteristics and biodegradability collected from soil pore waters and dominant vegetation species in four boreal black spruce forest sites in Alaska spanning a range of hydrologic regimes and permafrost extents (Well Drained, Moderately Well Drained, Poorly Drained, and Thermokarst Wetlands). DOC chemistry was characterized using fractionation, UV–Vis absorbance, and fluorescence measurements. Potential biodegradability was assessed by incubating the samples and measuring CO2 production over 1 month. Soil pore water DOC from all sites was dominated by hydrophobic acids and was highly aromatic, whereas the chemical composition of vegetation leachate DOC varied significantly with species. There was no seasonal variability in soil pore water DOC chemical characteristics or biodegradability; however, DOC collected from the Poorly Drained site was significantly less biodegradable than DOC from the other three sites (6% loss vs. 13–15% loss). The biodegradability of vegetation-derived DOC ranged from 10 to 90% loss, and was strongly correlated with hydrophilic DOC content. Vegetation such as Sphagnum moss and feathermosses yielded DOC that was quickly metabolized and respired. In contrast, the DOC leached from vegetation such as black spruce was moderately recalcitrant. Changes in DOC chemical characteristics that occurred during microbial metabolism of DOC were quantified using fractionation and fluorescence. The chemical characteristics and biodegradability of DOC in soil pore waters were most similar to the moderately recalcitrant vegetation leachates, and to the microbially altered DOC from all vegetation leachates. [ABSTRACT FROM AUTHOR]

The factors controlling spatial and temporal patterns in soil solution and streamwater chemistry are highly uncertain in northern hardwood forest ecosystems in the northeastern United States, where concentrations of reactive nitrogen (Nr) in streams have surprisingly declined over recent decades in the face of persistent high rates of atmospheric Nr deposition and aging forests. Reactive nitrogen includes inorganic species (e.g., ammonium [NH4+], nitrate [NO3-]) and some organic forms (e.g., amino acids) available to support the growth of plants and microbes. The objective of this study was to examine controls on the spatial and temporal patterns in the concentrations and fluxes of nitrogen (N) species and dissolved organic carbon (DOC) in a 12-year record of soil solutions and streamwater along an elevational gradient (540–800 m) of a forested watershed at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire, USA. Dissolved organic N and DOC concentrations were elevated in the high-elevation spruce—fir—white birch (SFB) zone of the watershed, while NO3- was the dominant N species in the lower elevation hardwood portion of the watershed. Within the soil profile, N retention was centered in the mineral horizon, and significant amounts of N were retained between the lower mineral soil and the stream, supporting the idea that near- and in-stream processes are significant sinks for N at the HBEF. Temporal analysis suggested that hydrologic flow paths can override both abiotic and biotic retention mechanisms (i.e., during the non-growing season when most hydrologic export occurs, or during years with high rainfall), there appears to be direct flushing of N from the organic horizons into the stream via horizontal flow. Significant correlations between soil NO3- concentrations, nitrification rates and streamwater NO3- exports show the importance of biological production as a regulator of inorganic N export. The lack of internal production response (e.g., mineralization, nitrification) to a severe ice Storm in 1998 reinforces the idea that plant uptake is the dominant regulator of export response to disturbance. [ABSTRACT FROM AUTHOR]

