Dissolved Organic Matter Dynamics Research Articles

Assessing the Dynamics of Dissolved Organic Matter (DOM) in the Coastal Environments Dominated by Mangroves, Indian Sundarbans

Tidal transport from coastal wetlands (“outwelling”) together with riverine fluxes provide most important sources of terrigenous organic matter (OM) to the ocean. The flux of dissolved organic carbon (DOC) from the mangrove swamps accounts for 10% of the terrestrial DOC flux to the coastal water. This study examines the sources, distribution, and export of dissolved OM at inter-annual, seasonal and diurnal basis along the estuaries located at the Sundarbans, world’s largest deltaic mangrove and heritage site. Sampling was carried out from riverine (Hooghly) and mangrove-dominated tidal estuaries (Saptamukhi, Thakuran, Matla) covering all three seasons (pre-monsoon, monsoon and post-monsoon) during 2012-2017. DOC varied at a broad range from 109 to 462 µM (n = 146) with higher concentration observed in the Hooghly (383±120 µM, n = 35) than the mangrove estuaries (246±82 to 266±120 µM, n = 111). Nonconservative mixing of DOC along the salinity gradient attested to mangrove input, particularly in the polyhaline waters. Upper and mid estuarine zones of the mangrove estuaries showed slightly higher DOC concentration (270±92 µM, n = 84, salinity range 18-25) than the mouth (250±85 µM, n = 27, salinity range 26-27) because of the dilution with marine waters having low DOC concentration and shorter water residence time downstream. Seasonally, higher DOC concentration during the post-monsoon might be linked to higher litterfall promoting leaching of organic compounds to the water. In that connection, colored dissolved organic matter (CDOM) could be the by-product of mangrove litter leaching, and its absorption coefficient (at 350 nm) exhibited non conservative mixing pattern at wide ranges from 2.5 to 7.6 m-1 (n = 40). CDOM enrichment was observed in the surface water during the low tide when outwelling maximized. Overall, central and eastern part of the Indian Sundarbans showed enrichment of more terrigenous type CDOM (evident from optical proxies e.g. S275-295 and SUVA254) than the western part, probably due to greater mangrove productivity in the eastern side. Flux estimates of DOM resulted in higher yield and export of mangrove-derived DOC but lower export of CDOM to the Bay of Bengal as compared to their riverine transport.

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.

Comparing the performance of three methods to assess DOM dynamics within two distinct glacierized watersheds of the tropical Andes

Dissolved organic matter (DOM) is recognized as a good indicator of water quality as its concentration is influenced by land use, rainwater, windborne material and anthropogenic activities. Recent technological advances make it possible to characterize fluorescent dissolved organic matter (FDOM), the fraction of DOM that fluoresces. Among these advances, portable fluorometers and benchtop fluorescence excitation and emission spectroscopy coupled with a parallel factor analysis (EEM-PARAFAC) have shown to be reliable. Despite their rising popularity, there is still a need to evaluate the extent to which these techniques can assess DOM dynamics at the watershed scale. We compare the performance of in-situ measurements of FDOM with laboratory measurements of fluorescence spectroscopy within the context of two distinct glacierized watersheds in Peru. Glacierized watersheds represent unique testing environments with contrasting DOM conditions, flowing from pristine, vegetation-free headwaters through locations with obvious anthropogenic influences. We used an in-situ fluorometer and a portable multimeter to take 38 measurements of FDOM, pH and turbidity throughout the two catchments. Additionally, samples were analyzed in the laboratory using the EEM-PARAFAC method. Results were compared to dissolved organic carbon (DOC) measurements using standard high-temperature catalytic oxidation. Our results show that the three techniques together were able to capture the DOM dynamics for both studied watersheds. Taken individually, all three methods allowed detection of the watershed DOM main points of sources but in a more limited way. Due to the narrow bandwidth of the portable fluorometer used in the study, FDOM measurements were almost non-detectable to protein-like substances. Indeed, the more demanding EEM-PARAFAC was able to both differentiate between potential sources of DOM and provide an estimate of relative concentrations of different organic components. Finally, similar to FDOM but to a lesser extent, the DOC measurements showed some limits where protein-like substances make up most of the DOM composition.

