Dissolved Organic Matter In Surface Waters Research Articles

Carbonate-Associated Organic Matter Is a Detectable Dissolved Organic Matter Source in a Subtropical Seagrass Meadow

Seagrasses can enhance carbonate sediment dissolution on diel timescales through oxidation of the rhizosphere and production of acidic exudates of dissolved organic matter (DOM). Carbonates can also associate with DOM either from biogenesis or later adsorption. However, the impact of mineral dissolution on the release of carbonate-associated DOM and on surface water DOM quantity and quality is unclear. We analyzed sub-daily changes in EEMS-PARAFAC components (excitation-emission matrices with parallel factor analysis), fluorescence, and absorbance properties of surface waters over adjacent low- and high-density (LD and HD) Thalassia testudinum seagrass meadows in Florida Bay, USA. We compared fluorescent DOM characteristics of seagrass leaves, acidified (dissolved) sediment leachates, and surface water samples collected from the HD and LD sites with surface water from a nearby mangrove island. The HD site was higher in humic-like PARAFAC components, specific ultraviolet absorbance, and humification index. We did not observe changes in EEMs indices or PARAFAC components with cumulative photosynthetically active radiation, indicating that photodegradation was unlikely to contribute to temporal variability in DOM. Similarities among DOM optical properties from acidified sediment leachates and surface waters at both sites suggest the importance of carbonate dissolution/reprecipitation for DOM cycling, while seagrass leaf leachates were markedly dissimilar to surface waters. We observed similarities among the acidified sediment leachate, surface water, and porewater elsewhere in Florida Bay, indicating dynamic coupling between these DOM pools. From this short study, Florida Bay DOM cycling appears to be more sensitive to carbonate dissolution than to additional photodegradation or authigenic seagrass leaching.

Labile Dissolved Organic Matter Compound Characteristics Select for Divergence in Marine Bacterial Activity and Transcription.

Bacteria play a key role in the planetary carbon cycle partly because they rapidly assimilate labile dissolved organic matter (DOM) in the ocean. However, knowledge of the molecular mechanisms at work when bacterioplankton metabolize distinct components of the DOM pool is still limited. We, therefore, conducted seawater culture enrichment experiments with ecologically relevant DOM, combining both polymer and monomer model compounds for distinct compound classes. This included carbohydrates (polysaccharides vs. monosaccharides), proteins (polypeptides vs. amino acids), and nucleic acids (DNA vs. nucleotides). We noted pronounced changes in bacterial growth, activity, and transcription related to DOM characteristics. Transcriptional responses differed between compound classes, with distinct gene sets (“core genes”) distinguishing carbohydrates, proteins, and nucleic acids. Moreover, we found a strong divergence in functional transcription at the level of particular monomers and polymers (i.e., the condensation state), primarily in the carbohydrates and protein compound classes. These specific responses included a variety of cellular and metabolic processes that were mediated by distinct bacterial taxa, suggesting pronounced functional partitioning of organic matter. Collectively, our findings show that two important facets of DOM, compound class and condensation state, shape bacterial gene expression, and ultimately select for distinct bacterial (functional) groups. This emphasizes the interdependency of marine bacteria and labile carbon compounds for regulating the transformation of DOM in surface waters.

Characterisation of groundwater dissolved organic matter using LC[sbnd]OCD: Implications for water treatment

