Variability Of Dissolved Organic Matter Research Articles

Stratification of dissolved organic matter in the upper 5000 m water column in the western Pacific Ocean

Marine dissolved organic matter (DOM) is a principal reservoir involved in biogeochemical cycles and exerts a pivotal influence on global carbon flux dynamics. In this study, excitation-emission matrix fluorescence spectroscopy combined with parallel factor analysis (EEM-PARAFAC) was conducted on 230 DOM samples collected from 21 sites between February and April 2022 in the Western Pacific Ocean (WPO). We identified five distinct fluorescence peaks (peaks B, T, A, C, and M), predominantly protein-like and humic-like components. These findings, marked by significant differences (p < 0.01) in fluorescence intensities and spectral indices, characterized the transformation of DOM with ocean depth, illustrating a transition from active to recalcitrant forms. Additionally, random forest analysis (RFA) identified depth as a key factor influencing marine dissolved organic carbon (DOC), with a 32.59% importance value. Correlations between hydrological and fluorescent parameters underscored the complexity of DOM sources and influencing processes. Overall, this work broadens our understanding of DOM variability in the upper 5000 m of the WPO, enhancing our knowledge of the marine environment's role in the global carbon cycle.

Carbon Stable Isotope Ratio of Dissolved Organic Matter as a Tool To Identify Its Sources and Transformations in a Tropical Montane Forest in Ecuador.

Dissolved organic matter (DOM) contributes to forest C cycling. We assessed temporal variability, sources, and transformations of DOM during four years in a tropical montane forest with the help of stable C isotope ratios (δ13C values). We measured δ13C values of DOM in rainfall (RF), throughfall (TF), stemflow (SF), litter leachate (LL), soil solutions at the 0.15 and 0.30 m depths (SS15, SS30), and streamflow (ST) with TOC-IRMS. The δ13C values of DOM did not vary seasonally. We detected an event with a high δ13C value likely attributable to black carbon from local pasture fires. The mean δ13C values of DOM outside the event decreased in the order, RF (-26.0 ± 1.3‰) > TF (-28.7 ± 0.3‰) > SF (-29.2 ± 0.2‰) > LL (-29.6 ± 0.2‰) because of increasing leaching of C-isotopically light compounds. The higher δ13C values of DOM in SS15 (-27.8 ± 1.0‰), SS30 (-27.6 ± 1.1‰), and ST (-27.9 ± 1.1‰) than in the above-ground solutions suggested that roots and root exudates are major belowground DOM sources. Although in DOM the C/N ratios correlated with the δ13C values when all solutions were considered, this was not the case for SS15, SS30, and ST alone. Thus, the δ13C values of DOM provide an additional tool to assess the sources and turnover of DOM.

Dissolved Organic Matter Fluorescence as a Tracer of Upwelling and Microbial Activities in Two Cyclonic Eddies in the Eastern Tropical North Atlantic

AbstractMesoscale eddies are frequently observed in the Eastern Tropical North Atlantic (ETNA), yet their effects on the transport and distribution of biogeochemical solutes, and specifically on the production and remineralization of dissolved organic matter (DOM) remain difficult to elucidate. Here, we investigated the submesoscale variability of chromophoric DOM (CDOM) and fluorescent DOM (FDOM) together with microbial production and remineralization processes in two cyclonic eddies (CEs) in the ETNA during summer and winter 2019. One CE, formed near the coast off Mauritania during the post‐upwelling season, was sampled along a ∼900 km zonal corridor between Mauritania and the Cape Verde Islands. The other CE, formed nearby Brava Island, was out of coastal influence. Four fluorescent components were identified with parallel factor analysis, two humic‐like, and two protein‐like components. Humic‐like FDOM components correlated to optode‐based community respiration and were also good indicators of upwelling associated with the Brava Island CE as they correlated to physical parameters (e.g., temperature) and to dissolved inorganic nitrogen. The tryptophan‐like FDOM components correlated with the carbon and nitrogen content of semi‐labile DOM, phytoplankton biomass, community respiration, and bacterial production. Overall, our study revealed that DOM optical properties are suitable for tracing freshly produced organic matter and the transport of remineralized DOM within offshore eddies.