Seasonal dynamics of dissolved organic matter in high arsenic shallow groundwater systems

Dissolved organic matter (DOM) plays critical roles in biogeochemistry of arsenic in groundwater systems. To characterize the spatial–temporal variations of groundwater DOM and its impacts on arsenic dynamics, surface water and shallow groundwater from two different depths were collected during monsoon and non-monsoon periods in Jianghan Plain, central Yangtze River Basin. Parallel factor analysis (PARAFAC) of the three-dimensional fluorescence excitation–emission matrix (EEM) reveals that DOM comprises three major components, one microbial humic-like component (C1) and two terrestrial humic-like components (C2 and C3). Abundant terrestrially-derived humic-like components were observed during monsoon while more microbially-derived DOM occurred in groundwater during non-monsoon season. These seasonal DOM variations are found to be related to groundwater-surface water (GW-SW) interactions, which is evidenced by stable H and O isotopes and Cl/Br mass ratios. During monsoon season, groundwater is recharged from dilute rainwater, evaporated surface water, and polluted surface water. More terrestrial humic-like components are introduced by precipitation and local agricultural activities (e.g. irrigation, paddy cultivation) during monsoon. The fluorescence spectroscopy and stable carbon isotopes (δ13CDIC and δ13CDOC) results indicate that seasonal As variations are affected by seasonal DOM dynamics. Smaller Δδ13C values (defined as δ13CDIC – δ13CDOC) with higher As concentrations are found during non-monsoon while larger Δδ13C values with relatively lower As concentrations are observed during monsoon season. The strong correlation between DOC and Fe(II) concentrations (r = 0.64, p < 0.01) and that between C2 maximum fluorescence intensity and As concentrations (r = 0.57, p < 0.01) clearly reflect the critical role of DOC in As mobilization. The seasonal variations of As concentrations are therefore correlated with biodegradation of labile organic matter, and terrestrially-derived high molecular weight humic-like components promote seasonal As mobilization via complexation reactions. These findings reveal the effects of seasonal DOM variations on As dynamics in shallow Quaternary alluvial aquifers and are applicable to similar geogenic As-affected Quaternary aquifers in floodplains and river delta regions of monsoon climate.

Measuring the influence of environmental conditions on dissolved organic matter biodegradability and optical properties: a combined field and laboratory study

Fluorescence spectroscopy is a common tool to assess optical dissolved organic matter (DOM) and a number of characteristics, including DOM biodegradability, have been inferred from these analyses. However, recent findings on soil and DOM dynamics emphasize the importance of ecosystem-scale factors, such as physical separation of substrate from soil microbial communities and soil physiochemical cycles driving DOM stability. We apply this principle to soil derived DOM and hypothesize that optical properties can only supply information on biodegradability when evaluated in the larger ecosystem because substrate composition and the activity/abundance of the microbial community ultimately drive DOM degradation. To evaluate biodegradability in this context, we assessed aqueous soil extracts for water extractable organic carbon (WEOC) content, biodegradability, microbial biomass and DOM characteristics using fluorescence spectroscopy across a range of environmental conditions (covariant with season and land use) in northern Vermont, USA. Our results indicate that changes in environmental conditions affect composition, quantity, and biodegradability of DOM. WEOC concentrations were highest in the fall and lowest in the summer, while no significant differences were found between land covers; however, DOM biodegradability was significantly higher in the agricultural site across seasons. Despite a shift in utilized substrate from less aromatic DOM in summer to more aromatic DOM in winter, biodegradability was similar for all seasons. The only exception was cold temperature incubations where microbial activity was depressed, and processing was slowed. These results provide examples on how fluorescence based metrics can be combined with context relevant environmental parameters to evaluate bioavailability in the context of the larger ecosystem.