The polarity and molecular weight of dissolved organic matter (DOM) is an important factor determining the treatability of water for domestic supply. DOM in surface water and groundwater is comprised of a mixture of carbon with varying molecular weight ranges, with its composition driven by DOM sources and processing. Here, we present the largest dataset of chromatographic DOM in surface and groundwater samples (n = 246) using liquid chromatography organic carbon detection (LCOCD). Our data represents four categories (surface water, hyporheic zone water, local groundwater, and regional groundwater) from five different sites across Australia. In all environments, high molecular weight hydrophilic DOM such as biopolymers (BP) and humic substances (HS) are present in surface waters and are processed out of groundwater as it moves from surface water and hyporheic zones into shallow local groundwater and deeper regional groundwaters. This results in a higher percentage of low molecular weight neutrals (LMWN) and hydrophobic organic carbon (HOC) in deeper regional groundwaters. Our findings indicate that the presence of sedimentary organic matter strongly influence the character of surface and groundwater DOM, resulting in groundwater with higher HS aromaticity and molecular weight, and reduced percentage of LMWNs. We also observe highly variable hydrophilic / HOC ratios in groundwater at all sites, with 9.60% and 25.64% of samples at sites containing sedimentary peat layers and non-sedimentary peat sites respectively containing only hydrophilic dissolved organic carbon (DOC). We identify average hydrophilic / HOC ratios of 4.35 ± 3.76 and 7.53 ± 5.32 at sites containing sedimentary peat layers and non-sedimentary peat sites respectively where both hydrophilic DOC and HOC are present. Overall our results suggest that fractured rock and alluvial aquifers in sedimentary organic carbon poor environments may contain DOC which is better suited to ozonation, biologically activated carbon filtration powdered activated carbon, suspended ion exchange treatment or magnetic ion exchange resin since DOC is more hydrophilic and of lower molecular weight and lower aromaticity. Aquifers located near sedimentary organic matter layers may benefit from pre-treatment by coagulation/flocculation, sedimentation and sand filtration which have high removal efficiency for high molecular weight and polar compounds.

Electrostratigraphy and hydrogeochemistry of hyporheic zone and water-bearing caches in the littoral shorefront of Akwa Ibom State University, Southern Nigeria.

Ground-based electrical geophysical data calibrated with borehole information are conveniently used to delineate subsurface strata because of their inherent capability to assess the lateral and vertical variations in the pore water. In this study, joined geophysical approach of vertical electrical sounding (VES) and electrical resistivity tomography (ERT) has been steered to define the strata in the hyporheic zone and in the water bearing caches in the Akwa Ibom State University's littoral shorefront. Four ERTs with each using Wenner array with 5m electrode spacing were conducted along four profiles at the same locations that VES were conducted. Twelve surface VES soundings were performed with maximum current electrode separations of (AB/2 = 150m). The integration of formation resistivity with six boreholes reveals motley topsoil/dry strata with resistivity value greater than 200Ω - m above water table; saturated clay/saline water depository with resistivity value less than 30 Ω - m below water table; fine-grained sand/brackish water depository with resistivity range spanning between 70 and 200Ω - m below water table; medium-grained sand/freshwater depository with resistivity ranging from 500 to 800Ω - m below water table and gravelly sand/freshwater depository with resistivity value greater 800Ω - m below water table were inferred from top to bottom within the maximum current electrode separations. These ranges of resistivity show lithological diversity in subsurface layer. Geochemical analysis was performed for main cations (magnesium, sodium, potassium, calcium, iron and manganese), anions (bicarbonates, sulphates, chloride, and fluoride) and other physical parameters such as, pH, electrical conductivity, total dissolved solids, dissolved oxygen, biochemical oxygen demand and chemical oxygen demand. The results of the interpretation of hydrochemical species of the groundwater samples revealed that the groundwater in most locations within the study area is fresh, slightly alkaline to acidic based on the EC, pH and TDS values. The order of abundance for anions and cations is HCO3- > Cl- > SO42- > F- and Na+ > Ca2+ > Mg2+ > K+ > Fe2+ > Mn2+ respectively. The observation of elevated BOD with lower DO even in the muddy area suggests anoxic condition (DO < 5mg/L) rather than oxic condition (DO > 5mg/L), based on the measured DO values (00.12-2.61mg/L). The elevated ferric iron concentrations on the surface water, which later seeps into the groundwater systems, are due to excessive accumulation of dissolved organic matter (DOM) and the consequent reduction reaction within the DOM in surface water.