In the United States, probability-based water quality surveys are typically used to meet the requirements of Section 305(b) of the Clean Water Act. The survey design allows an inference to be generated concerning regional stream condition, but it cannot be used to identify water quality impaired stream segments. Therefore, a rapid and cost-efficient method is needed to locate potentially impaired stream segments throughout large areas. We fit a set of geostatistical models to 312 samples of dissolved organic carbon (DOC) collected in 1996 for the Maryland Biological Stream Survey using coarse-scale watershed characteristics. The models were developed using two distance measures, straight-line distance (SLD) and weighted asymmetric hydrologic distance (WAHD). We used the Corrected Spatial Akaike Information Criterion and the mean square prediction error to compare models. The SLD models predicted more variability in DOC than models based on WAHD for every autocovariance model except the spherical model. The SLD model based on the Mariah autocovariance model showed the best fit (r2 = 0.72). DOC demonstrated a positive relationship with the watershed attributes percent water, percent wetlands, and mean minimum temperature, but was negatively correlated to percent felsic rock type. We used universal kriging to generate predictions and prediction variances for 3083 stream segments throughout Maryland. The model predicted that 90.2% of stream kilometers had DOC values less than 5 mg/l, 6.7% were between 5 and 8 mg/l, and 3.1% of streams produced values greater than 8 mg/l. The geostatistical model generated more accurate DOC predictions than previous models, but did not fit the data equally well throughout the state. Consequently, it may be necessary to develop more than one geostatistical model to predict stream DOC throughout Maryland. Our methodology is an improvement over previous methods because additional field sampling is not necessary, inferences about regional stream condition can be made, and it can be used to locate potentially impaired stream segments. Further, the model results can be displayed visually, which allows results to be presented to a wide variety of audiences easily. [ABSTRACT FROM AUTHOR]

Abstract: Soil organic matter is an important source of allochthonous dissolved organic matter inputs to the Sacramento–San Joaquin Delta waterways, which is a drinking water source for 22 million people in California, USA. Knowledge of trihalomethane (THM) formation potential of soil-derived organic carbon is important for developing effective strategies for organic carbon removal in drinking water treatment. In this study, soil organic carbon was extracted with electrolytes (deionized H2O and Na- or Ca-based electrolytes) of electrical conductivity bracketing those found in Delta leaching and runoff conditions. The extracts were physically and chemically separated into different fractions: colloidal organic carbon (0.45–0.1μm), fine colloidal organic carbon (0.1–0.025μm), and dissolved organic carbon (DOC) (<0.025μm); hydrophobic acid (HPOA), transphilic acid, and hydrophilic acid. Two representative Delta soils, Rindge Muck (a peat soil) and Scribner Clay Loam (a mineral soil) were examined. Results showed that less than 2% of soil organic carbon was electrolyte-extractable and heterogeneous organic fractions with distinct THM reactivity existed. Regardless of soil and electrolytes, DOC and HPOA fractions were dominant in terms of total concentration and THMFP. The amounts of extractable organic carbon and THMFP were dependent on the cation and to a lesser extent on electrical conductivity of electrolytes. Along with our previous study on temperature and moisture effects on DOC production, we propose a conceptual model to describe the impacts of agricultural practices on DOC production in the Delta. DOC is mainly produced in the surface peat soils during the summer and is immobilized by accumulated salt in the soils. DOC is leached from soils to drainage ditches and finally to the Delta channels during winter salt leaching practices. [Copyright &y& Elsevier]

The amount, chemical composition, and source of dissolved organic carbon (DOC), together with in situ ultraviolet (UV-B) attenuation, were measured at 1–2 week intervals throughout the summers of 1999, 2000, and 2001 at four sites in Rocky Mountain National Park (Colorado). Eight additional sites, four in Sequoia and Kings Canyon National Park/John Muir Wilderness (California) and four in Glacier National Park (Montana), were sampled during the summer of 2000. Attenuation of UV-B was significantly related to DOC concentrations over the three years in Rocky Mountain ( R2 = 0.39, F = 25.71, P < 0.0001) and across all parks in 2000 ( R2 = 0.44, F = 38.25, P < 0.0001). The relatively low R2 values, however, reflect significant temporal and spatial variability in the specific attenuation per unit DOC. Fluorescence analysis of the fulvic acid DOC fraction (roughly 600–2,000 Daltons) indicated that the source of DOC significantly affected the attenuation of UV-B. Sites in Sequoia–Kings Canyon were characterized by DOC derived primarily from algal sources and showed much deeper UV-B penetration, whereas sites in Glacier and Rocky Mountain contained a mix of algal and terrestrial DOC-dominated sites, with more terrestrially dominated sites characterized by greater UV-B attenuation per unit DOC. In general, site characteristics that promoted the accumulation of terrestrially derived DOC showed greater attenuation of UV-B per unit DOC; however, catchment vegetation and soil characteristics, precipitation, and local hydrology interacted to make it difficult to predict potential exposure from DOC concentrations. [ABSTRACT FROM AUTHOR]

