The Prairie Pothole Region (PPR) is the largest wetland complex in North America, with millions of wetlands punctuating the landscapes of Canada and the United States. Here, wetlands have been dramatically impacted by agricultural land use, with unclear implications for regional to global greenhouse gas (GHG) emissions budgets. By surveying wetlands across all three Canadian prairie provinces in the PPR, we show that emissions patterns of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) from aquatic habitats differ among wetlands embedded in cropland versus perennial landcover. Wetlands in cropped landscapes had double the aquatic diffusive emissions (20.6 ± 31.5 vs. 9.4 ± 17.3 g CO₂-eq m⁻² d⁻¹) largely driven by CH₄. Structural equation modeling showed that all three GHGs responded differently to the surrounding landscape properties. Emissions of CH₄ were the most sensitive to land use, responding positively to the elevated phosphorus content and lower sulfate content in cropped settings, despite higher organic matter content in wetlands in perennial landscapes. Aquatic N2O emissions were negligible, while CO₂ emissions were high, but not strongly related to agricultural land use. While our estimates of aquatic CH₄ emissions from PPR wetlands were high (18.2 ± 41.4 mmol CH₄ m⁻² d⁻¹), accounting for fluxes from vegetated and soil habitats would lead to whole-wetland emissions rates that are lower and comparable to wetlands in other biomes. Our study represents an important step toward understanding wetland emission responses to land use in the PPR and other wetland-rich agricultural landscapes.

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Riverine dissolved iron (Fe) affects water color, nutrients, and marine carbon cycling. Fe size and coupling with dissolved organic matter (DOM), in part, modulates the biogeochemical roles of riverine Fe. We used size fractionation to operationally define dissolved Fe (< 0.22 μm) into soluble (< 0.02 μm) and colloidal (0.02–0.22 μm) fractions in order to characterize the downstream drivers, concentrations, and fluxes of Fe across season and hydrologic regime at the freshwater Connecticut River mainstem, which we sampled bi-weekly for 2 yrs. Drivers of colloidal and soluble Fe concentrations were markedly different. The response of colloidal Fe concentration to changes in discharge was modulated by water temperature; colloidal Fe decreased with increasing discharge at temperatures < 10.5°C, but increased with increasing discharge at temperatures > 10.5°C. Conversely, soluble Fe concentrations were only positively correlated to discharge at high temperatures (> 20°C). Soluble Fe was strongly positively correlated to a humic-like DOM fluorescence component, suggesting coupling with DOM subsets, potentially through complexation. While average colloidal Fe fluxes varied twofold seasonally, soluble Fe fluxes varied ninefold; therefore, soluble Fe variability was more important to the overall dissolved Fe variability than colloidal Fe, despite lower concentrations. Seasonal Fe fluxes were decoupled from discharge: dissolved and soluble Fe fluxes were greatest in the fall, whereas discharge was greatest in the spring. Fluxes of soluble Fe, which may be more bioavailable and more likely to be exported to the ocean, were lowest in the summer when downstream biological demand is high, having implications for primary productivity and iron uptake.

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River networks transport dissolved organic carbon (DOC) from terrestrial uplands to the coastal ocean. The extent to which a reach or lake within a river network uptakes DOC depends on the stream order, the seasonal conditions, and the flow. At the watershed scale, it remains unclear whether DOC uptake is dominated by biological processes such as respiration, or abiotic processes like photomineralization. The partitioning of DOC uptake in lakes vs. rivers is also unclear. In this study, we present a new model that unifies year-round controls on DOC cycling for an entire river network, including river–lake connectivity, to elucidate the importance of biotic vs. abiotic controls on DOC uptake. We present the Catchment UPtake and Sinks by Season, Order, and Flow for DOC (CUPS-OF-DOC) model, which quantifies terrestrial DOC loading, gross primary productivity, and uptake via microbes and photomineralization. The model is applied to the Connecticut River Watershed, and accounts for cascading reach- and lake-scale DOC cycling across 98 scenarios spanning combinations of flows, seasons, and stream orders. We show that riverine DOC uptake is nearly constant with stream order, but the proportion of DOC uptake from photomineralization varies. Photomineralization dominates in rivers in most flow conditions and stream orders, especially in winter, accounting for at least half of whole-watershed DOC uptake in February across all flows. Whole-watershed summer DOC uptake occurs mostly via biomineralization in lakes, accounting for 80% of DOC uptake during the growing season, despite accounting for less than 6% of watershed open water surface area.