Processing of dissolved organic matter from surface waters to sediment pore waters in a temperate coastal wetland

Coastal wetlands are active transitional ecotones between land and ocean, and are considered as hot spots of organic matter processing within the global carbon cycle, which dissolved organic matter (DOM) plays a critical role. In this study, combined use of ultrahigh-resolution mass spectrometry (FT-ICR MS) and complementary optical techniques was conducted to assess the detailed molecular composition of DOM in the temperate Liaohe coastal wetland (LCW), NE China in respect to the differences in DOM composition from surface water to sediment pore water. Significant positive correlations between salinity and dissolved organic carbon (DOC) concentrations were observed in both surface waters and pore waters. Pore water DOM is generally characterized by lower protein-like fluorescence and biological index, but higher humification and humic-like fluorescent components than those in surface water DOM. Corresponding to the optical properties, FT-ICR MS measurements show that pore water DOM has higher proportions of heteroatoms, aromaticity index, O/C ratios, unsaturated aliphatics, and peptides, but lower average H/C ratios compared to surface water DOM across locations with different marsh plant species (rice (Oryza sativa), reed (Phragmites australis), Seablite (Suaeda Salsa)) and salinity (0.5 to 51.5 psu). The results suggest that selective preservation for polyphenols, lignin degradation intermediates (highly unsaturated compounds), and microbial resynthesis of heteroatomic compounds are involved in the processing of DOM from surface water to pore water, leading to the formation of higher molecular weight and sulfur-containing molecules. The abundant CHOS compounds could be related to the early diagenetic sulfurization of DOM in sediments. Our unique data set should provide new clues for a comprehensive understanding of the molecular dynamics of DOM in coastal wetlands.

Molecular-Level Chemical Characterization of Dissolved Organic Matter in the Ice Shelf Systems of King George Island, Antarctica

Antarctic shelf systems play an important role in organic matter circulation on Earth; hence, identifying the characteristics of dissolved organic matter (DOM) can be a good indicator for understanding its origin, as well as climate change. In this study, to identify the characteristics of DOM in the ice shelf systems, surface water was collected from the open sea (OS) and Marian cove (Fjord; FJ). Although there were no differences in DOM characteristics between sampling sites in quantitative analyses, the DOM in surface water of each region seemed to be more affected by terrestrial than marine biological sources in optical and molecular properties. This finding indicates that the terrestrial DOM related to mosses based on the molecular properties results; high levels of lipid-like (35–39%) and unsaturated hydrocarbon-like (UH; 27–34%) in both the OS and FJ regions, and significantly higher tannin-like substance and condensed aromatic structures (CAS) in the FJ than the OS region. When comparing the FJ transect samples, those nearest to a glacier (FJ1; 0.93 km from the glacier) showed relatively low salinity, high dissolved organic carbon (DOC), and high chromophoric DOM (CDOM), indicating that terrestrial DOM (possibly produced by moss) inflow occurred with the runoff from the freshly melting land ice and glacier. However, no significant differences in molecular composition were detected, suggesting that terrestrial DOM introduced into the ice shelf systems by melting land ice and glacier runoff could be a major source of DOM-rich seawater during austral fall, when low marine biological activity occurs. This study has a great significance as background data for DOM characteristics in the ice shelf systems due to the enhanced biological activity during the austral summer.

Optical Properties of Dissolved Organic Matter and Controlling Factors in Dianchi Lake Waters

Characterization of dissolved organic matter (DOM) is useful in understanding environment quality and carbon cycling in the lake system. In this study, the fluorescence of DOM, major ions, and nutrients in water were investigated to understand the sources and the transformation of DOM in Dianchi Lake, the sixth largest freshwater lake in China. The dissolved organic carbon content in water above the deposition layer was higher than 5 mg C∙L−1 but lower than that in pore water. Two primary components of humic (C1) and protein-like components (C2) were identified using parallel factor (PARAFAC) modeling on sample fluorescence spectra. Organic components were related to mineral structures, and encapsulation of bacterial or algal cells into particulates could be disintegrated to release DOM. The aromaticity and the hydrophobicity of optical properties were regulated by percentages of chromophores (CDOM) of DOM in surface water, whereas by percentages of fluorophores (FDOM) in DOM in pore water, the underlying water layer was defined as a belt of transition. The molecular weight enhanced with percentages of C1 in CDOM increased in water above the sediment layer and the pore water at the northern lake site, but molecular weight attenuated with percentages decreased in pore water at the southern lake site. DOM not only originated from particulate decomposition but also derived from internal transformation among different, dissolved organic molecules. Small molecules were aggregated into larger ones, and, conversely, large molecules decomposed into small sizes. Another speculation is that dissolved molecules adsorbed or were encapsulated into particulates or were degraded and released into dissolved phases. The precise factors regulated composition, structure, and spectral properties of dissolved organic matter in the Dianchi Lake. This study highlights that sources of DOM and transformation mechanisms in the lake water could be correlated with nutrients and primary geochemical factors for mobility and distribution in different water compartments.

