Terrigenous Dissolved Organic Matter Research Articles

Outwelling of reduced porewater drives the biogeochemistry of dissolved organic matter and trace metals in a major mangrove‐fringed estuary in Amazonia

AbstractMangrove‐fringed estuaries are intertidal ecosystems discharging significant amounts of dissolved organic matter (DOM) into coastal oceans. DOM in these ecosystems is derived from autochthonous production, fluvial input, and mangrove porewater outwelling; however, differentiating between these sources remains challenging. Our incomplete understanding of the biogeochemical factors controlling DOM dynamics and its relationship with nutrient and trace metal cycling still hinders the formulation of elemental budgets in coastal environments. Here, we relate the DOM composition in a mangrove‐fringed estuary in North Brazil (Amazonia) to the redox conditions at the formation sites. We combined molecular DOM analyses via ultrahigh‐resolution mass spectrometry (FT‐ICR‐MS) with parallel factor analysis of excitation–emission fluorescence matrices (EEM‐PARAFAC), nutrient and redox‐sensitive trace metal analyses. During low tide, the influx of oxygen‐depleted porewater carried terrigenous DOM, inorganic nutrients, and trace metals into the mangrove‐fringed creeks. Precipitation of metal(hydr)oxides and microbial turnover controlled nutrient and trace metal dynamics in the estuary. The highest inorganic nitrogen concentrations within the upper mangrove‐fringed estuary indicated outwelling from mangrove sediments as an essential source. Phosphate concentrations were highest within the lower mangrove‐fringed estuary, where available phosphate likely exceeded precipitation with iron(hydr)oxides. We tracked the DOM transport to the coastal ocean using a novel molecular index derived from sulfidic porewater (ISuP). Outwelling of recalcitrant DOM from mangrove habitats is relevant in the context of blue carbon storage. Therefore, applying our molecular proxy (ISuP), together with trace‐metal and optical DOM analyses, is a powerful approach for differentiating contributions of diverse DOM sources in highly complex coastal ecosystems.

Dissolved organic matter composition and characteristics during extreme flood events in the Yangtze River Estuary

Understanding the molecular composition and fate of dissolved organic matter (DOM) during transport in estuaries is essential for gaining a comprehensive understanding of its role within the global biogeochemical cycle. In 2020, a catastrophic flood occurred in the Yangtze River basin. It is currently unknown whether differences in hydrologic conditions due to extreme flooding will significantly impact the estuarine to oceanic DOM cycle. We determined the DOM composition in the Yangtze River estuary (YRE) to the East China Sea by using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) during the high discharge and the flood period (monthly average discharge was 1.2 times higher) on the same trajectory. Our study found that the composition of DOM is more diverse, and more DOM molecules were introduced to the YRE during the flood, especially in the freshwater end member. The result revealed that the DOM was significantly labile and unstable during the flood period. A total of 1840 unique molecular formulas were identified during the flood period, most of which were CHON, CHONS, and CHOS compounds, most likely resulting from anthropogenic inputs from upstream. Only 194 of these molecules were detected in the seawater end member after transporting to the sea, suggesting that the YRE served as a ‘filter’ of DOM. However, the flood enhances the transport of a group of terrigenous DOM, that is resistant to photodegradation and biodegradation. As a result, YRE experienced ~1.6 times higher terrigenous DOC flux than high discharge period. Considering the increased frequency of future floods, our study provides a preliminary basis for further research on how floods affect the composition and characteristics of estuarine DOM. With the help of the FT-ICR MS technique, we can now better understand the dynamic of DOM composition and characteristics in large river estuaries.

Land use regulates the spectroscopic properties and sources of dissolved organic matter in the inflowing rivers of a large plateau lake in southwestern China: implication for organic pollution control.