The influence of sources and solid phase extraction criteria on dissolved organic matter optical properties in aquatic systems

The integrity of studies investigating the composition of dissolved organic matter (DOM) is heavily dependent on the sampling and instrumental techniques employed. In this study, we aimed to assess the impacts of both the source of the DOM and the solid phase extraction (SPE) criteria on changes to the optical properties of DOM. Samples were collected from various aquatic environments, ranging from headwaters to the open ocean. The extraction kinetics of DOM were monitored throughout the SPE process, with alterations to the DOM verified using optical properties derived from chromophoric dissolved organic matter (CDOM), including absorbance and fluorescence measurements. The fluorescence excitation-emission matrices (EEMs) were further analyzed using PARAllel FACtor analysis (PARAFAC) to resolve the underlying chemical components. Results indicate that the reaction of DOM to acidification required for polystyrene-divinylbenzene polymer sorbent SPE varies across samples. And this variability may stem from differences in functional groups and DOM sources. The maximum carbon loading observed was 9.7 mg C per 1 g PPL, which is a widely used styrene-divinylbenzene polymer sorbent, and no overloading was noted. The dissolved organic carbon (DOC) extraction efficiency (EE) ranged from 51.5-61.5% and 27.6-40.0% for post- and extract- calculations, respectively, with no significant differences in DOC EE observed among different initial DOC concentrations, salinities, and flowrates. The differences in DOC EE across diverse sources and changes in optical proxies resulting from PPL extraction were mainly due to the heterogeneity of DOM. PPL extraction altered the overall molecular characteristics of DOM from more labile to more refractory or degraded in the open ocean and headwater source samples. Notably, high and low flowrates yielded identical SPE-DOM extraction and alterations. The results indicate that DOM sources primarily control SPE EE rather than methodological or other environmental factors. PPL SPE results in a convergence of DOM variability. These findings provide crucial insights into inter-study comparisons and the interpretability of SPE-derived DOM data from samples across diverse aquatic environments.

A snapshot on vertical variability of dissolved organic matter in the epilagic zone of the eastern Indian Ocean

Marine dissolved organic matter (DOM) plays an important role in aquatic ecosystems and is an essential reservoir of organic carbon in the marine carbon cycle. In this study, seawater DOM samples from 33 stations were collected in spring 2022 (April to May, 20 stations) and autumn 2020 (October to November, 13 stations) to better characterize and compare DOM variability within 200 m water layer in the eastern Indian Ocean (EIO). Hydrological parameters, nutrients and spectroscopic indices were calculated and evaluated for two cruises. In addition, excitation emission matrix spectroscopy combined with parallel factor analysis (EEM-PARAFAC) was used to directly analyse seawater DOM samples. The sources and processes of DOM in the EIO were assessed by fluorescence index (FI), freshness index (β/α), Biological index (BIX), and humification index (HIX). Three fluorescent components were identified in DOM samples from two cruises, including: C1 (tryptophan- and tyrosine-like), C2 (marine and/or terrestrial humic-like), and C3 (terrestrial humic-like). The components of C1 accounted for 39.45 % ± 8.79 %, C2 for 33.05 % ± 6.42 %, and C3 for 27.20 % ± 4.47 % within 200 m water layer. The intensity of the DOM fluorescence seems to varied due to seasonal differences. In particular, the source of the DOM fraction varied at <100 m layer, which may also be related to the structure of the microbial community. Further, there is a strong correlation between the depth of seawater and hydrographic parameters, fluorescence indices and fluorescence components. Nutrients (nitrate, dissolved inorganic phosphate, and dissolved silicate) and humic-like fractions are more likely to accumulate in the deeper layers of the ocean. Thus, these results provide some data support for the variability of DOM fractions on a vertical scale in the EIO.