Dissolved Organic Matter in the Gulf of Cádiz: Distribution and Drivers of Chromophoric and Fluorescent Properties

The Gulf of Cadiz (GoC) connects the Mediterranean Sea with the Atlantic Ocean through the Strait of Gibraltar. Particular hydrographic processes take place in the GoC, such as riverine discharges and surface circulation marked by wind-induced seasonal upwelling. Although physical processes have been widely studied, little is known about the biogeochemical processes that occur in the basin, especially those involving organic matter. Therefore, vertical and seasonal dynamics of dissolved organic carbon (DOC) and dissolved organic matter (DOM) optical properties (absorption and fluorescence) were measured in 766 samples collected between 5 and 800 m depth during 4 oceanographic cruises to obtain quantitative and qualitative information about DOM in the GoC. We performed parallel factor analysis (PARAFAC) to identify the main fluorophores present in the GoC, and an optimum multiparameter water mass analysis to differentiate the effect of water mass mixing from the biogeochemical processes in deep waters. PARAFAC analysis validated six fluorescent components; three humic-like, two protein-like, and a possible mixture of polycyclic aromatic hydrocarbon-like with protein-like material. DOC average concentration was 77.0 ± 12.7 µM, with higher values in surface and coastal waters during summer, mainly related to primary production. Linear relationships between DOC and apparent oxygen utilization indicate differences in oxygen consumption within the deep waters, which could be related to upwelling zones. Seasonal and spatial differences were also observed in fluorescent DOM distribution. Protein-like components were the most abundant fraction, with an average contribution of 64.75 % ± 7.85 %, being higher in summer and surface waters, associated with an increase in biological activity. Our results indicate that water mass mixing is the main driver of the major humic-like components, while biogeochemical processes at a local scale explain DOC and protein-like components distribution. Our findings suggest that modeling DOM dynamics in the GoC is complicated due to its complex hydrography and the presence of multiple sources and sinks of DOM.

The interruption of longitudinal hydrological connectivity causes delayed responses in dissolved organic matter

Hydrology is the main driver of dissolved organic matter (DOM) dynamics in intermittent rivers and ephemeral streams. However, it is still unclear how the timing and the spatial variation in flow connectivity affect the dynamics of DOM and inorganic solutes. This study focuses on the impact of flow cessation on the temporal and spatial heterogeneity of DOM quantity and quality along an intermittent stream. We monitored a headwater intermittent stream at high spatial and temporal frequencies during a summer drying episode and analysed dissolved organic carbon (DOC) and its spectroscopic properties, inorganic solutes and dissolved CO2. The drying period determined the disruption of the fluvial continuum with a recession of stream continuum at a rate of ~60 m/d and the gradual formation of a patched system of isolated pools of different sizes. Our results showed that the period of time that had elapsed since isolated pool formation (CI-days) was an essential factor for understanding how drying shaped the biogeochemistry of the fluvial system. Overall, drying caused a high DOC concentration and an increase in the humic-like fluorescence signal. Additionally, solutes showed contrasting responses to hydrological disconnection. Electrical conductivity, for instance, is a clear “sentinel” of the fragmentation process because it starts to increase before the hydrological disruption occurs. In contrast, DOC, most spectroscopic DOM descriptors and CO2 showed delayed responses of approximately 5–21 days after the formation of isolated pools. Furthermore, the spatial location and volume of each isolated pool seemed to exert a significant impact on most variables. In contrast, the temperature did not follow a clear pattern. These findings indicate that the fragmentation of longitudinal hydrological connectivity does not induce a single biogeochemical response but rather stimulates a set of solute-specific responses that generates a complex biogeochemical mosaic in a single fluvial unit.

Fundamental drivers of dissolved organic matter composition across an Arctic effective precipitation gradient

