Chromophoric Dissolved Organic Matter Research Articles

Seasonal controls override forest harvesting effects on the composition of dissolved organic matter mobilized from boreal forest soil organic horizons

Abstract. Dissolved organic matter (DOM) mobilized from the organic (O) horizons of forest soils is a temporally dynamic flux of carbon (C) and nutrients, and the fate of this DOM in downstream pools is dependent on the rate and pathways of water flow as well as its chemical composition. Here, we present observations of the composition of DOM mobilized weekly to monthly from O horizons in mature forest and adjacent harvested treatment plots. The study site was experimentally harvested, without replanting, 10-years prior to this study. Thus, the treatments differ significantly in terms of forest stand and soil properties, and they interact differently with the regional hydrometeorological conditions. This presented an opportunity to investigate the role of forest structure relative to environmental variation on soil DOM mobilization. On an annual basis, fluxes of total dissolved nitrogen (TDN) and dissolved organic nitrogen (DON) were largest from the warmer and thinner O horizons of the harvested (H) treatment compared to the forest (F) treatment; however, neither phosphate nor ammonium fluxes differed by treatment type. On a short-term basis in both H and F treatments, all fluxes were positively correlated to water input, and all concentrations were positively correlated to soil temperature and negatively correlated to water input. Soil moisture was negatively correlated to the C : N of DOM. These results suggest common seasonal controls on DOM mobilization regardless of harvesting treatment. Optical characterization of seasonally representative samples additionally supported a stronger control of season over harvesting. The chemical character of DOM mobilized during winter and snowmelt: lower C : N, higher specific ultraviolet absorbance and lower molecular weight of chromophoric DOM (CDOM; higher spectral slope ratio) were representative of relatively more decomposed DOM compared to that mobilized in summer and autumn. This shows that the decomposition of soil organic matter underneath a consistently deep snowpack is a key determinant of the composition of DOM mobilized from O horizons during winter and the hydrologically significant snowmelt period regardless of harvesting impact. Despite the higher proportion of aromatic DOM in the snowmelt samples, its lower molecular weight and rapid delivery from O to mineral horizons suggests that the snowmelt period is not likely to be a significant period of DOM sequestration by mineral soil. Rather, the higher-molecular-weight, high-C : N DOM mobilized during slow and relatively infrequent delivery during summer and rapid, frequent delivery during autumn are more likely to support periods of mineral soil sequestration and increased export of fresher terrestrial DOM, respectively. These observed seasonal dynamics in O horizon DOM suggest the predicted decreases in winter and spring snowfall and increasing autumn and winter rainfall with climate warming in this region will enhance mobilization of DOM that is more reactive to mineral interactions in deeper soil, but also more biological and photoreactive in the aquatic environment. Understanding the downstream consequences of this mobilized DOM in response to these shifts in precipitation timing and form can improve our ability to predict and manage forest C balance but requires understanding the response of landscape hydrology to these changing precipitation regimes.

Optical Proxies of Euxinia: Spectroscopic Studies of CDOM, Chlorophyll, and Bacteriochlorophylls in the Lagoon on Zeleny Cape (the White Sea)

Along the shoreline of the White Sea, due to the post-glacial uplift of the coast, some water bodies with stable stratification have been formed. They have been classified as meromictic as they are at different stages of isolation from the Sea. As separation progresses, significant changes occur in the water column, including the composition of chromophoric dissolved organic matter (CDOM) and the structure of the aquatic microbial community. In this work, we searched for optical proxies of euxinia (anoxic conditions with accumulated hydrogen sulfide) in the water column of the meromictic lagoon on Zeleny Cape. The lagoon is separated from the White Sea basin by a shallow threshold that completely isolates the lagoon during low tide, but marine water enters the lagoon during high tide. The ecosystem in the lagoon is characterized by the marine salinity of water and a high organic matter content in the bottom water and sediments. In this study, spectral methods were used to obtain the depth distribution of CDOM, chlorophyll, and bacteriochlorophyll in the lagoon with strong water stratification and euxinic conditions in the bottom water. The measured optical CDOM characteristics were compared with hydrochemical data (water salinity, Eh, pH, dissolved oxygen), phytoplankton (oxygenic phototrophs), and green sulfur bacteria (anoxygenic phototrophs) distribution along the water column. The spectroscopic methods showed to have the advantages of not requiring water sample pre-treatment and allowing rapid sensing of CDOM and photosynthetic pigments at each horizon.