Chromophoric dissolved organic matter (CDOM) was measured in the spring and summer in the northern Gulf of Mexico with the ECOShuttle, a towed, instrumented, undulating vehicle. A submersible pump mounted on the vehicle supplied continuously flowing, uncontaminated seawater to online instruments in the shipboard laboratory and allowed discrete samples to be taken for further analysis. CDOM in the northern Gulf of Mexico was dominated by freshwater inputs from the Mississippi River through the Birdfoot region and to the west by discharge from the Atchafalaya River. CDOM was more extensively dispersed in the high-flow period in the spring but in both time periods was limited by stratification to the upper 12 m or so. Thin, subsurface CDOM maxima were observed below the plume during the highly stratified summer period but were absent in the spring. However, there was evidence of significant in situ biological production of CDOM in both seasons.The Mississippi River freshwater end member was similar in spring and summer, while the Atchafalaya end member was significantly higher in the spring. In both time periods, the Atchafalaya was significantly higher in CDOM and dissolved organic carbon (DOC) than the Mississippi presumably due to local production and exchange within the coastal wetlands along the lower Atchafalaya which are absent along the lower Mississippi. Nearshore waters may also have higher CDOM due to outwelling from coastal wetlands. High-resolution measurements allow the differentiation of various water masses and are indicative of rapidly varying (days to weeks) source waters. Highly dynamic but conservative mixing between various freshwater and marine end members apparently dominates CDOM distributions in the area with significant in situ biological inputs (bacterial degradation of phytoplankton detritus), evidence of flocculation, and minor photobleaching effects also observed. It is clear that high-resolution measurements and adaptive sampling strategies allow a more detailed examination of the processes that control CDOM distributions in river-dominated systems. [Copyright &y& Elsevier]

Dissolved and particulate organic carbon concentrations were measured and annual loads estimated for the Trinity River, the main freshwater input source to Galveston Bay, which lies on the upper Gulf coast of Texas, USA, during 2000–2001. This river drains the forested lowlands south of a relatively large reservoir lake, Lake Livingston. A weak relationship between dissolved organic carbon (DOC) and QTR indicated hydrologic control but separation of the data, based on individual discharge events, was necessary to improve interpretation. For instance, the first rain of the season resulted in only a modest increase in DOC concentrations and led to an inverse relationship with discharge, due to decreased lateral flow and increased infiltration of rainwater, with the lower flows being more efficient at DOC leaching from soils. In contrast, a long duration high discharge river crest event resulted in an opposite trend, i.e. a linear increase in DOC with increasing discharge rates. A short duration high discharge tropical storm showed reduced Trinity River DOC concentrations and the highest POC concentrations measured, likely resulting from the relatively short duration, and minimal contact time, of this event. In contrast to DOC, the concentrations of particulate organic carbon, POC (mg C l-1) were linearly correlated to suspended particulate matter (SPM) concentrations and accounted for between 10 and 12% of the total suspended load at low discharge but decreased to ∼2% at high discharge. This suggests dilution by larger particles with a reduced organic carbon content, possibly silicate minerals, more readily resuspended at elevated levels of discharge. The annual total organic carbon (TOC) load to Galveston Bay, estimated from the slope of the daily load vs. discharge relationship, was 11.2×1010 g C and calculated export coefficients (g C m-2 year-1) were in good agreement with previous results. Using this relationship, accurate assessments of TOC flux inputs to Galveston Bay over the past quarter-century and in the future are possible by obtaining annual Trinity River discharge rates, which are readily available from the USGS. Comparing DOC riverine inputs to benthic sources in Trinity Bay, measured directly on the same day, indicates that the sediments contribute approximately 20% of total inputs of DOC to Trinity Bay. However, assuming a constant benthic source during low-flow conditions, which can occur for periods of up to 14 months in this region of Texas, benthic fluxes would account for >80% of the total inputs into Trinity Bay. At high levels of discharge, the Trinity River discharges ∼1.0×109 g C day-1 and dominates DOC inputs to Trinity Bay. [Copyright &y& Elsevier]