Onsite quantifying electron donating capacity of dissolved organic matter

Electron donating capacity (EDC) of dissolved organic matter (DOM) impacts the redox behaviors of DOM in surface waters, groundwaters, wetlands, sediments and soils but lacks applicable onsite quantification methods. To address these disadvantages, a simple and portable device with pre-injected [2,2′-azino-bis(3-ethylbenzothiazoline-6-sulfonicacid), ABTS·+] was developed that can be used for EDC onsite measurements of DOM in this work. The proposed device and method had better limits of quantification of Trolox (0.2 nmol) and more flexible DOC concentration requirement of 0.5–20 mg L−1 than that of flow injection analysis (FIA) (5–10 mg L−1) or mediated electrochemical oxidation (MEO) (>20 mg L−1). The proposed device and method greatly reduced the preparation and measurement time for sample tests compared to MEO or FIA method, enabling time-efficient EDC determination for large amount of samples. Meanwhile, the proposed device presented comparable accuracy with established MEO method when quantifying the EDCs of 7 standard humic and fulvic acids. Humic acids with higher molecular weight (MW) (<15,000 Da) had higher EDC than that with low MW (<5000 Da). EDCs of DOM in natural and reclaim water samples were both presented significantly positive correlations with their corresponding chemical oxygen demand, chromophoric DOM content, molecular weight and humification of the DOM in water samples. These results suggested that our device could accurately quantify the EDCs of DOM onsite and had promising applications on the fast quality assessment of natural and reclaimed waters.

Effect of dissolved organic matter fractions on photodegradation of phenanthrene in ice.

The effect of dissolved organic matter (DOM) fractions on photodegradation of phenanthrene (PHE) in ice was investigated. DOM in surface water and wastewater samples was fractionated using XAD-8/XAD-4 resins into five fractions: hydrophobic acid (HPO-A), hydrophobic neutral (HPO-N), transphilic acid (TPI-A), transphilic neutral (TPI-N) and hydrophilic fraction (HPI). The photodegradation rate of PHE in ice was about 40% greater than that in water. The screening effect and quenching effect contributed 3-12% and 88-97% toward the inhibition of DOM fractions on PHE photodegradation in ice, respectively. The contribution ratios of singlet oxygen (1O2) and hydroxyl radical (OH) produced from DOM fractions to PHE photodegradation rates in ice were 9-31% and 2-13%, respectively. Among five DOM fractions, HPO-A was most efficient in advancing PHE photodegradation in ice through 1O2 mechanism. When excluding the photosensitized effect of 1O2 and OH produced from DOM fractions, the quencing effect of DOM fractions on PHE photodegradation in ice was closely related to their PHE binding ability.

Effect of silver sulfide nanoparticles on photochemical degradation of dissolved organic matter in surface water