Rapid Correction of Turbidity and CDOM Interference on Three-Dimensional Fluorescence Spectra of Live Algae Based on Deep Learning

In natural aquatic environments, the existence of colored dissolved organic matter (CDOM), suspended particles, and colloids can cause scattering and reflection of light and even emit fluorescence itself. Such interference negatively impacts algal fluorescence, further making it unreliable to measure the algal concentration using three-dimensional excitation–emission matrix (3D-EEM) fluorescence spectroscopy. In this study, we proposed a novel algal fluorescence anti-interference network (AFAI-Net) based on a convolutional neural network. The main procedure of this model can be divided into two parts: (1) to quickly determine if there is an interference of CDOM or turbidity in the detected algal samples; (2) to correct the interfered samples and output the fluorescent components of the algae. We trained the model using the 3D-EEMs of pure algal samples (non-interfered) and mixed samples of algae and CDOM or turbidity (interfered); as a result, the well-trained model achieved a total classification accuracy of 96.82%, and the RMSE of CDOM and turbidity removal fitting effects were 0.2274 and 0.3423, respectively. Compared with the non-negative weighted least squares (NNLS) regression analysis method, using the CNN model for CDOM correction resulted in 13.11%, 0.65%, and 5.69% reductions in the average deviation rate for PD, PG, and CM, respectively. Furthermore, the spectra corrected by the model predicted algal densities that were closer to the true algal densities. This study provides a new way to remove non-algal factors that affect algal fluorescence spectra in water bodies, which is beneficial to monitoring eutrophication and red tide in aquatic systems.

Kinetic and Mechanistic Considerations of the Photosensitized Transformation of Chlorine in Chromophoric Dissolved Organic Matter Solutions under Simulated Solar Irradiation.

Chlorination is one of the most common disinfection methods for water treatments. Although the direct photolysis of free available chlorine (FAC) induced by solar irradiation has been extensively investigated, the photosensitized transformation of FAC caused by chromophoric dissolved organic matter (CDOM) has not previously been examined. Our results suggest that the photosensitized transformation of FAC can occur in sunlit CDOM-enriched solutions. Interestingly, the photosensitized decay of FAC can be fitted using a combined zero- and first-order kinetic model. The photogenerated O2•- from CDOM contributes to the zero-order kinetic component. The reductive triplet CDOM (3CDOM*) contributes to the pseudo-first-order decay kinetic component. The bimolecular reaction rate constants of the model triplet (3-methoxyacetophenone) with HOCl and OCl- were (3.6 ± 0.2) × 109 and (2.7 ± 0.3) × 109 M-1 s-1, respectively. Under simulated solar irradiation, the quantum yield coefficient of the reductive 3CDOM* toward FAC attenuation (fFAC = 840 ± 40 M-1) was 13 times greater than that of the oxidative 3CDOM* toward trimethylphenol (TMP) attenuation (fTMP = 64 ± 4 M-1). This study provides new insights into the photochemical transformation of FAC in sunlit surface waters, and the results are applicable when sunlight/FAC system is employed as an advanced oxidation process.