A 5-year program of study was conducted at the Sweetwater Recharge Facilities (SRF) to assess the performance of surface spreading operations for organics attenuation during field-scale soil-aquifer treatment (SAT) of municipal wastewater. Studies were conducted utilizing both mature (∼10 yr old) and new infiltration basins. Removals of dissolved organic carbon (DOC) were robust, averaging >90 percent during percolation through the local 37-m vadose zone. The hydrophilic (most polar) fraction of DOC was preferentially removed during SAT; removals were attributed primarily to biodegradation. Reductions in trihalomethane formation potential (THMFP) averaged 91 percent across the vadose zone profile. The reactivity (specific THMFP) of post-SAT organic residuals with chlorine decreased slightly from pre-SAT levels (60 vs. 72 μg THM per mg DOC, respectively). Variations in the duration of wetting/drying periods did not significantly impact organic removal efficiencies. [Copyright &y& Elsevier]

The coupled carbon (C) cycle across terrestrial and aquatic environments at the watershed scale has been identified as an important, but poorly constrained component of the global carbon budget. Here, we extended Soil and Water Assessment Tool (SWAT) with coupled riverine particulate organic carbon (POC) and dissolved organic carbon (DOC) modules (referred to as SWAT-C hereafter). Results show that SWAT-C reproduced daily POC and DOC fluxes well in two watersheds in the Northeastern United States. We found that SWAT-C tended to underestimate high flows and peak DOC and POC fluxes. Uncertainty analysis indicated flux uncertainties associated with POC and DOC simulations were larger than those for flow simulation. Sensitive parameters controlling POC and DOC biogeochemical processes were identified along with how these parameters influence mechanisms underlying C cycling. We anticipate that the tool developed and applied here will inform C related ecosystem services in watershed assessment and planning. • We added new processes into the SWAT model for riverine POC and DOC modeling. • The SWAT-C model performed well for simulating daily POC and DOC fluxes in two watersheds. • Uncertainty and sensitivity analyses were performed to understand key parameters controlling riverine carbon fluxes. • We anticipate the new capabilities of SWAT-C help inform watershed scale carbon management. [ABSTRACT FROM AUTHOR]

1. We surveyed eighty-five lakes located in the Adirondack Mountain Region of New York State, U.S.A., to characterize the attenuation of photosynthetically active (PAR) and ultraviolet radiation (UVR) in relation to dissolved organic carbon (DOC) concentrations and pH. Attenuation of PAR was quantified in situ. Attenuation was also inferred by measuring the light absorption of filtered lake water samples at wavelengths (300, 340 and 440 nm) representing UV-B, UV-A and PAR. 2. Substantial variation in transparency was observed among lakes in this region. Attenuation depths (z1%) for PAR ranged from 0.5 to greater than 20 m, while inferred values for UV-B and UV-A ranged from a few centimetres to > 5 m. Median values of UV-A penetration (0.75 m) and UV-B penetration (0.45 m) corresponded to 11% (UV-A) and 6% (UV-B) of lake maximum depth. 3. Much of the variation in PAR and UVR attenuation was explained by differences in lake DOC. Univariate power models based solely on DOC accounted for 85% (PAR), 90% (UV-A) and 91% (UV-B) of the variation in absorption. 4. Attenuation and absorption coefficients were generally lower for recently acidified lakes compared to acidic and circumneutral lakes which have not undergone recent acidification. However, differences among these three groups of lakes were not statistically significant. Our results suggest that the effects of acidification on the optical properties of a regional population of lakes, even in an area experiencing widespread acidification, are relatively subtle in comparison with other factors contributing to inter-lake variability. 5. The presence of near-shore wetlands is probably a key factor influencing regional variability in DOC and light climate among Adirondack lakes. Temporal variability in climatic factors influencing wetland DOC production and export may mask more subtle influences on lake DOC associated with anthropogenic acidification. [ABSTRACT FROM AUTHOR]