AbstractThe standard model for aquatic ecosystems is to link hydrologic connectivity to dissolved organic carbon (DOC) concentration and dissolved organic matter (DOM) composition and, ultimately, reactivity. Studies across effective precipitation gradients have been suggested as models for predicting how carbon cycling will change in Arctic aquatic ecosystems with projected drying (i.e., reduced hydrologic connectivity). To evaluate links between DOM dynamics and hydrologic connectivity, 41 stream samples from Greenland were analyzed across an effective precipitation gradient for DOM optical properties and elemental composition using ultrahigh‐resolution mass spectrometry. Sites with negative effective precipitation and decreased hydrologic connectivity exhibited elevated specific conductivity (SpC) and DOC concentrations as well as DOM composition indicative of decreased hydrologic connectivity, for example, lower aromaticity, assessed using carbon‐specific UV absorbance at 254 nm, decreased relative abundances of polyphenolic and condensed aromatic compounds, and increased relative abundances of highly unsaturated and phenolic compounds. Allochthonous inputs decreased as the summer progressed as exhibited by decreases in aromatic compounds. A decrease in molecular richness and N‐containing compounds coincided with the decrease in allochthonous inputs. DOC concentrations increased over the summer but more slowly than SpC, suggesting degradation processes outweighed combined evapoconcentration and production. The patterns in DOM composition suggest evapoconcentration and photodegradation are dominant controls. However, when hydrologic connectivity was high, regardless of effective precipitation, DOM reflected allochthonous sources such as snowmelt‐fed wetlands. These results highlight the challenges of modeling carbon cycling in aquatic ecosystems across effective precipitation gradients, particularly those with strong seasonality and regional variability in hydrologic inputs.

Spatial Variations of Trace Metals and Their Complexation Behavior with DOM in the Water of Dianchi Lake, China

The dynamics of trace metals and the complexation behavior related to organic matter in the interface between water and sediment would influence water quality and evolution in the lake system. This study characterized the distribution of trace metals and the optical properties of dissolved organic matter (DOM) on the surface, and the underlying and pore water of Dianchi Lake (DC) to understand the origin of metals and complexation mechanisms to DOM. Some species of trace metals were detected and Al, Ti, Fe, Zn, Sr and Ba were found to be the main types of metals in the aquatic environment of DC. Ti, Fe, Sr and Ba predominated in water above the depositional layer. Al, Ti, Fe and Sr were the most abundant metallic types in pore water. Mn and Zn were the main type found at the southern lake site, reflecting the contribution of pollution from an inflowing river. The correlations between DOM and metals suggested that both originated from the major source as particulate organic matter (POM), associated with weathering of Ca-, Mg-carbonate detritus and Fe- or Mn-bearing minerals. High dynamics of DOM and hydrochemical conditions would change most metal contents and speciation in different water compartments. Proportions of trace metals in dissolved organic carbon (DOC) in natural waters were correlated with both DOM molecular weight and structure, different metals were regulated by different organic properties, and the same metal also had specific binding characteristic with DOM in various water compartments. This study highlighted the interrelation of DOM and metals, as well as the pivotal role that organic matter and nutrients played during input, migrations and transformations of metals, thereby reflecting water quality evolution in the lake systems.

Non‐target screening of leaf litter‐derived dissolved organic matter using liquid chromatography coupled to high‐resolution mass spectrometry (LC‐QTOF‐MS)

AbstractLeaf litter‐derived dissolved organic matter (DOM) is an important source of organic matter entering the mineral soil, but characterization of leaf litter DOM is often not detailed enough to understand DOM dynamics and processes at the molecular level. (Ultra‐) high‐resolution techniques such as Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR‐MS), Orbitrap mass spectrometry and quadrupole time‐of‐flight (QTOF) mass spectrometry have brought a new level of detail to (D)OM research by providing molecular formulae and information on molecular structures. We present a novel approach for leaf litter DOM characterization that includes non‐target screening using liquid chromatography (LC) coupled to high‐resolution QTOF‐MS. A method validation test showed that out of 26 spiked naturally occurring DOM molecules in a leaf litter DOM sample, 23 were found as features. After implementation of additional filtering to exclude certain combinations of heteroatoms (that are difficult to resolve even by the newest high‐resolution MS instruments), including molecular formulae with P atoms, 22 molecular formulae were correctly assigned. Although a large part of the annotated molecular formulae was unique for the respective litter sources, the molecular compound class distribution was similar for deciduous, coniferous and mixed leaf litter DOM. Most intense masses were detected in the 100–300 Da mass range, showing a bias of QTOF‐MS towards lower masses compared to FT‐ICR‐MS. The use of LC in combination with QTOF‐MS for leaf litter (D)OM characterization provides, together with Orbitrap‐MS, a more widely available and lower cost high‐resolution MS alternative to FT‐ICR‐MS. Novel approach to the characterization and analysis of leaf litter derived DOM. Non‐target screening using LC‐QTOF‐MS and the patRoon R package. Similar molecular compound class distribution for three leaf litter sources. High‐resolution DOM characterization alternative to FT‐ICR‐MS.Highlights Novel approach to the characterization and analysis of leaf litter‐derived DOM. Non‐target screening using LC‐QTOF‐MS and the patRoon R package. Similar molecular compound class distribution for three leaf litter sources. High‐resolution DOM characterization alternative to FT‐ICR‐MS.

