In some high arsenic (As) groundwater systems, correlations are observed between dissolved organic matter (DOM) and As concentrations, but in other systems, such relationships are absent. The role of ...labile DOM as the main driver of microbial reductive dissolution is not sufficient to explain the variation in DOM–As relationships. Other processes that may also influence As mobility include complexation of As by dissolved humic substances, and competitive sorption and electron shuttling reactions mediated by humics. To evaluate such humic DOM influences, we characterized the optical properties of filtered surface water (n = 10) and groundwater (n = 24) samples spanning an age gradient in Araihazar, Bangladesh. Further, we analyzed large volume fulvic acid (FA) isolates (n = 6) for optical properties, C and N content, and 13C NMR spectroscopic distribution. Old groundwater (>30 years old) contained primarily sediment-derived DOM and had significantly higher (p < 0.001) dissolved As concentration than groundwater that was younger than 5 years old. Younger groundwater had DOM spectroscopic signatures similar to surface water DOM and characteristic of a sewage pollution influence. Associations between dissolved As, iron (Fe), and FA concentration and fluorescence properties of isolated FA in this field study suggest that aromatic, terrestrially derived FAs promote As–Fe–FA complexation reactions that may enhance As mobility.
Northern permafrost soils store a vast reservoir of carbon, nearly twice that of the present atmosphere. Current and projected climate warming threatens widespread thaw of these frozen, organic ...carbon (OC)-rich soils. Upon thaw, mobilized permafrost OC in dissolved and particulate forms can enter streams and rivers, which are important processors of OC and conduits for carbon dioxide (CO₂) to the atmosphere. Here, we demonstrate that ancient dissolved organic carbon (DOC) leached from 35,800 y B.P. permafrost soils is rapidly mineralized to CO₂. During 200-h experiments in a novel high–temporal-resolution bioreactor, DOC concentration decreased by an average of 53%, fueling a more than sevenfold increase in dissolved inorganic carbon (DIC) concentration. Eighty-seven percent of the DOC loss to microbial uptake was derived from the low–molecular-weight (LMW) organic acids acetate and butyrate. To our knowledge, our study is the first to directly quantify high CO₂ production rates from permafrost-derived LMW DOC mineralization. The observed DOC loss rates are among the highest reported for permafrost carbon and demonstrate the potential importance of LMW DOC in driving the rapid metabolism of Pleistocene-age permafrost carbon upon thawand the outgassing of CO₂ to the atmosphere by soils and nearby inland waters.
•IHSS Pony Lake and Suwannee River fulvic acid DOM reference samples compared.•Molecular constituents assigned and characterized using FT-ICR MS.•Complementary data obtained from fluorescence ...spectroscopy.•Proteinaceous signatures of Pony Lake DOM assigned from both analytical methods.•More reactive/microbial character for Pony Lake DOM than for Suwannee River DOM.
We present the extensive characterization of Antarctic Pony Lake (PL) dissolved organic matter (DOM), an International Humic Substance Society (IHSS) fulvic acid (FA) reference standard, using electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (ESI FT-ICR MS) and excitation–emission matrix fluorescence spectroscopy (EEMS). PLFA is the first reference standard available through IHSS derived solely from a microbial source. A number of factors differentiate PLFA from other IHSS standards, including source material, geographic location, sunlight exposure, freeze–thaw conditions, and other in situ environmental influences. ESI FT-ICR MS and EEMS were used to compare the PLFA microbial DOM compositional signature with the IHSS Suwannee River (SR) FA, a standard frequently studied for environmental DOM analysis. Although CcHhOoNnSs (n=0, 1, or 2 and s=0 or 1) constituents were present in both IHSS samples, PLFA contained more N and S molecular species, whereas SRFA was dominated by CcHhOo compounds. Proteinaceous character was detected with both methods, in greater abundance for PLFA, which we attributed to its microbial source material and labile, potentially more reactive nature than SRFA. Characterization from both analytical techniques resulted in complementary data that reinforce the importance of PLFA as an IHSS reference standard that should be utilized for other microbiological environmental DOM comparisons.