Silver sulfide nanoparticles (Ag2SNPs) have shown photocatalytic activity, yet little is known about the effect of Ag2SNPs on the photochemical degradation of dissolved organic matter (DOM) in surface water, which seriously impairs understanding of Ag2SNPs' environmental risks. Herein, this study on the basis of electrospray ionization coupled with Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FTICR-MS) showed for the first time that photodegradation of natural organic matter (NOM, 2R101 N) could be accelerated by both bared and polyvinylpyrrolidone (PVP)-coated Ag2SNPs; the NOM with Ag2SNPs (e.g., 500 μg/L) exposed to light irradiation for 96 h showed molecular formulas with lower O/C ratios as compared to the NOM alone. Also, added number of points (ranging from 1 to 2 carboxyl groups) having the same Kendrick mass defect (KMD) (COO) values and higher intensity in smaller Kendrick mass (KM) (COO) values were observed in NOM with Ag2SNPs compared to NOM alone. However, negligible effects of Ag2SNPs on photodegradation of humic acid (HA, 2S101H) were observed, even when the concentration of Ag2SNPs was as high as 5 mg/L. Besides molecular characteristics, a great reduction in organic carbon content of NOM within 96 h was only observed in the presence of Ag2SNPs under light condition. More importantly, the enhanced photodegradation of DOM by Ag2SNPs even at a concentration of 100 μg/L was also validated in surface water. These findings suggest that Ag2SNPs have the potential to accelerate the photochemical degradation of DOM in surface water.

Transport and transformation of dissolved organic matter in the Neuse River estuarine system, NC, USA, following Hurricane Irene (2011)

Rivers are major conduits for the transport of allochthonous dissolved organic matter (DOM) to the ocean in coupled land–coastal systems. DOM can regulate biogeochemical processes and affect water quality, depending on the concentration and quality of DOM. By using spectral parameters calculated from chromophoric dissolved organic matter (CDOM) ultraviolet-visible absorption spectra, along with dissolved organic carbon (DOC) concentrations, we examined the input and change in the amount and quality of DOM in surface waters of the lower Neuse River and upper–middle regions of the Neuse Estuary following a major rainfall (30cm in 1 day) associated with Hurricane Irene (2011). CDOM and DOC nearly tripled in the 3 days following the storm. Although a strong linear relationship was observed between DOC and CDOM absorption coefficient at 350nm (R2=0.85), a higher fraction of non-chromophoric DOC to CDOM was observed during the rising river discharge. The spectral slope at 275–295nm and the slope ratio (275–295:350–400nm) indicated a shift from higher to lower molecular-weight DOM as it transited through the estuary, probably as a result of photodegradation. The present study demonstrated the utility of using CDOM spectral parameters for examining the flux and transformation of DOM in coastal waters following major rain events.

Controls on dissolved organic matter (DOM) degradation in a headwater stream: the influence of photochemical and hydrological conditions in determining light-limitation or substrate-limitation of photo-degradation

Abstract. We investigated how absorption of sunlight by chromophoric dissolved organic matter (CDOM) controls the degradation and export of DOM from Imnavait Creek, a beaded stream in the Alaskan Arctic. We measured concentrations of dissolved organic carbon (DOC), as well as concentrations and characteristics of CDOM and fluorescent dissolved organic matter (FDOM), during ice-free periods of 2011–2012 in the pools of Imnavait Creek and in soil waters draining to the creek. Spatial and temporal patterns in CDOM and FDOM in Imnavait Creek were analyzed in conjunction with measures of DOM degradation by sunlight and bacteria and assessments of hydrologic residence times and in situ UV exposure. CDOM was the dominant light attenuating constituent in the UV and visible portion of the solar spectrum, with high attenuation coefficients ranging from 86 ± 12 m−1 at 305 nm to 3 ± 1 m−1 in the photosynthetically active region (PAR). High rates of light absorption and thus light attenuation by CDOM contributed to thermal stratification in the majority of pools in Imnavait Creek under low-flow conditions. In turn, thermal stratification increased the residence time of water and DOM, and resulted in a separation of water masses distinguished by contrasting UV exposure (i.e., UV attenuation by CDOM with depth resulted in bottom waters receiving less UV than surface waters). When the pools in Imnavait Creek were stratified, DOM in the pool bottom water closely resembled soil water DOM in character, while the concentration and character of DOM in surface water was reproduced by experimental photo-degradation of bottom water. These results, in combination with water column rates of DOM degradation by sunlight and bacteria, suggest that photo-degradation is the dominant process controlling DOM fate and export in Imnavait Creek. A conceptual model is presented showing how CDOM amount and lability interact with incident UV light and water residence time to determine whether photo-degradation is "light-limited" or "substrate-limited". We suggest that degradation of DOM in CDOM-rich streams or ponds similar to Imnavait is typically light-limited under most flow conditions. Thus, export of DOM from this stream will be less under conditions that increase the light available for DOM photo-degradation (i.e., low flows, sunny days).