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Dissolved organic matter (DOM) impacts the structure and function of aquatic ecosystems. DOM absorbs light in the UV and visible (UV–Vis) wavelengths, thus impacting light attenuation. Because absorption by DOM depends on its composition, UV–Vis absorbance is used to constrain DOM composition, source, and amount. Ferric iron, Fe(III), also absorbs in the UV–Vis; when Fe(III) is present, DOM-attributed absorbance is overestimated. Here, we explore how differing behavior of DOM and Fe(III) at the catchment scale impacts UV–Vis absorbance and evaluate how system-specific variability impacts the effectiveness of existing Fe(III) correction factors in a temperate watershed. We sampled five sites in the Connecticut River mainstem bi-weekly for ~ 1.5 years, and seven sites in the Connecticut River watershed once during the summer 2019. We utilized size fractionation to isolate the impact of DOM and Fe(III) on absorbance and show that variable contributions of Fe(III) to absorbance at 254 nm (a₂₅₄) and 412 nm (a₄₁₂) by size fraction complicates correction for Fe(III). We demonstrate that the overestimation of DOM-attributed absorbance by Fe(III) is correlated to the Fe(III):dissolved organic carbon concentration ratio; thus, overestimation can be high even when Fe(III) is low. a₂₅₄ overestimation is highly variable even within a single system, but can be as high as 53%. Finally, we illustrate that UV-Vis overestimation might impart bias to seasonal, discharge, and land-use trends in DOM quality. Together, these findings argue that Fe(III) should be measured in tandem with UV–Vis absorbance for estimates of CDOM composition or amount.

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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.

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Aquatic primary productivity produces oxygen (O₂) and consumes carbon dioxide (CO₂) in a ratio of ~1.2. However, in aquatic ecosystems, dissolved CO₂ concentrations can be low, potentially limiting primary productivity. Here, results show that a large drainage basin maintains its highest levels of gross primary productivity (GPP) when dissolved CO₂ is diminished or undetectable due to photosynthetic uptake. Data show that, after CO_{2 is depleted, bicarbonate, an ionized form of inorganic carbon, supports these high levels of productivity. In fact, outputs from a process-based model suggest that bicarbonate can support up to ~58% of GPP under the most productive conditions. This is the first evidence that high levels of aquatic GPP are sustained in a riverine drainage network despite CO2 depletion, which has implications for freshwater ecology, biogeochemistry, and isotopic analysis. </p> Read more}

Wetlands export chromophoric dissolved organic matter (CDOM) to estuaries, where CDOM is removed and transformed through biotic and abiotic process, subsequently impacting nutrient cycling, light availability, ecosystem metabolism, and phytoplankton activity. We examined the bioavailability and photoreactivity of CDOM exported from four Chesapeake Bay tidal marshes across three seasons and along an estuarine salinity gradient using three incubation treatments: 14‐day microbial (MD), 7‐day combined photochemical/microbial (PB+MD), and 7‐day microbial incubation after photobleaching (MD after PB). CDOM absorption at 300 nm (a_CDOM300) and dissolved organic carbon (DOC) concentrations showed strong seasonality, with minima in winter, but CDOM quality (absorption spectral slopes, fluorescence component ratios) was less variable seasonally. PB+MD over 7 days decreased a_CDOM300 (–56.0%), humic‐like fluorescence (–67.6%), and DOC (–17.8%), but increased the spectral slope ratio SR (=S_275‐295/S_300‐350) (+94.8%), suggesting a decrease in CDOM molecular weight. Photochemistry dominated the PB+MD treatment. Photoreactivity was greater during the winter and in marsh/watershed versus down‐estuary sites, likely due to less previous light exposure. Prior photobleaching increased the bioavailability of marsh‐exported CDOM, resulting in a greater loss of a_CDOM300 and DOC, and a greater increase in humic‐like fluorescence (–6.0%, –5.9%, and +18.4% change, respectively, over 7‐day MD after PB incubations, versus –2.8%, –5.5%, and +2.6% change, respectively, over 14‐day MD incubations). CDOM exported from a marsh downstream of a major wastewater treatment plant showed the greatest photoreactivity and bioavailability. This highlights the significance of human activity on estuarine CDOM quality and biogeochemical cycles.

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May 2020 · Science of the Total Environment

Narasin is an antibiotic administered to broiler chickens to prevent coccidiosis. After storage, excreta containing parent narasin compounds is commonly spread as nitrogen fertilizer, yielding initial soil concentrations in the low μg·kg⁻¹ range. In soil, antibiotics have been found to modify one or more pathways in the biogeochemical nitrogen cycle. The concentrations tested are often too high to be considered environmentally relevant, despite evidence that sub-therapeutic doses may also be disruptive. We conducted soil mesocosm experiments to determine the overall impact of trace narasin on major nitrogen pools and fluxes in soils treated with 0, 1, 10, 100, or 1000 ng·kg⁻¹ narasin. Water content in the mesocosms varied from 40% to 80% water-filled pore space (WFPS), simulating a range of different redox conditions. Under aerobic conditions (40% WFPS), exposure to narasin inhibited nitrification, yielding increases in soil ammonium by up to 76%, perhaps by targeting nitrifying fungi. Under the same conditions, narasin caused soil nitrate concentrations to decline 17–39%. When the soil was near saturation (80% WFPS), nitrate increased by an average of 30%. Mass balances and isotopic enrichment of N₂O indicate that NAR may also affect anammox and the rate of nitrifier nitrification/denitrification. In aerobic soils, N₂O flux increased with antibiotic dose and the rise in flux strongly correlates to the N2O:N2 product ratio from dentification. This relationship suggests that N2O flux may increase in soils exposed to narasin even when total denitrification is modestly inhibited. We conclude that trace concentrations of narasin can significantly modify biogeochemical activities in soil on short timescales. Our results indicate the potential for extremely low concentrations of antibiotics to impact agricultural productivity, terrestrial N<sub<2</sub>O flux, and non-point source nitrogen pollution.

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