Dissolved organic matter (DOM) transported by inflowing rivers can considerably contribute to the organic loadings of lakes. The current study characterized the DOM properties and source apportionment in the inflowing rivers of Dianchi Lake, the sixth largest freshwater lake in China suffering from organic pollution, during the rainy season by using spectroscopic and carbon stable isotope techniques, and the regulation role of land use was assessed. The results showed that land use (urbanized, agricultural, or mixed) largely affected DOM properties. Greater concentrations and fluorescence intensities of DOM with low aromaticity and dominant autochthonous sources were observed in the urban rivers than in the agricultural rivers. The proportion of humic-like substances increased, while that of tryptophan-like matter decreased from upstream to downstream of two main urban rivers. DOM in the agricultural rivers was characterized by more amounts of aromatic humic-like substances with dominant allochthonous sources compared to that in the urban rivers. Stable isotope analysis showed that the decomposition of macrophytes and input of terrestrial sources from C3 plant-dominated soil and sewage were the major DOM origins in the rivers. The positive linear relationship between the chemical oxygen demand (COD) concentration and fluorescence intensities of terrigenous DOM components implied the necessity of controlling exogenous inputs to alleviate organic pollution in the Dianchi Lake.

Linkages between fluorescent dissolved organic matter and lignin phenols in the Canada Basin, Arctic Ocean

Lignin is one of the more refractory components of dissolved organic matter (DOM) and its concentration and composition are used to trace terrigenous DOM in the ocean. In this study, lignin oxidation products (LOP), and chromophoric and fluorescent DOM were measured in Polar Mixed Layer (PML), Atlantic water (AW), and Canada Basin Deep Water (CBDW) layers in the Canada Basin to elucidate the composition and source of terrestrial DOM. Significant differences in total lignin phenol concentration (Σ11) and cinnamyl to vanillyl ratio (C/V) measured with CuO oxidation in PML, AW, and CBDW samples were found. The fluorescence intensities of humic-like C1 were correlated with Σ11 in PML, congruent with the humic-like terrestrial origin. However, no significant correlation was found when all layers are considered (p > 0.05), suggesting a change in the molecular composition of C1. The protein-like C3 and C5 intensities were strongly correlated with Σ11 (rs = 0.55–0.64) corroborating that lignins can contribute to the protein-like fluorescence in the water column. The principal component analysis (PCA) confirmed the origin and composition of DOM vary between the different water masses, especially in terms of C/V, DOC, and fluorescent components. Significant relationships between humic-like fluorescence intensities and C/V reveal the significant contribution of woody sources to humic material in the Canada Basin. Elevated S/V and humic-like C6 levels were associated with a cold eddy DOM sample but further studies are required to validate the use of these DOM characteristics to trace deep eddy.

Indirect photodegradation of ofloxacin in simulated seawater: Important roles of DOM and environmental factors

Dissolved organic matter (DOM) plays a non-negligible role in the indirect photodegradation of organic contaminants. This research investigated the roles of DOM and the environmental factors (salinity, pH, NO3−, and HCO3−) in the indirect photodegradation of ofloxacin (OFX) in simulated seawater. Results showed that DOM can significantly accelerate the indirect photodegradation of OFX, and 1O2 and 3DOM* were the main reactive intermediates (RIs) that could promote the indirect photodegradation of OFX. Fluorescence excitation–emission matrix spectroscopy–parallel factor analysis (EEMs-PARAFAC) was used to divide DOM into four fluorescence components. The indirect photodegradation rate of OFX was affected by DOM structure, and terrigenous DOM usually produced more RIs to promote the indirect photodegradation of OFX. Increased salinity significantly promotes the indirect photodegradation of OFX, while increased NO3− concentration had no effect on the OFX indirect photodegradation. pH affected the formation of RIs and the structure of OFX, thereby affecting the indirect photodegradation of OFX. The indirect photodegradation rate of OFX increased in the HCO3− solution, which is due to the formation of carbonate radical (CO3−). This study is essential in understanding the role of DOM in OFX indirect photodegradation and providing a novel insight into the fate, removal, and transformation of OFX.