Excitation−emission matrixes (EEMs) of 379 dissolved organic matter (DOM) samples from diverse aquatic environments were modeled by parallel factor analysis (PARAFAC). Thirteen components likely ...representing groups of similarly fluorescing moieties were found to explain the variation in this data set. Seven of the thirteen components were identified as quinone-like based on comparison of their excitation and emission spectra to spectra of model quinones. These quinone-like fluorophores were found to vary in redox state and degree of conjugation. Two components were identified as amino acid-like based on comparison to tyrosine and tryptophan fluorescence spectra. The other four components are not yet associated with any class of molecules. The quinone-like fluorophores account for about 50% of the fluorescence for every sample analyzed, showing that quinone-like fluorophores are an important and ubiquitous fluorescing moiety and in natural waters. Further, the distribution of the quinone-like fluorophores was evaluated as a function of environmental and laboratory redox gradients. Under reducing conditions, the contribution of the reduced quinone-like fluorophores increased concurrent with a decrease in the oxidized quinone-like fluorophores, indicating that DOM fluorescence is a function of redox state of quinone-like moieties. Lastly, a ratio of two quinone-like fluorophores was found to explain the variation in the fluorescence index. These results provide new insight into the redox reactivity of DOM and have implications for the application of fluorescence spectroscopy as a tool to characterize DOM.
It was demonstrated more than two decades ago that microorganisms use humic substances, including fulvic acid (FA), as electron shuttles during iron (Fe) reduction in anaerobic soils and sediments. ...The relevance of this mechanism for the acceleration of Fe(III) reduction in arsenic-laden groundwater environments is gaining wider attention. Here we provide new evidence that dissolved FAs isolated from sediment-influenced surface water and groundwater in the Bengal Basin were capable of electron shuttling between Geobacter metallireducens and Fe(III). Moreover, all four Bangladesh sediment-derived dissolved FAs investigated in this study had higher electron accepting capacity (176 to 245μmol/g) compared to aquatic FAs, such as Suwanee River Fulvic Acid (67μmol/g). Our direct evidence that Bangladesh FAs are capable of intermediate electron transfer to Fe(III) supports other studies that implicate electron shuttling by sediment-derived aqueous humics to enhance Fe reduction and, in turn, As mobility. Overall, the finding of greater electron accepting capacity by dissolved FAs from groundwater and other sediment-influenced environments advances our understanding of mechanisms that control Fe reduction under conditions where electron transfer is the rate limiting step.
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•Fulvic acids from Bangladesh groundwater (BFA) were capable of electron shuttling to Fe(III).•Bangladesh FA had ~3× higher electron accepting capacity than Suwanee River FA.•Sedimentary sources, low C:N, and high aromaticity may contribute to high electron accepting capacity of BFAs.•Electron shuttling by BFAs may influence arsenic mobilization in reducing groundwater.
A better understanding is needed of how hydrological and biogeochemical processes control dissolved organic carbon (DOC) concentrations and dissolved organic matter (DOM) composition from headwaters ...downstream to large rivers. We examined a large DOM dataset from the National Water Information System of the US Geological Survey, which represents approximately 100 000 measurements of DOC concentration and DOM composition at many sites along rivers across the United States. Application of quantile regression revealed a tendency towards downstream spatial and temporal homogenization of DOC concentrations and a shift from dominance of aromatic DOM in headwaters to more aliphatic DOM downstream. The DOC concentration–discharge (C-Q) relationships at each site revealed a downstream tendency towards a slope of zero. We propose that despite complexities in river networks that have driven many revisions to the River Continuum Concept, rivers show a tendency towards chemostasis (C-Q slope of zero) because of a downstream shift from a dominance of hydrologic drivers that connect terrestrial DOM sources to streams in the headwaters towards a dominance of instream and near-stream biogeochemical processes that result in preferential losses of aromatic DOM and preferential gains of aliphatic DOM.
Northern ecosystems are experiencing some of the most dramatic impacts of global change on Earth. Rising temperatures, hydrological intensification, changes in atmospheric acid deposition and ...associated acidification recovery, and changes in vegetative cover are resulting in fundamental changes in terrestrial–aquatic biogeochemical linkages. The effects of global change are readily observed in alterations in the supply of dissolved organic matter (DOM)—the messenger between terrestrial and lake ecosystems—with potentially profound effects on the structure and function of lakes. Northern terrestrial ecosystems contain substantial stores of organic matter and filter or funnel DOM, affecting the timing and magnitude of DOM delivery to surface waters. This terrestrial DOM is processed in streams, rivers, and lakes, ultimately shifting its composition, stoichiometry, and bioavailability. Here, we explore the potential consequences of these global change‐driven effects for lake food webs at northern latitudes. Notably, we provide evidence that increased allochthonous DOM supply to lakes is overwhelming increased autochthonous DOM supply that potentially results from earlier ice‐out and a longer growing season. Furthermore, we assess the potential implications of this shift for the nutritional quality of autotrophs in terms of their stoichiometry, fatty acid composition, toxin production, and methylmercury concentration, and therefore, contaminant transfer through the food web. We conclude that global change in northern regions leads not only to reduced primary productivity but also to nutritionally poorer lake food webs, with discernible consequences for the trophic web to fish and humans.