Controls of Land Use and the River Continuum Concept on Dissolved Organic Matter Composition in an Anthropogenically Disturbed Subtropical Watershed.

About 250 Tg of dissolved organic carbon are annually transported from inland waters to coastal systems making rivers a critical link between terrestrial and ocean carbon pools. During transport through fluvial systems, various biogeochemical processes selectively remove or transform labile material, effectively altering the composition of dissolved organic matter (DOM) exported to the ocean. The river continuum concept (RCC) has been historically used as a model to predict the fate and quality of organic matter along a river continuum. However, the conversion of natural landscapes for urban and agricultural practices can also alter the sources and quality of DOM exported from fluvial systems, and the RCC may be significantly limited in predicting DOM quality in anthropogenically impacted watersheds. Here, we studied DOM dynamics in the Altamaha River watershed in Georgia, USA, a fluvial system where headwater streams are highly impacted by anthropogenic activities. The primary goal of this study was to quantitatively assess the importance of both the RCC and land use as environmental drivers controlling DOM composition. Land use was a stronger predictor of spatial variation (∼50%) in DOM composition defined by both excitation-emission matrix-parallel factor analysis (EEM-PARAFAC) and ultrahigh-resolution mass spectrometry. This is compared to an 8% explained variability that can be attributed to the RCC. This study highlights the importance of incorporating land use among other controls into the RCC to better predict the fate and quality of DOM exported from terrestrial to coastal systems.

Assessment of Spatial Variability and Temporal Dynamics of Dissolved Organic Matter (DOM) at Lower Kinabatangan River Catchment, Sabah.

The spatial and temporal variability of dissolved organic matter (DOM) characteristics and surface water quality in the Lower Kinabatangan River Catchment were determined between October 2015 and May 2016. The objectives of this study were: (i) to distinguish the DOM absorption characteristics and physicochemical quality of surface water draining from different types of land use: oil palm plantation (OP), secondary forest (SF) and semi-natural vegetation (SV); and to examine its temporal variations during dry and wet periods. The collected physicochemical parameters data was analysed and classified based on the Malaysian National Water Quality Standard (NWQS). Findings indicated all the parameters fall into Class I, except for pH, total suspended solids (TSS) and concentration dissolved oxygen (DO). Linear discriminant analysis has been applied to distinguished the physico-chemical and absorption DOM properties data into mutually-exclusive spatial and temporal groups. Interestingly, the pH, DO and total nitrogen values were exhibited as dominant parameters at SV during both low and high rainfall periods. The dominance of these parameters suggested that the spatially and temporally varied water quality were influenced by both natural processes (e.g precipitation rate) and anthropogenic factor (e.g land use change). Whereas, both absorption coefficients (a340) and spectral slope (S275-295) were more dominant at SF and OP respectively. This might be due to increasing terrestrial DOM loadings as well as significant degradation of DOM via microbial and/or photochemical reaction.

Migration and evolution of dissolved organic matter in landfill leachate-contaminated groundwater plume

Groundwater pollution around landfill sites is becoming a global environmental problem. The hydrochemistry of dissolved organic matter (DOM) in landfill leachate-contaminated groundwater plume has been investigated extensively. The spatial migration and evolution of DOM, the relationship between hydrochemistry and DOM dynamics, and the role of microorganisms in DOM transformation are still largely unclear. In this study, UV–vis, 3D-EEM PARAFAC and principal components analyses were combined to investigate the migration and evolution of DOM. The results show that when the contamination plume gradually diffuses, the molecular weight of DOM in downstream groundwater tends to be high, while the humification degree is low and DOM is mostly derived from microbial metabolism. Compared with humic-like and fulvic-like substances, protein-like substances have a higher migration capability, making them suitable indicators of groundwater pollution. Under low-permeability aquifer medium (K < 10−7 cm/s), the DOM pollution was easy to rebound. An evident positive correlation exists between electrical conductivity and the maximum fluorescence intensity of humic-like and fulvic-like substances. Venn diagram shows that 15.31% (117 of 764) of the species were unique in the upstream and could not be detected in downstream groundwater. In comparison, 43.05% (489 of 1136) of the species were unique in the downstream and 382 were not detected in upstream groundwater. The main microorganisms (such as Polynucleobacter, Comamonadaceae, Desulfobulbus, and Magnetovibrio) play a pivotal role in DOM transformation in the aquifer. This study provides a novel insight into the monitoring, early warning and remediation of groundwater pollution in landfills.