Origins and transformations of terrigenous dissolved organic matter in a transgressive coastal system

Hydrogeological, geochemical, and optical approaches were combined to explore how a transgressive coastal system, where 2-kyr-old organic matter is preserved, contributes to dissolved organic matter (DOM) and dissolved organic carbon (DOC) flux to coastal waters. The sand spit system, located on the St. Lawrence shore (Chaleur Bay, Québec, Canada), was mainly composed of saline groundwater from seawater recirculation, and we estimated submarine groundwater discharge (SGD) velocities fluctuating from 0 during the rising tide to 106 cm d−1 during the ebb tide. The radon-222 (222Rn) activities revealed the presence of two distinct water masses in the spit: a surface recirculated saline groundwater cell with a short residence time and low or no 222Rn activity, and a deeper recirculated saline groundwater with a longer residence time, from a few hours to a few days, and thus exhibited higher 222Rn activities. At the falling tide, the upward flow of this 222Rn-rich groundwater contributes to volumetric discharges ranging from ∼6 to ∼17 m3 d−1, corresponding between 10 and 30% of the total SGD. Both DOC and DOM were produced in the subsurface. However, despite the concentrations being high in the discharge zone, the DOC export remained weak, with mean fluxes of 60.7 (±15.5) and 55.8 (±36.5) mol C yr− 1 in 2018 and 2019, respectively. While surface recirculated saline groundwater dominates the SGD, absorbance and fluorescent indices indicated a strong terrestrial character in the DOM pool likely originated from material sources with a high degree of humification. An EEM-PARAFAC model reveals the dominance of humic-like components of high molecular weights and the occurrence of degraded material, whatever the tidal regime. When the residence time of the groundwater was longer, with higher radon activities, protein-like components with lower molecular weights were microbially produced, probably enhancing the bioavailability of the DOM exported to surrounding seawater. Our results suggest that transgressive coastal systems can be a hot spot for terrigenous DOM transformation which could affect the optical and chemical properties of coastal waters.

Spatial Complexity in Dissolved Organic Matter and Trace Elements Driven by Hydrography and Freshwater Input Across the Arctic Ocean During 2015 Arctic GEOTRACES Expeditions

AbstractThis study traces dissolved organic matter (DOM) in different water masses of the Arctic Ocean and its effect on the distributions of trace elements (TEs; Fe, Cu, Mn, Ni, Zn, Cd) using fluorescent properties of DOM and the terrigenous biomarker lignin. The Nansen, Amundsen, and Makarov Basins were characterized by the influence of Atlantic water and the fluvial discharge of the Siberian Rivers with high concentrations of terrigenous DOM (tDOM). The Canada Basin and the Chukchi Sea were characterized by Pacific water, modified through contact with productive shelf sediments with elevated levels of marine DOM. Within the surface layer of the Beaufort Gyre, meteoric water (river water and precipitation) was characterized by low concentrations of lignin and tDOM fluorescence proxies as DOM is removed during freezing. High‐resolution in situ fluorescence profiles revealed that DOM distribution closely followed isopycnals, indicating the strong influence of sea‐ice formation and melt, which was also reflected in strong correlations between DOM fluorescence and brine contributions. The relationship of DOM and hydrography to TEs showed that terrigenous and marine DOM were likely carriers of dissolved Fe, Ni, Cu from the Eurasian shelves into the central Arctic Ocean. Chukchi shelf sediments were important sources of dCd, dZn, and dNi, as well as marine ligands that bind and carry these TEs offshore within the upper halocline in the Canada Basin. Our data suggest that tDOM components represent stronger ligands relative to marine DOM components, potentially facilitating the long‐range transport of TE to the North Atlantic.

Effect of dissolved organic matter and its fractions on disinfection by-products formation upon karst surface water

