Dissolved Organic Matter Dynamics Research Articles

Response of dissolved organic matter chemistry to flood control of a large river reservoir during an extreme storm event

With the frequent occurrence of extreme floods under global climate change-induced storm events, reservoir operation has been highlighted for river flood control, complicating the transport and transformation of riverine dissolved organic matter (DOM), one of the largest reactive carbon pools on earth. In particular, the response of riverine DOM chemistry to reservoir flood control during extreme storm events is still unclear. To fill this knowledge gap, the mechanism of DOM variation in Yangtze River with the world's largest Three Gorges Reservoir (TGR) operation during an extreme storm event was explored. Optical and molecular properties of DOM varied significantly from upstream to downstream in non-TGR area, while no significant variation in DOM chemistry was observed in TGR area. The results uncovered a short time transformation of DOM from non-TGR area to TGR area, demonstrating that although storm event induced chemodiversity bloom of riverine DOM, flood control of TGR “re-constrained” DOM to more similar chemistry mainly under the influence of turbidity involved DOM transformation (e.g., adsorption/desorption and flocculation). Furthermore, combined with the hydrological information, we found that although TGR temporarily blocked dissolved organic carbon (DOC) flow during the flood event, the abundance of biologically recalcitrant DOC increased in TGR, which would contribute to its further transportation to downstream watershed. This study emphasizes the impact of TGR on extreme storm event-induced DOM dynamics, which also hints a better understanding of the crucial role of anthropogenic activity in affecting carbon cycling under extreme climate change.

Revealing the Inner Changes of Component Composition Derived from DOM PARAFAC Based on Two-Dimensional Correlation Spectroscopy.

Plenty of humic acid components compositions are contained in dissolved organic matter (DOM) derived from composting. Fluorescence signals were employed to characterize the changes in DOM components in the component process. In the composting process, five individual DOM fluorescence parallel factor analysis (PARAFAC) components were identified. At the end of the composting, PARAFAC component C5, which represented high humification and complex structure compounds, was detected, but the simple structure DOM PARAFAC component C1 was absent. In this study, a technique combining EEM-PARAFAC with two-dimensional correlation spectroscopy (2DCOS) further supplied detailed information about the dynamics of DOM peaks in PARAFAC components. 2DCOS results showed that the variation of the peaks in PARAFAC components was different in the composting process. The formation of a complex DOM fluorescence substance was attributed to the residues from the simple fluorescence peak degradation. The evolution of the DOM fluorescence peaks in each PARAFAC component indicated that simple structure compounds helped the formation of the complex DOM fluorescence substance in the composting process. These results revealed that EEM/PARAFAC combined with 2DCOS could be used to track the evolution of DOM PARAFAC components during the composting process.

Oxygen availability regulates the quality of soil dissolved organic matter by mediating microbial metabolism and iron oxidation

Dissolved organic matter (DOM) plays a vital role in biogeochemical processes and in determining the responses of soil organic matter (SOM) to global change. Although the quantity of soil DOM has been inventoried across diverse spatio-temporal scales, the underlying mechanisms accounting for variability in DOM dynamics remain unclear especially in upland ecosystems. Here, a gradient of SOM storage across 12 croplands in northeast China was used to understand links between DOM dynamics, microbial metabolism, and abiotic conditions. We assessed the composition, biodegradability, and key biodegradable components of DOM. In addition, SOM and mineral-associated organic matter (MAOM) composition, soil enzyme activities, oxygen availability, soil texture, and iron (Fe), Fe-bound organic matter, and nutrient concentrations were quantified to clarify the drivers of DOM quality (composition and biodegradability). The proportion of biodegradable DOM increased exponentially with decreasing initial DOM concentration due to larger fractions of depolymerized DOM that was rich in small-molecular phenols and proteinaceous components. Unexpectedly, the composition of DOM was decoupled from that of SOM or MAOM, but significantly related to enzymatic properties. These results indicate that microbial metabolism exhibited a dominant role in DOM generation. As DOM concentration declined, increased soil oxygen availability regulated DOM composition and enhanced its biodegradability mainly through mediating microbial metabolism and Fe oxidation. The oxygen-induced oxidation of Fe(II) to Fe(III) removed complex DOM compounds with large molecular weight. Moreover, increased oxygen availability stimulated oxidase-catalyzed depolymerization of aromatic substances, and promoted production of protein-like DOM components due to lower enzymatic C/N acquisition ratio. As global changes in temperature and moisture will have large impacts on soil oxygen availability, the role of oxygen in regulating DOM dynamics highlights the importance of integrating soil oxygen supply with microbial metabolism and Fe redox status to improve model predictions of soil carbon under climate change.