Dissolved Organic Matter Quality in a Shallow Aquifer of Bangladesh: Implications for Arsenic Mobility.

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 (13)C 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.

Long-term effect of a simple storage protocol on fluorescence fingerprint of dissolved organic matter

Fitting a pre-established region-specific Parallel Factor Analysis (PARAFAC) model to new dissolved organic matter (DOM) samples has become a popular approach in DOM studies. A key step during the development of the pre-established model is to identify the fluorescence fingerprint, i.e. the number of fluorescent DOM (FDOM) components as well as their excitation and emission spectral features. In practice the samples to use for model development may not be measured immediately after sampling and will instead be stored for a relatively long time, thus raising the question whether the storage would change the intrinsic fingerprints. In this study, two PARAFAC models were separately developed and validated for the same set of surface water DOM samples from an estuary and its river, immediately after sampling and after 9-month storage respectively. The fingerprint did not change after storage, regardless of the change of the intensity of some components. The findings in this study highlighted that DOM samples stored using a simple protocol (i.e. filtration to 0.45 μm pore size without any preservatives and placed at 4 °C in the dark under airtight conditions) for a few months would not change fluorescence fingerprints for PARAFAC and broaden our understanding of the application of PARAFAC to DOM studies.

Ultrahigh resolution mass spectrometric differentiation of dissolved organic matter isolated by coupled reverse osmosis-electrodialysis from various major oceanic water masses

A high-recovery technique of dissolved organic matter (DOM) isolation – reverse osmosis coupled with electrodialysis (RO/ED) – was used to isolate DOM from the North Atlantic Senegal-Mauritanian upwelling area surface water (5m), North Atlantic oxygen minimum water (415m) and deep water (3000m), North Pacific subtropical gyre surface water (5m), and North Pacific intermediate water (674m) and deep water (3500m). Samples were characterized by ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry with electrospray ionization (ESI FTICR-MS). RO/ED isolated DOM samples share a significant number of common formulas accounting for 54–79% of formulas in each sample. Total dissolved carbohydrate (TCHO) concentrations in RO/ED isolated DOM were specifically measured using a colorimetric method, and were found to have higher contribution to DOC than estimated by FTICR-MS data. Percentages of TCHO-C in DOC are in the range of 3.7–19.6% in all samples, with the North Pacific deep (3500m) water having the lowest % and the North Atlantic upwelling core surface water having the highest %. Principal component analysis (PCA) using the relative magnitudes of MS peaks facilitated identification of specific peaks that are enriched in different samples. Peaks enriched in surface samples have higher H/C values than peaks enriched in deep samples, in both the North Atlantic DOM and the North Pacific DOM. This enrichment pattern is likely due to the selective photo-degradation of aromatic compounds and the bio-production of aliphatic and carbohydrate-like compounds in surface waters, and the selective bio-degradation of aliphatic and carbohydrate-like compounds with increasing depth. In further support of a photo-degraded signature for DOM in surface waters, photo-resistant and photo-produced molecular formulas were present in the highest numbers in the surface North Pacific subtropical gyre DOM. Peaks enriched in the North Pacific intermediate and deep DOM have significantly higher O/C values than the North Atlantic oxygen minimum layer and deep DOM, for both CHO formula compounds and CHON formula compounds. This difference in O/C values observed for the deep Pacific vs. Atlantic suggests oxidation of DOM, possibly via microbial activity during the ageing of DOM or the preferential remineralization of DOM from sinking particles at depth in the Pacific.