In this study, disinfection by-products (DBP) formation from dissolved organic matter (DOM) and its fractions, including both hydrophilic and hydrophobic components, were investigated at a typical karst surface water. The subsequent DBP formation potential was evaluated by deducing chemical characteristics of DOM fractions and representative algal organic matter (Chlorella sp. AOM) under the influence of divalent ions (Ca2+ and Mg2+) via spectra analysis. Both terrigenous and autochthonous DOM performed as critical DBP precursors, and DBP formation patterns were tightly correlated to organic matter chemical variations. DBP formation was significantly higher in drought period compared to that in wet period (P < 0.05). Particularly, trichloromethane (TCM) and dichloroacetonitrile (DCAN) showed distinct formation patterns compared to the scenarios in non-karst water. For DOM fractions, hydrophobic components showed higher DBP formation compared to hydrophilic counterparts, hydrophilic neutral enriched more reactive organic nitrogen for N-DBPs production. It was preferable to enrich humic-like substances after Ca2+ and Mg2+complexation in Chlorella sp. AOM, TCM formation increased whereas DCAN production remained unchanged in the presence of divalent ions. This study innovatively provided a linkage between chemical characteristics of DOM and understanding of DBP formation in karst surface water.

The Effects of Hurricanes and Storms on the Composition of Dissolved Organic Matter in a Southeastern U.S. Estuary

Extreme events such as hurricanes and tropical storms often result in large fluxes of dissolved organic carbon (DOC) to estuaries. Precipitation associated with tropical storms may be increasing in the southeastern U.S., which can potentially impact dissolved organic matter (DOM) dynamics and cycling in coastal systems. Here, DOM composition at the Altamaha River and Estuary (Georgia, U.S.A.) was investigated over multiple years capturing seasonal variations in river discharge, high precipitation events, and the passage of two hurricanes which resulted in substantial storm surges. Optical measurements of DOM indicate that the terrigenous signature in the estuary is linearly related to freshwater content and is similar after extreme events with or without a storm surge and during peak river flow. Molecular level analysis revealed significant differences, however, with a large increase of highly aromatic compounds after extreme events exceeding what would be expected by freshwater content alone. Although extreme events are often followed by increased DOC biodegradation, the terrigenous material added during those events does not appear to be more labile than the remainder of the DOM pool that was captured by ultrahigh-resolution mass spectrometry analysis. This suggests that the added terrigenous organic matter may be exported to the coastal ocean, while a fraction of the organic matter that co-varied with the terrigenous DOM may contribute to the increased biomineralization in the estuary, with implications to carbon processing in coastal areas.

Distribution and Dynamics of Dissolved Organic Matter in the Changjiang Estuary and Adjacent Sea

AbstractLarge, river‐dominated estuaries provide important land‐ocean interfaces for biogenic element fluxes that impact global marine biogeochemistry. However, the reactivity and fate of dissolved organic matter (DOM) in this dynamic region remain not well understood. In this study, three cruises were conducted in February, May, and July 2017 in the Changjiang Estuary and adjacent sea (CEAS). Incubation experiments to determine photochemically and/or biologically mediated degradation of terrigenous and marine DOM were performed. Dissolved organic carbon (DOC), chromophoric DOM (CDOM) absorption, and total dissolved amino acids (TDAAs) were measured. DOM photodegradation and transformation were indicated by the CDOM spectral slope (S275−295) and diagenetic state by DOC‐normalized yields of TDAA [TDAA (%DOC)]. From the river to the open sea, DOC showed non‐conservative mixing and was rapidly removed in low salinity areas. Photodegradation, biodegradation, and photoenhanced biodegradation are important DOC removal mechanisms and exhibit specific responses depending on DOC source and composition. Terrigenous DOM is vulnerable to photodegradation, while marine DOM experiences rapid biodegradation. Contrary to DOC, TDAA (%DOC) increased from the river to the nearshore and then decreased toward the open sea, indicating the contribution of fresh marine plankton production. A box model showed that the annual DOC removal of CEAS was 2.6 ± 1.9 × 106 tons. Seaward increase of ammonium (NH4‐N) was observed in the CEAS; photoammonification, biomineralization of autochthonous and allochthonous DOM are potential NH4‐N regeneration mechanisms. A model was developed to calculate surface DOC concentrations based on CDOM absorption coefficients allowing high‐resolution monitoring of DOC in the CEAS.