Impact of environmental factors and tributary contributions on tidal dissolved organic matter dynamics

Riverine dissolved organic matter (DOM) transport was a key step in the carbon biogeochemical cycle while we had limited understanding of its contribution to the estuary DOM dynamics. This study focused on the river downstream-to-tidal estuary DOM variation and the control of environmental factors on it. The contributions of three tributaries with varing urbanization degrees to the tidal DOM dynamics were evaluated. Though more aromatics were introduced to the urban tributary, the A250/A365 values and fluorescent index values indicated the DOM molecular size was uniformly reduced due to the enhanced microbial degradation during transport. The tidal DOM showed less varied spectroscopic indexes than the tributary DOM, but tidal cycles strongly impacted the fluorescent DOM quantified by the fluorescence regional integration (FRI). Salinity range can differentiate the fluorescent DOM variation patterns in river tributaries (e.g., <2.5, positive correlations; >2.5, negative correlations) and tidal cycles (>10, negative correlations). For tidal DOM, the high salinity decreased more humic-related components, resulting in increased proportions of protein-related components in high tides. The dissolved oxygen and nitrogen contents were negatively correlated with salinity, suggesting the microbial contributions and anthropogenic inputs in tributaries increased the tidal DOM quantity. The less urbanized tributaries contributed more to the low-tide DOM compositions/properties while the dynamic contribution of the urban tributary impacted more the tidal DOM dynamics. Our results highlighted the uneven declines of FRI values of different components by freshwater-saltwater mixing in estuaries and suggested the different functioning of urban, agro-urban, and suburban tributaries contributed to tidal DOM dynamics.

Fluorescence and molecular signatures of dissolved organic matter to monitor and assess its multiple sources from a polluted river in the farming-pastoral ecotone of northern China.

The sources and composition of dissolved organic matter (DOM) in rivers are critical to water quality and aquatic ecosystems. Studies on detailed composition of organic matter in rivers in the farming-pastoral ecotone are relatively limited in the research community. To better understand the characteristics and dynamics of DOM, Yang River in North China was selected as the study area because of its profound influences on the farming-pastoral ecotone nearby. A combination of fluorescence spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) techniques revealed that the DOM composition of Yang River is driven by land use. DOM in Yang River is predominantly imported from allochthonous inputs, together with agricultural runoff, pastureland, and urban sewage, causing a comprehensive impact on DOM. In detail, DOM associated with cropland inputs was dominated by lignin-like species, with higher nitrogen content. In comparison, DOM related to grassland is more diverse and susceptible to degradation. An increase in urban areas led to an increase in sulfur-containing compounds, while their oxygen, nitrogen, and aromaticity contents were significantly lower than those in cropland. Interestingly, urban-influenced lignin-like compounds may be associated with the effluents from the pulp and paper mill. Additionally, synthetic surfactants from the lower section of the river were also structurally identified by tandem mass spectrometry. Overall, this study could provide valuable insights into the DOM sources and their transformation dynamics at a molecular level, which could be an indicator for riverine water quality management and be applied to other farming-pastoral ecotones straightforward.

Do long-term high nitrogen inputs change the composition of soil dissolved organic matter in a primary tropical forest?