Spatial variability of dissolved organic matter in the water-sediment systems in the Baiyangdian Lake

Dissolved organic matter (DOM) is widespread in natural water bodies and plays an essential role in ecological processes of aquatic ecosystems. However, spatial variability of DOM in shallow lakes is still kept unclear. Surface water, overlaying water and sediment samples were collected at 61 sampling sites in the whole Baiyangdian Lake to investigate spatial variability and distribution patterns of organic matter contents in three environmental compartments using geostatistical analysis. Results showed that organic matter (OM) contents in sediments were higher with medium variability, and lower DOM content occurred in surface water, with medium variability in overlying waters and high variability in surface waters. The highest DOM contents in waters and OM in sediments appeared in Zaolinzhung (ZLZ) and Zaozhadian (ZZD), respectively, while a relatively low contents appeared in ZLZ sediments. DOM in surface water and overlying water were negatively correlated with OM in sediments. Semi-variance functions showed that a strong spatial autocorrelation for DOM in waters, while OM in sediments had a medium spatial correlation. The autocorrelation distance of DOM in waters was between 1.16 km and 11.70 km, and OM in sediments was 4.25 km. The findings of this work can contribute to better understanding spatial variability of OM, which would provide a guidance for the sampling optimization of waters and sediments in the Baiyangdian Lake and lake sustainable management.

Marine Dissolved Organic Matter Shares Thousands of Molecular Formulae Yet Differs Structurally across Major Water Masses.

Most oceanic dissolved organic matter (DOM) is still not fully molecularly characterized. We combined high-field nuclear magnetic resonance (NMR) and ultrahigh-resolution mass spectrometry (Fourier-transform ion cyclotron resonance mass spectrometry, FT-ICR-MS) for the structural and molecular formula-level characterization of solid-phase extracted (SPE) DOM from surface, mesopelagic, and bathypelagic Atlantic and Pacific Ocean samples. Using a MicroCryoProbe, unprecedented low amounts of SPE-DOM (∼1 mg carbon) were sufficient for two-dimensional NMR analysis. Low proportions of olefinic and aromatic relative to aliphatic and carboxylated structures (NMR) at the sea surface were likely related to photochemical transformations. This was consistent with lower molecular masses and higher degrees of saturation and oxygenation (FT-ICR-MS) compared to those of the deep sea. Carbohydrate structures in the mesopelagic North Pacific Ocean suggest export and release from sinking particles. In our sample set, the universal molecular DOM composition, as captured by FT-ICR-MS, appears to be structurally more diverse when analyzed by NMR, suggesting DOM variability across oceanic provinces to be more pronounced than previously assumed. As a proof of concept, our study takes advantage of new complementary approaches resolving thousands of structural and molecular DOM features while applying reasonable instrument times, allowing for the analysis of large oceanic data sets to increase our understanding of marine DOM biogeochemistry.

Effects of ant colonies on molecular characteristics of dissolved organic matter in peatland soils, Northeast China

Increasing water discharge has resulted in rising dissolved organic matter (DOM) in draining peatlands. However, how contents and spectral characteristics of DOM changed by increasing terrestrial ants in peatland with water level decreasing, were not well revealed. In the present work, chemical and molecular composition of DOM in ant and nearby soils were characterized using UV–Visible absorption and fluorescence excitation-emission matrix (EEM) combined with parallel factor analysis (PARAFAC). Results indicated that four fluorescence components were identified, of which endogenetic fluorophores accounted for a higher proportion (79.21 ± 0.06%) of the total variability of all four. DOM from ant mound soils had lower aromaticity, humification degree, and smaller molecular size than those from control soil. The influence of microorganisms on the content and structure of DOM was crucial in the ant mound soil, whereas the effect from soil physicochemical properties was apparently more pronounced in the control soil. The study demonstrated that the endogenetic and microbial substances were closely related to the compositional variability of DOM in ant and control soils of peatlands.