When Forests Take Over After Land Abandonment: Dissolved Organic Matter Response in Headwater Mountain Streams

Dissolved organic matter (DOM) represents the largest pool of organic carbon in fluvial ecosystems. The majority of DOM in rivers is of terrigenous origin—making DOM composition highly dependent on vegetation cover and soil properties. While deforestation is still a worldwide anthropogenic phenomenon, current land cover change in temperate regions is often characterized by secondary succession processes following the abandonment of agricultural activities including grazing on pasturelands. This results in (secondary) forest expansion with a consequent, time-lagged transformation of soil properties. Predicting the time scale and spatial scale (i.e., location in the catchment: riparian vs. upslope areas) at which such land cover changes affect the terrestrial-aquatic carbon linkage and concomitantly alter properties of fluvial DOM as drivers of carbon cycling in freshwater ecosystems represents a new scientific challenge. In an attempt to identify potential legacy effects of land cover, i.e., reaction delays of fluvial DOM to changes in land cover, we here investigate the influence of specific current and historic (2 decade-old) land cover types on molecularly resolved fluvial DOM composition in headwater mountain streams. Our analysis is based on a scale-sensitive approach weighing in the distance of land cover (changes) to the stream and ultrahigh-resolution mass spectrometric analyses. Results identified the dominance of terrigenous DOM, with phenolic and polyphenolic sum formulae commonly associated to lignins and tannins, in all the studied streams. DOM properties mostly reflected present-day gradients of forest cover in the riparian area. In more forested catchments, DOM had on average higher molecular weight and a greater abundance of O-rich phenols and polyphenols but less aliphatics. Besides the modulation of the DOM source, our results also point to an important influence of photodegradation associated to variation in light exposition with riparian land cover in defining fluvial DOM properties. Despite expectations, we were unable to detect an effect of historic land cover on present-day DOM composition, at least at the investigated baseflow conditions, probably because of an overriding effect of current riparian vegetation.

Conversion of Forest to Agriculture Increases Colored Dissolved Organic Matter in a Subtropical Catchment and Adjacent Coastal Environment

AbstractLand‐ocean dissolved organic matter (DOM) transport is a significant and changing term in global biogeochemical cycles which is increasing as a result of human perturbation, including land‐use change. Knowledge of the behavior and fate of transported DOM is lacking, particularly in the tropics and subtropics where land‐use change is occurring rapidly. We used Parallel Factor (PARAFAC) Analysis to investigate how land‐use influenced the composition of the DOM pool along a subtropical land‐use gradient (from near‐pristine broadleaf forest to agri‐urban settings) in Belize, Central America. Three humic‐like and two protein‐like components were identified, each of which was present across land uses and environments. Land‐use mapping identified a strong (R2 = 0.81) negative correlation between broadleaf forest and agri‐urban land. All PARAFAC components were positively associated with agri‐urban land‐use classes (cropland, grassland, and/or urban land), indicating that land‐use change from forested to agri‐urban exerts influence on the composition of the DOM pool. Humic‐like DOM exhibited linear accumulation with distance downstream and behaved conservatively in the coastal zone whilst protein‐like DOM exhibited nonlinear accumulation within the main river and nonconservative mixing in coastal waters, indicative of differences in reactivity. We used a hydrodynamic model to explore the potential of conservative humics to reach the region's environmentally and economically valuable coral reefs. We find that offshore corals experience short exposures (10 ± 11 days yr−1) to large (∼120%) terrigenous DOM increases, whilst nearshore corals experience prolonged exposure (113 ± 24 days yr−1) to relatively small (∼30%) terrigenous DOM increases.

Insights into the origins, molecular characteristics and distribution of iron-binding ligands in the Arctic Ocean