Dissolved organic matter (DOM) plays a key role in forest carbon biogeochemistry by linking soil organic carbon (SOC) sequestration and water fluxes, which is further shaped by elevated atmospheric nitrogen (N) deposition. Although enhanced SOC sequestration was evidenced in tropical forests due to rising N deposition, it remains unclear how long-term N inputs affect soil DOM composition, which regulates SOC sequestration capability due to its mobility and biological instability. Here, the quantity, optical properties, and molecular-level characteristics of soil DOM based on a simulative N deposition experiment with four N addition levels (0, 5, 10, and 15 g m−2 yr−1) were studied in a primary tropical forest in south China. Results showed that 18 year N additions significantly altered soil DOM composition, with an increasing trend in soil dissolved organic carbon content. Medium- (10 g m−2 yr−1) and high-N addition (15 g m−2 yr−1) markedly elevated DOM average molecular weight by 12% and aromaticity, with specific ultraviolet absorbance at 254 nm increasing by 17%, modified aromatic index by 35%, and condensed aromatics by 67%. Medium- and high-N addition also increased recalcitrant DOM components but decreased other DOM components, with increasing percentages of lignin-like, tannin-like, and carboxylic-rich alicyclic molecule-like compounds, and decreasing percentage of more bioavailable contributions with H/C ratio >1.5. Importantly, significant correlations of the SOC content of the heavy fraction with optical properties and with recalcitrant DOM components were observed. These findings suggest that long-term N additions may alter soil DOM composition in a way to benefit soil OC storage in the primary tropical forests. It merits focusing on the mechanisms to association of soil DOM dynamics with SOC sequestration.

Spatiotemporal Controls on the Delivery of Dissolved Organic Matter to Streams Following a Wildfire

AbstractWarmer and drier climate has contributed to increased occurrence of large, high severity wildfires in the Pacific Northwest, drawing concerns for water quality and ecosystem recovery. While nutrient fluxes generally increase post‐fire, the composition of organic matter (OM) transported to streams immediately following a fire is poorly constrained, yet can play an integral role in downstream water quality and biogeochemistry. Here, we quantified the spatiotemporal patterns of dissolved OM (DOM) chemistry for five streams burned by wildfires in Oregon, USA in 2020. We sampled over a 24 hr storm event 1 month after the fire revealing DOM dynamics were temporally variable, but spatially linked with burn severity. Specifically, nitrogen and aromatic character of DOM increased in streams burned at greater severity. Our results suggest spatially distinct gradients of burn severity impact DOM dynamics immediately following fire activity and highlight a key gap in our knowledge of post‐fire DOM transport to streams.

Spatial dynamics of dissolved organic matter among different segments of a large-scale reservoir in the water-level declining period

Large-scale reservoirs exhibit complex hydrological conditions and exert a significant alteration on river flowing. Although dissolved organic matter (DOM) is noted to involve in biogeochemical processes, the variation mechanism of DOM chemistry across a large-scale reservoir is not well assessed. Here, we investigated four tributaries across different segments (e.g., the front and non-front areas) of the world’s largest Three Gorges Reservoir (TGR). Optical techniques and ultrahigh-resolution mass spectrometry were used to comprehensively explore the variation of DOM chemistry across TGR in the water-level declining period, and biological incubation experiments were conducted to trace its biogeochemical influences. We found that the variation of DOM composition and property between tributary and river mouth sites show different patterns between front and non-front areas of TGR. In particular, there was more terrestrial derived and biologically recalcitrant DOM in the river mouth than tributaries in the front area, while the opposite variation was observed in the non-front area. Integrated with hydrological information, the results demonstrated that the density current exerts a significant influence on DOM dynamics. Furthermore, the biological incubation experiments suggested that this variation of DOM property among tributaries would involve in the spatial dynamics of carbon dioxide (CO2) was emitted in TGR that more CO2 was emitted in the tributary of the front area than of the non-front area during the water-level declining period.

Metabolomics analysis of unresolved molecular variability in stoichiometry dynamics of a stream dissolved organic matter