Spatial Variation in the Molecular Composition of Dissolved Organic Matter from the Podzol Soils of a Temperate Pine Forest

In podzols, the production and mobilization of dissolved organic matter (DOM) account for a considerable loss of carbon from the forest floor. The dynamics of DOM in the podzol soils of a pine fore...

Quantity and quality variations of dissolved organic matter (DOM) in column leaching process from agricultural soil: Hydrochemical effects and DOM fractionation.

Leaching dynamics of dissolved organic matter (DOM) from agricultural soil are influenced by the chemistry of irrigation water. This study used multiple spectroscopy methods to investigate the dynamics of DOM in column leaching process by utilizing selected solutions. DOC was leached out by Na solutions more effectively than by Ca, despite the additional contributions of the ligand exchange of SO42- and the hydrolysis of HCO3-. The concentration ranking corresponding to solutions is Na2SO4 > NaHCO3 > NaCl > CaCl2. Furthermore, the aromaticity and molecular weights of DOM leached by Na solutions increased with the leaching, whereas contrary trends were reared by Ca. Thus, specific cation substantially affects DOM concentration and composition. Parallel factor analysis of the three-dimensional fluorescence spectra (EEM-PARAFAC) determined the characteristics of DOM components. (1) The long-wavelength, humic-like components (HLC, i.e., C1, C3, and C5) were largely accumulated at the early water saturating stage but attenuated toward dynamic stability in the salt-solution leaching (SSL) stage. (2) Reactive functional group-related HLC (i.e., C2, and C4) remarkably increased in response to the solutions chemistry. (3) The protein-like component (PLC, i.e., C6) was released with insignificant dynamic differences between the solutions. Despite approximately stable DOC concentration at the late stage of each SSL, HLC exhibited accelerating release in sodium salts but declining trend in calcium salt whereas PLC showed a roughly constant dynamic in the both. Ca-bridging reactions with the detachment of OM-Ca-clay complex prompting HLC release and its formation to stabilize the HLC well explain the contrast dynamics of HLC with Na or Ca leaching. The diffusion process that was defined for the slow, lasting and small concentration of leaching of inactive components could contribute to the steady dynamic of PLC, given its insensitivity to either the salt cation. This study identified distinct release kinetics of DOM compositions from those of DOC by EEM-PARAFAC and correspondingly the potential mechanisms, which offers promising advantages in monitoring DOM transport and fractionation in dynamic leaching process.

Immobilization of relic anthropogenic dissolved organic matter from alpine rivers in the Himalayan-Tibetan Plateau in winter

The Tibetan Plateau is a critical ecosystem that sensitively responds to ongoing glacier shrinkage and permafrost thaw. Dissolved organic matter (DOM) in Tibetan Alpine rivers plays pivotal roles in the biogeochemical cycling of elements and nutrients at regional and even global scales, impacting water quality, downstream environments, and climate. However, little is known about the characteristics and dynamics of DOM in these watersheds. We investigated five major Himalayan rivers in the southern Tibetan Plateau, utilizing bulk dissolved organic carbon (DOC), optical properties, and molecular formulas. We found extremely low DOC and fluorescent DOM (FDOM) levels in the rivers (average DOC: 0.25–0.87 mg L−1, FDOM: 0.02–0.05 RU) with a highly degraded molecular signature, which was enriched with heteroatomic molecular formulas (S-containing: 58–72%, N-containing: 61–86%) and condensed aromatics (31–39% vs. ∼11% in world major rivers). Further, 81–99% of the condensed aromatics was identified as dissolved black nitrogen (DBN) with multiple nitrogen atoms, typical for grassy biomass combustion. The findings highlighted potentially fast DOM remineralization leading to the release of CO2 and enriched apparently anthropogenic condensed aromatics and heteroatomic formulas in what have been considered pristine Tibetan rivers. These findings should be considered in future biogeochemical models and ecosystem management.