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.

Chemodiversity of Soil Dissolved Organic Matter and Its Association With Soil Microbial Communities Along a Chronosequence of Chinese Fir Monoculture Plantations.

The total dissolved organic matter (DOM) content of soil changes after vegetation transformation, but the diversity of the underlying chemical composition has not been explored in detail. Characterizing the molecular diversity of DOM and its fate enables a better understanding of the soil quality of monoculture forest plantations. This study characterized the chemodiversity of soil DOM, assessed the variation of the soil microbial community composition, and identified specific linkages between DOM molecules and microbial community composition in soil samples from a 100-year chronosequence of Chinese fir monoculture plantations. With increasing plantation age, soil total carbon and dissolved organic carbon first decreased and then increased, while soil nutrients, such as available potassium and phosphorus and total nitrogen, potassium, and phosphorus, increased significantly. Lignin/carboxylic-rich alicyclic molecule (CRAM)-like structures accounted for the largest proportion of DOM, while aliphatic/proteins and carbohydrates showed a decreasing trend along the chronosequence. DOM high in H/C (such as lipids and aliphatic/proteins) degraded preferentially, while low-H/C DOM (such as lignin/CRAM-like structures and tannins) showed recalcitrance during stand development. Soil bacterial richness and diversity increased significantly as stand age increased, while soil fungal diversity tended to increase during early stand development and then decrease. The soil microbial community had a complex connectivity and strong interaction with DOM during stand development. Most bacterial phyla, such as Acidobacteria, Chloroflexi, and Firmicutes, were very significantly and positively correlated with DOM molecules. However, Verrucomicrobia and almost all fungi, such as Basidiomycota and Ascomycota, were significantly negatively correlated with DOM molecules. Overall, the community of soil microorganisms interacted closely with the compositional variability of DOM in the monoculture plantations investigated, both by producing and consuming DOM. This suggests that DOM is not intrinsically recalcitrant but instead persists in soils as a result of simultaneous consumption, transformation, and formation by soil microorganisms with extended stand ages of Chinese fir plantations.

In-Situ Variability of DOM in Relation with Biogeochemical and Physical Parameters in December 2017 in Laucala Bay (Fiji Islands) after a Strong Rain Event

Heavy rain events alter the biogeochemical outflows, affects water quality and ecosystem health within the coastal waters of small Pacific Islands. We characterized snapshots of the optical fingerprints of dissolved organic matter (DOM) sources together with the select nutrients, biogeochemical and physical variables for 10 stations in December 2017 in Laucala Bay, Fiji Islands. DOM absorption coefficients and fluorescence components were determined via spectrofluorometry and Parallel Factor Analysis identifying four components: Type M, two terrestrial (humic, fulvic) components, and a protein component linked to marine biological activity. Associations of DOM together with climate variables and the other tested variables were determined via principal component, hierarchical cluster, and cross-correlation (Pearson) analysis. All component s (together with most tested variables) displayed higher values (plumes) at the southwest coast consistent with surface currents outflow during the wet season. Type M component associated with two allochthonous fluorescent components signaling anthropogenic forcings via riverine outflows. Terrigenous inputs association with autochthonous chromophoric dissolved organic matter (CDOM) is indicative of tidal mixing, dilution, and bottom resuspension processes. Positive correlations of dissolved organic carbon (DOC) with nutrients (NOx, PO4) elucidates DOM being utilized as energy sources. The positive correlation of DON with nutrients (NOx, PO4, Si(OH)4) reflects the role of DON as a nutrient source consistent with chlorophyll plume formation.