Broad molecular classification based on stoichiometric ratio relationships has been used extensively to characterize the chemical diversity of aquatic dissolved organic matter (DOM). However, variability in the molecular composition within this classification has remained elusive, thus limiting the interpretation of DOM dynamics, especially with respect to transport versus transformation patterns in response to hydrologic or landscape changes. Here, leveraging high-frequency spatiotemporal sampling during rainfall events at a Critical Zone Observatory project site in Clear Creek, Iowa, we apply a metabolomics-based analysis validated with fragmentation using tandem mass spectrometry to uncover patterns in the molecular features of the DOM composition that were not resolved by classification based on stoichiometric ratios in the chemical formulae. From upstream to downstream sites, beyond the increased aromaticity implied by changes in the stoichiometric ratios, we identified an increased abundance of flavonoids and other phenylpropanoids, two important subgroups of aromatic compounds. The stoichiometric analysis also proposed a localized decline in the abundance of lipid-like compounds, which we attributed specifically to medium-chain and short-chain fatty acids; other lipids such as long-chain fatty acids and sterol lipids remained unchanged. We further determined in-stream molecular transitions and specific compound degradation by capturing changes in the molecular masses of terpenoids, phenylpropanoids, fatty acids, and amino acids. In sum, the metabolomics analysis of the chemical formulae resolved molecular variability imprinted on the stoichiometric DOM composition to implicate key molecular subgroups underlying carbon transport and cycling dynamics in the stream.

DOM in the long arc of environmental science: looking back and thinking ahead

Dissolved organic matter (DOM) is a heterogeneous mixture of organic compounds that is produced through both microbial degradation and abiotic leaching of solid phase organic matter, and by a wide range of metabolic processes in algae and higher plants. DOM is ubiquitous throughout the hydrologic cycle and plays an important role in watershed management for drinking water supply as well as many aspects of aquatic ecology and geochemistry. Due to its wide-ranging effects in natural waters and analytical challenges, the focal research questions regarding DOM have varied since the 1920s. A standard catchment-scale model has emerged to describe the environmental controls on DOM concentrations. Modest concentrations of DOM are found in atmospheric deposition, large increases occur in throughfall and shallow soil flow paths, and variable concentrations in surface waters occur largely as a result of the extent to which hydrologic flow paths encounter deeper mineral soils, wetlands or shallow organic-rich riparian soils. Both production and consumption of DOM occur in surface waters but appear to frequently balance, resulting in relatively constant concentrations with distance downstream in most streams and rivers. Across biomes the concentration and composition of DOM in flowing waters is driven largely by soil processes or direct inputs to channels, but high levels can be found in streams and rivers from the tropics to the poles. Seven central challenges and opportunities in the study of DOM should frame ongoing research. These include maintaining or establishing long-term records of changes in concentrations and fluxes over time, capitalizing on the use of sensors to describe short-term DOM dynamics in aquatic systems, integrating the full carbon cycle into understanding of watershed and aquatic DOM dynamics, understanding the role of DOM in evasion of greenhouse gases from inland waters, unraveling the enigma of dissolved organic nitrogen, documenting gross versus net DOM fluxes, and moving beyond an emphasis on functional ecological significance to understanding the evolutionary significance of DOM in a wide range of environments.

Oxygen availability driven trends in DOM molecular composition and reactivity in a seasonally stratified fjord

Ocean deoxygenation could potentially trigger substantial changes in the composition and reactivity of dissolved organic matter (DOM) pool, which plays an important role in the global carbon cycle. To evaluate links between DOM dynamics and oxygen availability, we investigated the DOM composition under varying levels of oxygen in a seasonally hypoxic fjord through a monthly time-series over two years. We used ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to characterize DOM on a molecular level. We find a clear trend both in diversity and molecular composition of the DOM along the oxygen gradient. As oxygen decreased, the chemodiversity was significantly increased, along with accumulation of relatively high-molecular-weight, reduced and unsaturated compounds enriched with carboxyl-group structures, which were also thermodynamically less favorable to biodegradation. Our results suggested that oxygen depletion selectively protected otherwise bioavailable compounds from decomposition and may promote the accumulation of a larger recalcitrant DOM pool in the global ocean, which could provide negative feedback to the ocean carbon sequestration and climate change.