Dissolved lignin phenols, chromophoric dissolved organic matter (DOM), and in situ fluorescence were determined in waters of the Laptev Sea and major Arctic basins, and they were compared with dissolved iron (dFe) distributions to elucidate the sources, molecular characteristics and distributions of iron-binding ligands in the Arctic Ocean. In the Transpolar Drift region (TPD), concentrations of dFe were positively correlated with concentrations of lignin phenols and multiple optical proxies of DOM composition and source. Strong relationships between dFe and visible and ultraviolet wavelength fluorescent DOM indicated that vascular plant and algal-derived DOM contributed to the dFe-ligand pool. These observations are consistent with previous studies suggesting the association of dFe with humic terrigenous and marine organic ligands. The primary sources of iron-binding ligands appear to be the riverine discharge of terrigenous DOM, marine organic matter produced on the shelves, and degradation products of plankton-derived organic matter in the shelf sediments. A stronger relationship between dFe and visible wavelength CDOM fluorescence than with lignin phenols suggested the presence of multiple terrigenous ligands, such as aromatic tannins. The aromatic nature of these terrigenous ligands was indicated by a strong relationship between dFe and the absorption coefficient at 254 nm. A strong negative correlation between the p-hydroxyl to vanillyl lignin phenols ratio and dissolved iron concentrations indicated recently-discharged terrigenous DOM (tDOM) was an important source of iron-binding ligands. Given the strong relationships of marine and terrigenous DOM with dissolved iron, iron-binding functional groups appear to occur in diverse molecules of multiple sources. Examples of such iron-binding functional groups included catechols and carboxylates found in lignins and tannins of terrigenous origins and carboxyl-rich alicyclic molecules (CRAM) of terrigenous and marine origins. The observed dFe distributions in the Arctic Ocean could not be explained by the presence of a single ligand type, but rather by a potpourri of ligand molecules of varying concentrations and binding strengths. This molecular diversity of ligands and associated binding strengths ultimately controls the distribution and transport of dFe in the Arctic Ocean and beyond.

Experimental metatranscriptomics reveals the costs and benefits of dissolved organic matter photo-alteration for freshwater microbes.

Microbes and sunlight convert terrigenous dissolved organic matter (DOM) in surface waters to greenhouse gases. Prior studies show contrasting results about how biological and photochemical processes interact to contribute to the degradation of DOM. In this study, DOM leached from the organic layer of tundra soil was exposed to natural sunlight or kept in the dark, incubated in the dark with the natural microbial community, and analysed for gene expression and DOM chemical composition. Microbial gene expression (metatranscriptomics) in light and dark treatments diverged substantially after 4 h. Gene expression suggested that sunlight exposure of DOM initially stimulated microbial growth by (i) replacing the function of enzymes that degrade higher molecular weight DOM such as enzymes for aromatic carbon degradation, oxygenation, and decarboxylation, and (ii) releasing low molecular weight compounds and inorganic nutrients from DOM. However, growth stimulation following sunlight exposure of DOM came at a cost. Sunlight depleted the pool of aromatic compounds that supported microbial growth in the dark treatment, ultimately causing slower growth in the light treatment over 5 days. These first measurements of microbial metatranscriptomic responses to photo-alteration of DOM provide a mechanistic explanation for how sunlight exposure of terrigenous DOM alters microbial processing and respiration of DOM.

Source Switching Maintains Dissolved Organic Matter Chemostasis Across Discharge Levels in a Large Temperate River Network

Dissolved organic matter (DOM) helps regulate aquatic ecosystem structure and function. In small streams, DOM concentrations are controlled by transport of terrestrial materials to waterways, and are thus highly variable. As rivers become larger, the River Continuum Concept hypothesizes that internal primary production is an increasingly important DOM source, but direct evidence is limited. Recently, the Pulse-Shunt Concept postulated that terrestrial DOM concentrations in larger rivers increase with flow and temperature, which seemingly contradicts previously reported DOM chemostasis in large rivers. This study estimates daily gross primary production (GPP) in 13 streams and rivers across the Connecticut River watershed (watershed areas 0.4–25,019 km2) from 2015 through 2017. Chemostasis of DOM concentrations is maintained by a switch from autochthonous sources of DOM at low flows to terrestrial sources of DOM at high flows in a large temperate river and to a lesser degree in smaller tributaries. At low flow, autochthonous DOM linked to aquatic GPP is the dominant fraction of the DOM pool in large rivers. This autochthonous DOM maintains chemostasis in the main stem and to a lesser extent upstream. Thus, in larger rivers, low-flow autochthonous production stabilizes DOM concentrations during the summer, a critical time for riverine ecology. Consistent with the Pulse-Shunt Concept, terrigenous DOM is the dominant fraction of DOM during higher flow periods and about 70% of annual DOM fluxes to the coast are terrestrial. This pattern of DOM switching is potentially widespread in temperate watersheds with implications to both inland waters and coastal ecosystems.