Multi-decadal trends and influences on dissolved organic carbon distribution in the Barataria Basin, Louisiana from in-situ and Landsat/MODIS observations

Barataria Basin, a deltaic marsh-estuary system in the northern Gulf of Mexico (nGOM), is an important source of dissolved organic matter (DOM) to the coastal ecosystem. To assess DOM dynamics, monthly measurements of dissolved organic carbon (DOC) concentrations and colored DOM (CDOM; an optical proxy for DOC) absorption were obtained over a three-year period (2008–2011) along a transect encompassing the upper, middle and lower regions of the Barataria Basin estuary. While CDOM absorption coefficient at 355 nm (ag355) generally decreased and spectral slope at 275–295 nm (S275–295) increased along the transect from the upper to lower basin, deviations were observed associated with hydrologic and meteorological influences. An application of wavelet analysis to the time-series of ag355 and various freshwater sources revealed a spatiotemporal resonance of ag355 with freshwater input from the Mississippi River (MR), the controlled freshwater discharge from the Davis Pond freshwater diversion, and rainfall in the lower, middle and upper basins, respectively. Field measurements and Landsat/MODIS data spanning across different seasons between 2008 and 2011 were used to develop optimized ag355 empirical algorithms for the high river flow, cold front, and normal conditions. Power law models relating a band ratio (green/red) of a satellite sensor to ag355 showed good performance for the MODIS-Aqua (R2~0.81–0.83, N = 581) and Landsat-5 TM (R2~0.72–0.79, N = 304) imagery. ag355-DOC relationships (R2~0.77–0.84, N = 306) applied to historical Landsat-5 TM (1985–2011) and MODIS-Aqua imagery (2002−2012) showed strong environmental influences on multi-decadal CDOM and DOC spatiotemporal trends. Satellite-derived DOC agreed reasonably well with in-situ measured DOC (averaged R2 = 0.71; RMSE = 2.13). While the time-series DOC showed seasonality and decreasing gradients from the upper to lower basin, an overall increase in DOC was observed in the upper and middle basins from 1985 to 2012. However, in the lower basin, DOC increased from 1985 to 2006 but decreased from 2007 to 2012, likely due to the impact of two major hurricanes in 2005. Furthermore, relationships between satellite-derived DOC and land cover variations (1985–2011) along the wetland-estuary interfaces derived from Landsat-5 TM supervised classification showed an increase in DOC with corresponding increase in developed area in the upper basin, while in the lower basin, DOC increased by 41% (R2 = 0.54) between 1985 and 2006 corresponding to a 17% decrease in salt marsh area, suggesting strong land use/land loss impact on the long-term DOC trends in Barataria Basin.

Shining light on the storm: in-stream optics reveal hysteresis of dissolved organic matter character

The quantity and character of dissolved organic matter (DOM) can change rapidly during storm events, affecting key biogeochemical processes, carbon bioavailability, metal pollutant transport, and disinfection byproduct formation during drinking water treatment. We used in situ ultraviolet–visible spectrophotometers to concurrently measure dissolved organic carbon (DOC) concentration and spectral slope ratio, a proxy for DOM molecular weight. Measurements were made at 15-minute intervals over three years in three streams draining primarily agricultural, urban, and forested watersheds. We describe storm event dynamics by calculating hysteresis indices for DOC concentration and spectral slope ratio for 220 storms and present a novel analytical framework that can be used to interpret these metrics together. DOC concentration and spectral slope ratio differed significantly among sites, and individual storm DOM dynamics were remarkably variable at each site and among the three sites. Distinct patterns emerged for storm DOM dynamics depending on land use/land cover (LULC) of each watershed. In agricultural and forested streams, DOC concentration increased after the time of peak discharge, and spectral slope ratio dynamics indicate that this delayed flux was of relatively higher molecular weight material compared to the beginning of each storm. In contrast, DOM character during storms at the urban stream generally shifted to lower molecular weight while DOC concentration increased on the falling limb, indicating either the introduction of lower molecular weight DOM, the exhaustion of a higher molecular weight DOM sources, or a combination of these factors. We show that the combination of high-frequency DOM character and quantity metrics have the potential to provide new insight into short-timescale DOM dynamics and can reveal previously unknown effects of LULC on the chemical nature, source, and timing of DOM export during storms.