Influence of land cover types and phytoplankton community on the distribution and fate of dissolved organic matter in a typical river located in the semi-arid regions of China

Land use can impact dissolved organic matter (DOM) dynamics directly by changing terrestrial loading in anthropogenic areas, and indirectly by promoting in-situ phytoplankton production due to the high load of nutrients. However, understanding how land use and phytoplankton community interact to affect DOM quantity and quality is deficient. To tease apart the synergistic effects of land use and phytoplankton on DOM dynamics, fluorescence spectroscopy with parallel factor analysis (PARAFAC) and mediation analysis was performed in this study. PARAFAC produced three components that had two humic-like (C1 and C2) and a protein-like (C3) fluorescent components, significantly related to anthropogenic land-use (cropland + urban area) and phytoplankton concentration (p < 0.05). Through the mediation analysis, the altered C1 and C3 at sites with a higher proportion of anthropogenic land-use appeared to originate partly as a result of the phytoplankton production fueled by a high load of nutrients from anthropogenic run-off (26.6% and 12.5%). These results suggested phytoplankton significantly mediated the effects of land use on microbial humic-like and protein-like DOM. Additionally, Redundancy analysis (RDA) results further showed that Phormidium (Cyanophyta) had a notable influence on the production of the protein-like component via phytoplankton. Conversely, the decreases in terrestrial humic-like C2 were mainly attributed to the reduced terrestrial loading in anthropogenic areas. These findings provide insights into the roles of land use and phytoplankton communities in DOM dynamics and advance our understanding of the mechanisms that drive DOM composition patterns.

Molecular-level composition of dissolved organic matter in distinct trophic states in Chinese lakes: Implications for eutrophic lake management and the global carbon cycle

Dissolved organic matter (DOM) is an abundant and mobile part of the aquatic environment and plays important roles in aquatic biogeochemical cycles and the global carbon cycle. Recently, eutrophication has become an important environmental issue in global lakes, but how eutrophication drives changes in the molecular composition of DOM along trophic gradients remains poorly understood. We thus characterized 67 DOM isolates from 11 lakes along a trophic gradient in China by using a combined approach including absorption spectroscopy, excitation-emission matrix fluorescence and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Our results indicated that dissolved organic carbon and absorption coefficients at 350 nm increased with increasing trophic status index. The ultraviolet absorbance at 254 nm and fluorescence intensity of all fluorescent components were higher in eutrophic lakes than in oligotrophic lakes. DOM in high trophic state lakes tended to be dominated by higher molecular weight, unsaturation degree, greater abundance of S-containing compounds, and condensed or polycyclic aromatic compounds than oligotrophic lakes. Additionally, autochthonous DOM characterized by more aliphatic compounds increased with the increasing trophic state. We concluded that nutrient input along with allochthonous DOM favors the lake eutrophication and subsequently increases the release and accumulation of autochthonous DOM. Consequently, eutrophication modifies the structure of the organic matter into more complex materials with increased input of allochthonous DOM and increased release of autochthonous DOM, which could accelerate global carbon cycle processes. Our results here have potential to contribute significantly to future studies of DOM dynamics in eutrophic lakes.

Linkages Among Dissolved Organic Matter Export, Dissolved Metabolites, and Associated Microbial Community Structure Response in the Northwestern Sargasso Sea on a Seasonal Scale.

Deep convective mixing of dissolved and suspended organic matter from the surface to depth can represent an important export pathway of the biological carbon pump. The seasonally oligotrophic Sargasso Sea experiences annual winter convective mixing to as deep as 300 m, providing a unique model system to examine dissolved organic matter (DOM) export and its subsequent compositional transformation by microbial oxidation. We analyzed biogeochemical and microbial parameters collected from the northwestern Sargasso Sea, including bulk dissolved organic carbon (DOC), total dissolved amino acids (TDAA), dissolved metabolites, bacterial abundance and production, and bacterial community structure, to assess the fate and compositional transformation of DOM by microbes on a seasonal time-scale in 2016–2017. DOM dynamics at the Bermuda Atlantic Time-series Study site followed a general annual trend of DOC accumulation in the surface during stratified periods followed by downward flux during winter convective mixing. Changes in the amino acid concentrations and compositions provide useful indices of diagenetic alteration of DOM. TDAA concentrations and degradation indices increased in the mesopelagic zone during mixing, indicating the export of a relatively less diagenetically altered (i.e., more labile) DOM. During periods of deep mixing, a unique subset of dissolved metabolites, such as amino acids, vitamins, and benzoic acids, was produced or lost. DOM export and compositional change were accompanied by mesopelagic bacterial growth and response of specific bacterial lineages in the SAR11, SAR202, and SAR86 clades, Acidimicrobiales, and Flavobacteria, during and shortly following deep mixing. Complementary DOM biogeochemistry and microbial measurements revealed seasonal changes in DOM composition and diagenetic state, highlighting microbial alteration of the quantity and quality of DOM in the ocean.