Use of optical absorption indices to assess seasonal variability of dissolved organic matter in Amazon floodplain lakes

Abstract. Given the importance of dissolved organic matter (DOM) in the carbon cycling of aquatic ecosystems, information on its seasonal variability is crucial. In this study we assess the use of optical absorption indices available in the literature based on in situ data to both characterize the seasonal variability of DOM in a highly complex environment and for application in large-scale studies using remote sensing data. The study area comprises four lakes located in the Mamirauá Sustainable Development Reserve (MSDR). Samples for the determination of colored dissolved organic matter (CDOM) and measurements of remote sensing reflectance (Rrs) were acquired in situ. The Rrs was used to simulate the response of the visible bands of the Sentinel-2 MultiSpectral Instrument (MSI), which was used in the proposed models. Differences between lakes were tested using the CDOM indices. The results highlight the role of the flood pulse in the DOM dynamics at the floodplain lakes. The validation results show that the use of the absorption coefficient of CDOM (aCDOM) as a proxy of the spectral slope between 275 and 295 nm (S275–295) during rising water is worthwhile, demonstrating its potential application to Sentinel-2 MSI imagery data for studying DOM dynamics on the large scale.

Two-Phase System Model to Assess Hydrophobic Organic Compound Sorption to Dissolved Organic Matter.

The equilibrium partition of organic compounds to dissolved organic matter (DOM) is an essential process that affects their environmental risks. Traditional models cannot accurately assess this process as the variability of DOM is not properly accounted for. The two-phase system (TPS) model was developed with the consideration of the variability that stems from both organic compounds and DOM. In this study, we examined the applicability of the TPS model for the prediction of the organic carbon-water partition coefficient (KOC) of hydrophobic organic compound (HOC) sorption to aquatic and sediment DOM using a diverse set of 17 organic compounds and 53 DOM samples. The TPS model showed good predictive power (RMSE < 0.20) without calibration, outperforming currently used linear free energy relationship models (RMSE > 0.28). The significance of DOM properties in the sorption behavior was quantitatively analyzed based on the TPS model. The spatial pattern of KOC for HOC sorption to aquatic DOM in Lake Taihu, the third largest freshwater lake in China, was assessed using the TPS model. Our results suggest that the TPS model has great potential to facilitate the routine assessment of the partition behavior of HOCs in aquatic systems for environmental risk assessment.

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.

Variability in Dissolved Organic Matter Composition and Biolability across Gradients of Glacial Coverage and Distance from Glacial Terminus on the Tibetan Plateau.

Globally, alpine glaciers hold a large quantity of dissolved organic matter (DOM) and are headwaters of numerous rivers supporting downstream heterotrophic metabolism. However, it remains unclear how glacial coverage and distance from the glacial terminus affect the fate of DOM. Here, we elucidate DOM variability in glacial-fed streams on the Tibetan Plateau using field sampling and bioincubation experiments and compare our findings with the existing literature. We found that dissolved organic carbon, DOM absorption a(254), DOM aromaticity, and the relative abundance of lignin compounds in glacial-fed streams and rivers all increased with increasing distance from the glacial terminus and with decreasing glacial coverage. We also found that contribution of protein-like components, the relative abundance of aliphatic compounds, and DOM biolability increased with increasing glacial coverage and with decreasing distance from the glacial terminus. The ratio of glacial coverage to the logarithmic transformed distance from the glacial terminus was better than that of actual glacial coverage and distance from the glacial terminus in tracing the variability of glacial-fed stream DOM. Microbes in surface ice can produce biolabile DOM that is exported downstream with meltwater. This glacial-fed stream and river DOM is an important source of the highly bioavailable material fueling downstream heterotrophic activity.