The influence of tillage on dissolved organic matter dynamics in a Mid-Atlantic agroecosystem

Dissolved organic matter (DOM) represents a fraction of soil organic matter essential to many biogeochemical processes in surface soils and subsoils. Given its dynamic nature, DOM is known to be sensitive to land management practices. This study investigates the influence of tillage on DOM dynamics, including amount and composition of DOM, through a soil profile up to 72 cm depth from moldboard plowed (MB) and no-till (NT) plots in a Mid-Atlantic agroecosystem. Composition of DOM is probed using UV/Visible spectrophotometry (UV/Vis), Fourier transform infrared spectroscopy (FTIR) and fluorescence excitation-emission matrices (EEMs). Tillage decreases extractable DOM at the topmost depth interval studied (0–12 cm), with similar amounts extracted at depths below this point from MB and NT soils. DOM content of MB and NT soils decreases with depth, following a gradual decline in NT soils and a sharp decrease below the plow layer (20 cm) in MB soils. In both soils, chemical composition of DOM becomes less aromatic and less polysaccharide-like in character with depth accompanied by a relative enrichment in carboxylate groups. EEMs demonstrate that species within DOM follow a corresponding transition from conjugated fluorophores to moieties attributable to simple phenol, protein, amino acid and nucleic acid-like DOM constituents. These depth trends in functional group composition follow a distribution similar to DOM content, with observed transitions more abrupt in the MB than NT soils and marked by the plow layer. The decrease in DOM content with depth and the aforementioned shifts in DOM characteristics point to sorptive fractionation of DOM by the soil matrix and/or a conversion from more plant-derived DOM near the surface to more microbially processed DOM in subsoils. Overall, this work further highlights the impact of tillage on DOM behavior below the zone of physical disruption, suggesting that tillage also influences downstream DOM dynamics.

Land-based dissolved organic nitrogen dynamics and bioavailability in Jiaozhou Bay, China

Dissolved organic matter (DOM) samples from forestry, livestock breeding, urban domestic and agricultural sources around Jiaozhou Bay were incubated in seawater to examine DOM dynamics and bioavailability, which is the availability of DOM to degradation by microbes. The fluorescence signals of protein- and humic-like compounds were used as a proxy for bioavailable and refractory materials, respectively, whereas the evolution of SUVA254, dissolved organic nitrogen (DON) and dissolved organic carbon (DOC) concentrations were used to discuss bioavailability. The high bioavailability DON sources were from livestock breeding and urban domestic sources, with no significant difference, followed by agricultural and forestry sources, with respective degradation rate constants of 0.36 ± 0.01 (mean ± SD) d−1, 0.32 ± 0.03 d−1, 0.22 ± 0.04 d−1, and 0.18 ± 0.01 d−1, and with bioavailable DON (BDON%) representing 70% ± 10%, 67% ± 7%, 62% ± 8% and 37% ± 12% of the total DON, respectively. The turnover times of BDON were 3–6 days, far less than the flushing time of Jiaozhou Bay. Approximately 66% of land-based DON may contribute to DIN by microbial re-mineralization in Jiaozhou Bay, with the main sources being urban domestic, agricultural and livestock breeding. DON seemed to exhibit higher bioavailability than DOC. The temporal evolution of fluorophores and optical indices revealed that bioavailable DOM from livestock breeding and urban domestic sources was high. Spectroscopic properties indicated that the degree of humification of the organic molecules changed significantly for DON from livestock breeding and urban domestic sources, followed by agricultural sources, and forestry sources were the lowest. SUVA254 values and HIX were negatively correlated with DON bioavailability, which can be used as a refractory index and for predicting bioavailable DON.