Influence of environmental parameters on bio-optical characteristics of colored dissolved organic matter in a complex tropical coastal and estuarine region

Dissolved organic matter (DOM) is an important source of carbon in aquatic ecosystems, and colored DOM (CDOM), which is smaller than 0.2 μm and interacts with ultraviolet (UV) and visible light, affects the spectral quality and quantity of light in water. In this study, the spatial and temporal variations of CDOM with changes in environmental conditions were investigated from March 2014 to May 2017 in the coastal waters and two estuaries (Zuari and Mandovi) of Goa, western India, and the major sources and sinks controlling the optical properties of these waters were identified. The CDOM absorption in the estuaries was two times higher than that of the coastal waters. It was also determined that the CDOM absorption at 412 nm (ag412) in the coastal and estuarine waters significantly varied between seasons. The ag412 was found to be higher in the coastal waters during the spring inter-monsoon (SIM) and fall inter-monsoon (FIM) than during the northeast monsoon (NEM). The high absorption during the SIM was of autochthonous origin, while terrigenous DOM was the primary contributor mainly during the FIM. The photobleaching of CDOM was highest during the SIM, resulting in the predominance of low-molecular-weight DOM in the coastal waters. This photobleaching of DOM also resulted in deeper UV light penetration, as indicated by the diffuse attenuation coefficient Kd at 350 nm. In the Mandovi and Zuari estuaries, higher levels of CDOM were observed during the southwest monsoon (SWM) and SIM than the FIM and NEM. The terrigenous DOM contribution was higher in the estuaries during the SWM, while phytoplankton contributed to a higher level of CDOM during the SIM. CDOM exhibited non-conservative mixing behavior in the study region, as it decreased in estuaries with lower salinities and increased at salinities between 20 and 31. Considering the importance of CDOM in the carbon cycle, this study highlights the various sources and sinks of CDOM controlling the optical properties and biogeochemical processes of coastal and estuarine waters.

The role of fluorescent dissolved organic matter on mercury photoreduction rates: A case study of three temperate lakes

The formation of dissolved gaseous mercury (DGM) in lake water is mainly attributed to photochemical processes mediated by dissolved organic matter (DOM). In this study, we explored how different DOM components influence the Hg(II) photoreduction rate constant (kr) in lake surface water. For this purpose, the kr and the fluorescence properties of DOM were obtained from three Korean lakes with different trophic states. Three major fluorophores were identified by excitation-emission matrix fluorescence spectroscopy combined with a parallel factor analysis: plant-derived terrigenous humic-like DOM (C1), autochthonous DOM (C2), and soil fulvic-like DOM (C3). The principal component analysis (PCA) loading matrix demonstrated that kr increases when bulk DOM has high flavin-like and soil-derived fulvic-like fractions. The results of a Pearson’s correlation agreed with the outcome of the PCA analysis: kr showed a strong positive correlation with the soil fulvic-like DOM component (r = 0.92) and the redox index (r = 0.92). This was further confirmed by a partial least squares-regression model that predicted kr (r = 0.99) using multiple DOM components. Overall, our results suggest that kr can be modeled using fluorescence intensities of diverse DOM components, which in turn has the potential to be incorporated into Hg biogeochemical models to better predict the variability of Hg redox rates across lake systems.