Influence of land use and rainfall on the optical properties of dissolved organic matter in a key drinking water reservoir in China

The concentration, source and composition of dissolved organic matter (DOM) in aquatic ecosystems are associated with land use and hydrological connectivity between terrestrial and aquatic systems. However, direct evidence of the effects of rainfall and land use on the variability of DOM in aquatic ecosystems is very limited. In this study, chromophoric DOM (CDOM) absorption and fluorescence spectroscopy were used to elucidate how rainfall and land use affect the variability of CDOM in the watershed of Lake Tianmu, a key drinking water reservoir in the Yangtze River Delta. The mean values of the fluorescence intensity (Fmax) of parallel factor analysis-derived humic-like components (C1, C3, C6) and tryptophan-like components C5 were higher in the southeastern inflowing river mouths than those downstream of the lake outlet regions. The upstream tributaries were mainly dominated by humic-like materials, while the lake was mainly dominated by protein-like materials. The Fmax values of four humic-like components and two tryptophan-like components all increased significantly as the %woodland decreased, but %anthropogenic land use (%cropland+%urban construction area) increased. The Fmax of the humic-like components at the inflowing tributaries and the lake increased with increasing rainfall during storm events, and the value was especially pronounced at the inflowing river mouths. We concluded that land use and hydrological conditions play an important role in influencing the CDOM source and optical composition, and these findings provide insights for the understanding of aquatic ecosystem metabolism and reservoir water quality management.

Microbial production and consumption of dissolved organic matter in glacial ecosystems on the Tibetan Plateau

Dissolved organic matter (DOM) from alpine glaciers is highly biolabile and plays a vital role in the biogeochemical cycle of meltwater-impacted environments. To unravel the composition and interactions of DOM with the bacterial community in glacier and glacier meltwater, we conducted sampling of two different Tibetan Plateau glaciers and carried out laboratory bio-incubation experiments. The field data revealed that four protein-like components accounted for 86.0 ± 11.9% of the total variability of all six fluorescence components, which suggests a predominantly microbial source of glacial chromophoric DOM (CDOM). The ice and meltwater samples displayed major contributions of molecular formulae associated with lipids and proteins (i.e. high H/C) as revealed by ultrahigh resolution mass spectrometry. Multiple linear regression models revealed that the abundant phyla explain 64.2%, 61.3%, and 65.0% of the variability of microbial and terrestrial humic-like, and protein-like components, respectively. Correlation-based network analysis determined the metabolic niches of the bacterial community members associated with different fluorescence types in biogeochemical processes. Furthermore, laboratory DOM bio-incubation experiments confirmed that sub-components of the CDOM pool differentially participate in bacterial metabolism. We therefore conclude that the bacterial community interacted closely with the compositional variability of DOM in the investigated alpine glacial environments by both producing and consuming of DOM.

Homogenization of dissolved organic matter within a river network occurs in the smallest headwaters

Understanding the drivers of variability in the concentration and composition of dissolved organic matter (DOM) in running waters is a fundamental challenge for aquatic biogeochemistry. Although multiple conceptual frameworks have been proposed to quantify expected changes in DOM within a river network, few studies have collected data to document the variability of DOM within such a network. We examine spatial and temporal variability in the concentration, optical properties, and biodegradability of DOM within a river network in New Hampshire, U.S.A. We examined dissolved organic carbon (DOC) concentration and DOM optical properties across 21 locations within the Lamprey River network (548 km2) with weekly to monthly sampling for 4 years, and biodegradable DOC (BDOC) at 7 mainstem locations for 1 year. DOC concentration and DOM character were highly heterogeneous at the scale of the smallest streams, where variability in land cover and nutrient concentrations were also greatest. At the landscape scale, DOC concentrations and DOM aromaticity increased with wetland cover and decreased with developed land cover and nitrate concentrations. Despite this heterogeneity across streams, we observed similar synchronous seasonal variation in DOC concentration and DOM character regardless of position within the river network, suggesting that broad environmental factors shape the sources and transport of DOM in river networks. In contrast, variation in BDOC over time or across sites was not strongly related to DOM quality or other predictors. Spatiotemporal patterns in DOM provide insight into the complex processes that influence terrestrial carbon export, in-stream processes, and DOM delivery to downstream ecosystems.