Inner filter correction of dissolved organic matter fluorescence

The fluorescence of dissolved organic matter (DOM) is suppressed by a phenomenon of self‐quenching known as the inner filter effect (IFE). Despite widespread use of fluorescence to characterize DOM in surface waters, the advantages and constraints of IFE correction are poorly defined. We assessed the effectiveness of a commonly used absorbance‐based approach (ABA), and a recently proposed controlled dilution approach (CDA) to correct for IFE. Linearity between corrected fluorescence and total absorbance (ATotal; the sum of absorbance at excitation and emission wavelengths) across the full excitation‐emission matrix (EEM) in dilution series of four samples indicated both ABA and CDA were effective to an absorbance of at least 1.5 in a 1 cm cell, regardless of wavelength positioning. In regions of the EEMs where signal to background noise (S/N) was low, CDA correction resulted in more variability than ABA correction. From the ABA algorithm, the onset of significant IFE (&gt;5%) occurs when absorbance exceeds 0.042. In these cases, IFE correction is required, which was the case for the vast majority (97%) of lakes in a nationwide survey (n= 554). For highly absorbing samples, undesirably large dilution factors would be necessary to reduce absorbance below 0.042. For rare EEMs with ATotal &gt; 1.5 (3.0% of the lakes in the Swedish survey), a 2‐fold dilution is recommended followed by ABA or CDA correction. This study shows that for the vast majority of natural DOM samples the most commonly applied ABA algorithm provides adequate correction without prior dilution.

Molecular transformation and degradation of refractory dissolved organic matter in the Atlantic and Southern Ocean

More than 90% of the global ocean dissolved organic carbon (DOC) is refractory, has an average age of 4000–6000years and a lifespan from months to millennia. The fraction of dissolved organic matter (DOM) that is resistant to degradation is a long-term buffer in the global carbon cycle but its chemical composition, structure, and biochemical formation and degradation mechanisms are still unresolved. We have compiled the most comprehensive molecular dataset of 197 Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) analyses from solid-phase extracted marine DOM covering two major oceans, the Atlantic sector of the Southern Ocean and the East Atlantic Ocean (ranging from 50°N to 70°S). Molecular trends and radiocarbon dating of 34 DOM samples (comprising Δ14C values from −229‰ to −495‰) were combined to model an integrated degradation rate for bulk DOC resulting in a predicted age of >24ka for the most persistent DOM fraction. First order kinetic degradation rates for 1557 mass peaks indicate that numerous DOM molecules cycle on timescales much longer than the turnover of the bulk DOC pool (estimated residence times of up to ~100ka) and the range of validity of radiocarbon dating. Changes in elemental composition were determined by assigning molecular formulae to the detected mass peaks. The combination of residence times with molecular information enabled modelling of the average elemental composition of the slowest degrading fraction of the DOM pool. In our dataset, a group of 361 molecular formulae represented the most stable composition in the oceanic environment (“island of stability”). These most persistent compounds encompass only a narrow range of the molecular elemental ratios H/C (average of 1.17±0.13), and O/C (average of 0.52±0.10) and molecular masses (360±28 and 497±51Da). In the Weddell Sea DOC concentrations in the surface waters were low (46.3±3.3μM) while the organic radiocarbon was significantly more depleted than that of the East Atlantic, representing average surface water DOM ages of 4920±180a. These results are in accordance with a highly degraded DOM in the Weddell Sea surface water as also shown by the molecular degradation index IDEG obtained from FT-ICR MS data. Further, we identified 339 molecular formulae which probably contribute to an increased DOC concentration in the Southern Ocean and potentially reflect an accumulation or enhanced sequestration of refractory DOC in the Weddell Sea. These results will contribute to a better understanding of the persistent nature of marine DOM and its role as an oceanic carbon buffer in a changing climate.

Comparative study of dissolved organic matter from groundwater and surface water in the Florida coastal Everglades using multi-dimensional spectrofluorometry combined with multivariate statistics