Unified concepts for understanding and modelling turnover of dissolved organic matter from freshwaters to the ocean: the UniDOM model

The transport of dissolved organic matter (DOM) across the land-ocean-aquatic-continuum (LOAC), from freshwater to the ocean, is an important yet poorly understood component of the global carbon budget. Exploring and quantifying this flux is a significant challenge given the complexities of DOM cycling across these contrasting environments. We developed a new model, UniDOM, that unifies concepts, state variables and parameterisations of DOM turnover across the LOAC. Terrigenous DOM is divided into two pools, T1 (strongly-UV-absorbing) and T2 (non- or weakly-UV-absorbing), that exhibit contrasting responses to microbial consumption, photooxidation and flocculation. Data are presented to show that these pools are amenable to routine measurement based on specific UV absorbance (SUVA). In addition, an autochtonous DOM pool is defined to account for aquatic DOM production. A novel aspect of UniDOM is that rates of photooxidation and microbial turnover are parameterised as an inverse function of DOM age. Model results, which indicate that ~ 5% of the DOM originating in streams may penetrate into the open ocean, are sensitive to this parameterisation, as well as rates assigned to turnover of freshly-produced DOM. The predicted contribution of flocculation to DOM turnover is remarkably low, although a mechanistic representation of this process in UniDOM was considered unachievable because of the complexities involved. Our work highlights the need for ongoing research into the mechanistic understanding and rates of photooxidation, microbial consumption and flocculation of DOM across the different environments of the LOAC, along with the development of models based on unified concepts and parameterisations.

Molecular Hysteresis: Hydrologically Driven Changes in Riverine Dissolved Organic Matter Chemistry During a Storm Event

AbstractHydrological events, driven by rainfall, control the amount and composition of dissolved organic matter (DOM) mobilized through river networks. In forested watersheds, the concentration, composition, and reactivity of DOM exported changes as baseflow transitions to storm flow, with major implications to downstream biogeochemistry. Hysteresis describes an observed difference between in‐stream solute concentration/signal and discharge. By studying the relationship between DOM and stream discharge, we refine our understanding of the environmental and hydrological factors that influence the quantity and quality of stream DOM. The main objective of this study was to track hysteretic changes in riverine DOM molecular composition during storm events. Samples were collected from nested sites within the Passumpsic River catchment (Vermont, USA), a tributary of the Connecticut River. High‐resolution monitoring of fluorescent DOM (via in situ sensors) and automated collection of discrete samples captured short‐term, hydrologically driven variations in DOM concentration and composition. Ultrahigh‐resolution mass spectrometry revealed an enrichment in aliphatic compounds at storm onset, while aromatic and polyphenolic compounds were more enriched at peak discharge. Molecular hysteresis patterns were similar across stream orders, indicating that fresh, terrigenous DOM is quickly shunted downstream, through the river network, during pulses of high discharge.

Molecular weight fractionated characterization of dissolved organic matter and its correlation with water quality in the Bahe River Basin, Northwest China

Dissolved organic matter (DOM) is one of the core carriers of organic matter decomposition and nutrient regeneration in water. It is an important link of biogeochemical cycle of carbon and nitrogen, and the key content of water environmental science research. We used liquid chromatography-organic carbon detection-organic nitrogen detection (LC-OCD-OND) to investigate the molecular weight distribution of riverine DOM in the Bahe River of Xi'an City and further analyzed its correlation with water quality. The results showed that riverine DOM was composed of biopolymers, humic substances, humus degradation products, and low molecular-weight neutrals, with the ave-rage concentration of 0.15, 1.75, 0.48, 0.36 and 0.002 mg·L-1, respectively. The DOM along the river was in order of urban＞ town＞ river source. The humic substances with molecular weight of 1000-20000 Da accounted for 49.0% of total DOM which followed the order of town＞ waste water treatment plants (WWTPs) effluents＞river estuary＞ river source. The biopolymers with the molecular weight of >20000Da accounted for 5.1% of total DOM, with the sequence of WWTPs effluents＞river estuary＞river source＞town. The allochthonous (terrigenous) DOM produced by WWTPs effluents had the greatest contribution to the riverine DOM. The fractions of DOM with different molecular weights had significant correlations with water quality parameters. The results showed that the molecular weight fractions and abundance of DOM based on LC-OCD-OND could be used as a comprehensive index for water quality monitoring, to characterize the spatial heterogeneity of river water quality, and used for quantitative identification and source apportionment of pollutant components.