Hydrological and Biogeochemical Controls of Seasonality in Dissolved Organic Matter Delivery to a Blackwater Estuary

Changes in riverine discharge of dissolved organic matter (DOM) serves as an indicator of linkages between terrestrial ecosystem and receiving aquatic environments. In this study, we test the hypothesis that the seasonal variability of DOM in an estuary fed by a blackwater river is primarily controlled by water discharge and also modified by photochemical and biological processes. We collected surface water samples during 4-week-long field campaigns to the lower Pearl River estuary located in southeastern Louisiana, two during high discharge in spring and two during low discharge in winter and summer, respectively. DOM composition was determined using spectrofluorometric indices and a site-specific parallel factor model, and dissolved organic carbon (DOC) concentrations. Spring samples with low salinity showed higher abundance of terrestrial, humic-like DOM and higher DOC concentrations, indicating the export of flood plain-derived DOM during high discharge. In contrast, summer and winter samples with high salinity had greater proportions of labile DOM and higher biological and fluorescence indices, which may reflect enhanced photochemical and biological degradation during summer and better preservation of labile DOM in winter. Spring DOM displayed highly variable source and quality character, relative to winter and summer samples. This observation suggests that river discharge acted as a more rapid and direct control of spatial variation in DOM and photochemical and biological degradation was responsible for removing this flow-related variation between seasons. The incubation experiments showed that natural light can remove terrestrial and microbial humic DOM, while bacterial degradation was responsible for degrading protein-like DOM. Our results provide new evidence that DOM seasonality in blackwater river estuarine environments is collectively regulated by discharge and photochemical and biological degradation.

Influence of land use and land cover on the spatial variability of dissolved organic matter in multiple aquatic environments.

Water quality of lakes, estuaries, and coastal areas serves as an indicator of the overall health of aquatic ecosystems as well as the health of the terrestrial ecosystem that drains to the water body. Land use and land cover plays not only a significant role in controlling the quantity of the exported dissolved organic matter (DOM) but also influences the quality of DOM via various biogeochemical and biodegradation processes. We examined the characteristics and spatial distribution of DOM in five major lakes, in an estuary, and in the coastal waters of the Mississippi, USA, and investigated the influence of the land use and land cover of their watersheds on the DOM composition. We employed absorption and fluorescence spectroscopy including excitation-emission matrix (EEM) combined with parallel factor (PARAFAC) analysis modeling techniques to determine optical properties of DOM and its characteristics in this study. We developed a site-specific PARAFAC model to evaluate DOM characteristics resulting in five diverse DOM compositions that included two terrestrial humic-like (C1 and C3), two microbial humic-like (C2 and C5), and one protein-like (C4) DOM. Our results showed elevated fluorescence levels of microbial humic-like or protein-like DOM in the lakes and coastal waters, while the estuarine waters showed relatively high fluorescence levels of terrestrial humic-like DOM. The results also showed that percent forest and wetland coverage explained 68 and 82% variability, respectively, in terrestrial humic-like DOM exports, while 87% variability in microbially derived humiclike DOM was explained by percent agricultural lands. Strong correlations between microbial humic-like DOM and fluorescence-derived DOM indices such as biological index (BIX) and fluorescence index (FI) indicated autochthonous characteristics in the lakes, while the estuary showed largely allochthonous DOM of terrestrial origin. We also observed higher concentrations of total dissolved phosphorous (TDP) and ammonium nitrogen (NH4-N) in coastal waters potentially due to photodegradation of refractory DOM derived from the sediment-bound organic matter in the coastal wetlands. This study highlights the relationships between the DOM compositions in the water and the land use and land cover in the watershed. The spatial variability of DOM in three different types of aquatic environments enhances the understanding of the role of land use and land cover in carbon cycling through export of organic matter to the aquatic ecosystems..