Biochar affects methylmercury production and bioaccumulation in paddy soils: Insights from soil-derived dissolved organic matter

Biochar has been used increasingly as a soil additive to control mercury (Hg) pollution in paddy rice fields. As the most active component of soil organic matter, soil dissolved organic matter (DOM) plays a vital role in the environmental fate of contaminants. However, there are very few studies to determine the impact of biochar on the Hg cycle in rice paddies using insights from DOM. This study used original and modified biochar to investigate their effect on DOM dynamics and their potential impact on methylmercury (MeHg) production and bioaccumulation in rice plants. Porewater DOM was collected to analyze the variations in soil-derived DOM in paddy soils. The results showed that the addition of biochar, whether in original or modified form, significantly reduced the bioaccumulation of MeHg in rice plants, especially in hulls and grains (p<0.05). However, MeHg production in soils was only inhibited by the modified biochar. Biochar addition induced a significant increase in DOM's aromaticity and molecular weight (p<0.05), which decreased Hg bioavailability. Furthermore, enhanced microbial activity was also observed in DOM (p<0.05), further increasing MeHg production in the soil. Thus, the effect of biochar on the fate of Hg cycle involves competition between the two different roles of DOM. This study identified a specific mechanism by which biochar affects Hg behavior in rice paddy soil and contributes to understanding the more general influence of biochar in agriculture and contaminant remediation.

The Optical Characterization and Distribution of Dissolved Organic Matter in Water Regimes of Qilian Mountains Watershed.

The constituents and content of dissolved organic matter (DOM) in the Qilian Mountain watershed were characterized with a spectroscopic technique, especially 3-DEEM fluorescence assisted by parallel factor (PARAFAC) analysis. The level of DOM in the surrounding area of Qinghai lake (thereafter the lake in this article specifically refers to Qinghai Lake)was highest at 9.45 mg C·L−1 and about 3 times less (3.09 mg C·L−1) in a cropland aquatic regime (the lowest value). In general, DOM was freshly autochthonously generated by plankton and plant debris, microorganisms and diagenetic effects in the aquatic environment (FI > 1.8). Component 1 (humic acid-like) and 3 (fulvic acid-like) determined the humification degree of chromophoric dissolved organic matter (CDOM). The spatial variation of sulfate and nitrate in the surrounding water regime of the lake revealed that organic molecules were mainly influenced by bacterial mediation. Mineral disintegration was an important and necessary process for fluorescent fraction formation in the cropland water regime. Exceptionally, organic moiety in the unused land area was affected by anespecially aridclimate in addition to microbial metabolic experience. Salinity became the critical factor determining the distribution of DOM, and the total normalized fluorescent intensity and CDOM level were lower in low-salinity circumstances (0.2–0.5 g·L−1) with 32.06 QSU and 1.38 m−1 in the grassland area, and higher salinity (0.6~0.8 g·L−1) resulted in abnormally high fluorescence of 150.62 QSU and absorption of 7.83 m−1 in the cropland water regime. Climatic conditions and microbial reactivity controlled by salinity were found to induce the above results. Our findings demonstrated that autochthonous inputs regulated DOM dynamics in the Qilian Mountains watershed of high altitude.