Global change effects are manifested in alterations to dissolved organic matter ‐ a messenger between terrestrial and aquatic ecosystems. Here, we explore the potential consequences of changing dissolved organic matter concentration and composition for lake food webs at northern latitudes. We conclude that increasing loads of refractory dissolved organic matter will adversely affect northern lake food webs by reducing primary production and the available pool of high quality fatty acids, and by increasing the risk of food‐web exposure to cyanobacteria toxins and methylmercury..
In the western USA, one legacy of historic mining is drainage of acidic, metal-rich water generated by exposure to oxygen of sulfide minerals in mine workings, referred to as acid mine drainage ...(AMD). Streams receiving AMD and natural acid rock drainage (ARD) have a low pH, high dissolved metal concentrations, and extensive streambed oxide deposits. Recently, enhanced ARD generation in the Snake River watershed in the Rocky Mountains has been shown to be associated with warmer summer air temperatures, which has been attributed to expanding weathering fronts that promote oxidation due to earlier drying of shallow soils. In mountain watersheds where complex orogeny disseminated minerals throughout the landscape, weathering processes may also mobilize rare earth elements (REEs). We report that in the Snake River REEs are currently distributed in streams at concentrations ranging from 1 to 100 μg/L. Further, analysis of archived sample indicates that REE increases over time are also associated with increased summer air temperatures. In downstream reaches where the Snake River discharges into a water supply reservoir, colloidal and particulate metal oxides are abundant and sorptive processes may influence REE speciation. We also show that REEs accumulate in benthic invertebrates at concentrations comparable to toxic metals associated with ARD.
Advances in hydrology are greatly needed and approaches that employ hypotheses to guide research have the potential to contribute to future advances. In this context, hypotheses can serve a range of ...purposes. Overarching hypotheses can provide a common integrating framework for collaborative research and can be revised as research progresses over time. Hypotheses that attempt to explain unexpected field observations or experimental results can provide a guide for designing further field studies. Focused testable hypotheses can facilitate effective presentation of proposed research, and clarify alternative hypotheses. Finally, the value of employing a hypothesis‐based approach depends upon the research environment, which can act as an “environmental filter” in determining successful research outcomes.
Key Points
Hydrologic hypotheses can contribute to advances in research depending upon the research environment
Overarching hypotheses and hypotheses to explain unexpected observations are particularly useful
Well‐instrumented, long‐term research programs can promote development of new hypotheses in hydrology
The spectral characteristics of whole water dissolved organic matter (DOM) and fulvic acid were studied in samples collected from an alpine lake, a subalpine lake, and a subalpine stream during ...snowmelt and the summer growing season. Excitation‐emission matrices of whole water DOM and fulvic acid were analyzed by parallel factor analysis (PARAFAC). Allochthonous inputs of terrestrially derived fulvic acid DOM were dominant during snowmelt at the alpine lake, and during both snowmelt and summer at the subalpine sites. At the alpine lake, autochthonous inputs of DOM dominated during the summer phytoplankton bloom, and the spectral characteristics of the whole water DOM diverged from those of the fulvic acid. For example, the quinone‐like fluorophores in whole water DOM at the alpine lake were more oxidized and microbially derived than the fulvic acid fraction during the summer. At the subalpine sites, the seasonal changes in the source and redox state of the quinone‐like fluorophores of the whole water DOM tracked those of the fulvic acid pool. However, at both lake sites there was a greater contribution of amino acid‐like fluorophores in the whole water DOM than the fulvic acid fraction. This trend was not observed at the subalpine stream site. Principal components analysis (PCA) of the PARAFAC components suggests that during snowmelt, the chemical quality of the DOM at the alpine lake was similar to that of the subalpine stream; whereas the alpine site was more similar to the subalpine lake during the summer. Spectral characterization and PCA of the PARAFAC components suggest that nonhumic quinone‐like and amino acid‐like fluorophores were produced in the alpine lake during the summer phytoplankton bloom. Our results show that different types of water bodies produce different seasonal patterns in whole water DOM and fulvic acid quantity and quality.
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