Dissolved organic matter (DOM) in groundwater and surface water samples from the Florida coastal Everglades were studied using excitation–emission matrix fluorescence modeled through parallel factor analysis (EEM-PARAFAC). DOM in both surface and groundwater from the eastern Everglades S332 basin reflected a terrestrial-derived fingerprint through dominantly higher abundances of humic-like PARAFAC components. In contrast, surface water DOM from northeastern Florida Bay featured a microbial-derived DOM signature based on the higher abundance of microbial humic-like and protein-like components consistent with its marine source. Surprisingly, groundwater DOM from northeastern Florida Bay reflected a terrestrial-derived source except for samples from central Florida Bay well, which mirrored a combination of terrestrial and marine end-member origin. Furthermore, surface water and groundwater displayed effects of different degradation pathways such as photodegradation and biodegradation as exemplified by two PARAFAC components seemingly indicative of such degradation processes. Finally, Principal Component Analysis of the EEM-PARAFAC data was able to distinguish and classify most of the samples according to DOM origins and degradation processes experienced, except for a small overlap of S332 surface water and groundwater, implying rather active surface-to-ground water interaction in some sites particularly during the rainy season. This study highlights that EEM-PARAFAC could be used successfully to trace and differentiate DOM from diverse sources across both horizontal and vertical flow profiles, and as such could be a convenient and useful tool for the better understanding of hydrological interactions and carbon biogeochemical cycling.

High‐frequency in situ optical measurements during a storm event: Assessing relationships between dissolved organic matter, sediment concentrations, and hydrologic processes

Dissolved organic matter (DOM) dynamics during storm events has received considerable attention in forested watersheds, but the extent to which storms impart rapid changes in DOM concentration and composition in highly disturbed agricultural watersheds remains poorly understood. In this study, we used identical in situ optical sensors for DOM fluorescence (FDOM) with and without filtration to continuously evaluate surface water DOM dynamics in a 415 km2 agricultural watershed over a 4 week period containing a short‐duration rainfall event. Peak turbidity preceded peak discharge by 4 h and increased by over 2 orders of magnitude, while the peak filtered FDOM lagged behind peak turbidity by 15 h. FDOM values reported using the filtered in situ fluorometer increased nearly fourfold and were highly correlated with dissolved organic carbon (DOC) concentrations (r2 = 0.97), providing a highly resolved proxy for DOC throughout the study period. Discrete optical properties including specific UV absorbance (SUVA254), spectral slope (S290–350), and fluorescence index (FI) were also strongly correlated with in situ FDOM and indicate a shift toward aromatic, high molecular weight DOM from terrestrially derived sources during the storm. The lag of the peak in FDOM behind peak discharge presumably reflects the draining of watershed soils from natural and agricultural landscapes. Field and experimental evidence showed that unfiltered FDOM measurements underestimated filtered FDOM concentrations by up to ∼60% at particle concentrations typical of many riverine systems during hydrologic events. Together, laboratory and in situ data provide insights into the timing and magnitude of changes in DOM quantity and quality during storm events in an agricultural watershed, and indicate the need for sample filtration in systems with moderate to high suspended sediment loads.

Dissolved organic matter characteristics within the Lake Superior watershed

Within the aquatic carbon cycle, dissolved organic carbon (DOC) plays key roles: as a link between terrestrial and aquatic systems, a major food source for biota, a player in photochemical reactions and a sunscreen/competitor for light for aquatic organisms. The molecular-level composition of the dissolved organic matter (DOM) is believed to determine the efficacy of DOM in these roles. While much detailed compositional information is available on selected DOM isolates from both freshwater and marine systems, relatively few studies have presented molecular information for multiple sites within the same system and few of these have focused upon large lakes. Therefore, we present an initial study of surface water DOM characteristics in a temperate large lake (Lake Superior, USA) and selected tributaries. UV–visible spectroscopy, electrospray mass spectrometry and Fourier transform infrared spectroscopy (FTIR) analyses of bulk DOM, along with FTIR and direct temperature-resolved mass spectrometry analyses of C 18 extracts, illustrate a shift from higher molecular weight organic matter enriched in protein and lignin at the tributary sites toward compositionally different material in the open lake. Differences in optical characteristics and DOC concentration between tributary and open lake sites are consistent with possible photodegradation of tributary DOM. Comparison of our FTIR data with those from a previous Chesapeake Bay study indicates that DOM in both the freshwater tributaries and the open lake contain a wider range of aromatic and carbohydrate components than that in the saltwater tributary and bay. This highlights the need for further cross system comparisons of natural organic matter composition.