Evidence of Covid-19 lockdown effects on riverine dissolved organic matter dynamics provides a proof-of-concept for needed regulations of anthropogenic emissions

The fast spread of SARS-CoV-2 virus in Italy resulted in a 3-months lockdown of the entire country. During this period, the effect of the relieved anthropogenic activities on the environment was plainly clear all over the country. Herein, we provide the first evidence of the lockdown effects on riverine dissolved organic matter (DOM) dynamics. The strong reduction in anthropogenic activities resulted in a marked decrease in dissolved organic carbon (DOC) concentration in the Arno River (−44%) and the coastal area affected by its input (−15%), compared to previous conditions. The DOM optical properties (absorption and fluorescence) showed a change in its quality, with a shift toward smaller and less aromatic molecules during the lockdown. The reduced human activity and the consequent change in DOM dynamics affected the abundance and annual dynamics of heterotrophic prokaryotes. The results of this study highlight the extent to which DOM dynamics in small rivers is affected by secondary and tertiary human activities as well as the quite short time scales to return to the impacted conditions. Our work also supports the importance of long-term research to disentangle the effects of casual events from the natural variability.

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.

Water Residence Time and Temperature Drive the Dynamics of Dissolved Organic Matter in Alpine Lakes in the Tibetan Plateau

AbstractLakes are hotspots of dissolved organic matter (DOM) degradation and play an active role in carbon cycles. Alpine lakes are under the most immediate impacts of climate change and act as sentinels of alpine ecosystem’s response to global warming, yet the primary environmental drivers of DOM variability in alpine lakes remain unidentified. Here, we sampled and analyzed DOM from 25 alpine lakes (&gt;4,000 m above mean sea level) across the Tibetan Plateau. We found that the water residence time (WRT) of lakes and mean annual temperature (MAT) accounted for 30%–59% of the variance in the abundance of chromophoric DOM (CDOM) and fluorescent DOM (FDOM). Both CDOM and FDOM decreased with increasing WRT and MAT. Molecular‐level analysis of selected DOM samples showed that a higher WRT corresponded to DOM with lower molecular intensities, lower proportions of aromatics, lower oxidized compounds, and higher proportions of reduced and nitrogen‐containing compounds. These results indicate that in‐lake DOM processing dominated the regional variability of CDOM and FDOM. Under the scenario of climate warming and lake enlargement due to increasing precipitation and glacier melting, increased amount of DOM could be processed and removed from Tibetan lakes. Our findings are the first that identified WRT as a significant predictor of the amount of various DOM pools in Tibetan lakes and this observation may be broadly applicable to alpine lakes since they share similar DOM source‐sink characteristics. The simple empirical models (DOM vs. WRT + MAT) established here can be integrated into more sophisticated numerical models in predicting changes in the carbon cycle of alpine lakes.

Variability of dissolved organic matter in two coastal wetlands along the Changjiang River Estuary: Responses to tidal cycles, seasons, and degradation processes

Dissolved organic matter (DOM) in the coastal tidal marsh–estuary systems are complex mixtures with different source materials that vary with hydrological regimes, seasons, and environmental conditions and are modified by removal processes including photochemical and microbial degradations. Here, monthly surveys of DOM and its optical properties (i.e., absorbance and fluorescence of DOM) covering a complete semi-diurnal tidal cycle were conducted in two coastal marshes with distinct hydrological regimes (i.e., one freshwater and one brackish marsh) in the Changjiang River Estuary (CRE). Four fluorescent components were identified by excitation–emission matrix fluoresces combined with parallel factor analysis (EEMs–PARAFAC) as two terrestrial humic-like components and two autochthonous protein-like components. Results indicated that ebbing waters draining the marshes were consistently enriched with highly absorbing, more humic and highly aromatic DOM compared to the flood tidewaters. On a seasonal basis, DOM dynamics were largely modulated by the marsh productivity and the seasonal Changjiang runoff. Protein-like fluorophores, however, demonstrated a constant, less variable pattern on both the tidal and seasonal timescales. Onsite water incubation experiments with photochemical and microbial alterations revealed that photochemistry was primarily responsible for the removal of optically active components in the marsh DOM pools whereas the impact of microbes was minor. Principal component analysis (PCA) illustrated the processes regulating the DOM dynamics at the marsh–estuarine interface and allowed a clear distinction of samples between the two marshes, in addition to the samples under the influence of episodic weather events (i.e., Super Typhoon Lekima in summer 2009 and the basin-wide severe flood event occurred in summer 2020). This study underscores the importance of world large-rivers such as Changjiang on estuarine marsh DOM dynamics and also highlights the substantial heterogeneity of the marsh DOM across a river-dominated estuary.