Green Synthesis of Silver Nanoparticles Using Aqueous Extract of Typha domingensis Pers. Pollen (qurraid) and Evaluate its Antibacterial Activity

In this study, the aqueous extract of (Typha domingensis Pers.) pollen grain (qurraid) to know its ability to manufacture silver nanoparticles. Qurraid is a semi-solid yellow food substance, sold in Basra markets and eaten by the local population. It is made from the pollen of the T. domingensis Pers. plant after being pressed and treated with water vapor. The Gas chromatography–mass spectrometry (GC-MS) reaction was done to identify the active compounds of qurraid aqueous extract. The ability of the aqueous extract of qurraid to manufacture silver nanoparticles was tested, and the construction of silver nanoparticles was inferred by the reaction mixture's color, which ranged from yellow to dark brown. The synthesized silver nanoparticles (AgNPs) were described by UV-Vis, FTIR, XRD, SEM, and EDX. Then its anti-bacterial activity was estimated by the agar well diffusion method. The findings of the GC-MS analysis of the qurraid aqueous extract showed the major components with their ratio were: 5-Hydroxymethylfurfural with RT% 13.6196, 3-Deoxy-d-mannoic lactone 6.4285,. alpha.-L-lyxo-Hexopyranoside, methyl 3-amino-2,3,6-trideoxy- 4.264, 4H-Pyran-4-one, 2,3-dihydro-3,5-dihydroxy-6-methyl- 3.2078, and 1,3-Methylene-d-arabitol 3.1257. The construction of silver nanoparticles was described by spectroscopic methods, where the highest peak was recorded at 400nm by UV-Vis spectrum, which indicates the silver spectrum. The mineral nature of AgNPs was confirmed by XRD analysis, in which the highest peaks were, 111, 300, and 330 were recorded. In addition, the qrdAgNPs nanoparticles were spherical with sizes ranging from 20-70nm. The results of the EDX confirmed that the chemical composition of AgNPs was silver. The ability of the AgNPs was tested against four bacterial species, three of which were Gram-negative Escherichia coli A1, Escherichia coli A2, Alcaligenes faecalis AL1, and the fourth was Gram-positive bacteria Bacillus zanthoxyli B1 , which were identified by traditional and molecular methods using 16SrRNA gene sequencing, antibacterial activity results of AgNPs showed that it increases with increasing of AgNPs concentration, and the most sensitive species to silver particles was Alcaligenes faecalis AL1bacteria.

Response of size-dependent chemical composition of dissolved organic and inorganic species to land use types in tropical coastal headstreams

Land-use types dramatically affect the source, composition, behavior and fate of headstream chemical species. However, the influence of land-use types on the size-dependent composition of dissolved organic matter (DOM) and dissolved inorganic matter (DIM) is still unclear. Here, we evaluated the size-dependent concentration, composition and relationship of chemical species from 9 coastal headstreams (i.e. forested, agricultural and wetland headstreams) using multiple spectroscopic techniques including absorbance, excitation-emission matrix, near-edge X-ray absorption fine structure spectroscopy (NEXAFS), X-ray diffraction (XRD), scanning electron microscopy combined with energy dispersive spectroscopy (SEM-EDS) and synchrotron-based micro-X-ray fluorescence (μ-XRF). Results showed that 68.57%-85.78% of dissolved organic carbon (DOC) with abundant humic/fulvic-like substances were distributed in the <1 kDa fraction in all samples, with a higher percentage in wetland samples. Agricultural land-use increased the export of aluminosilicate minerals in >1 kDa fractions and terrestrial fulvic/humic-like substances from land to stream. Forested streams showed high DOC in the 1 kDa-0.2 μm fraction, predominance of autochthonous DOM rich in amides, amines, carboxyl and O-alkyl groups, and abundance whitlockite and ferromanganese oxides. Wetland streams were characterized by abundant halite, sylvite, and microbially-transformed humic-like DOM rich in aromatic and phenolic groups. Morphology and correlation analysis implied DIM was closely combined with fluorescent DOM (FDOM) and colored DOM (CDOM), resulting in more irregular organo-inorganic complexes in forested and agricultural streams, and more halite and sylvite crystal blocks encapsulating by smooth and close organic–inorganic complexes in wetland streams. Redundancy analysis (RDA) analysis suggested that land-use types (17.20%) and molecular weight (14.29%) were the prominent factors impacting the composition and relationship of DOM and DIM. Land-use types altered the sources and transport patterns of chemical species in headstreams, influencing their size-dependent compositions and associated biogeochemical processes. Such information is imperative for monitoring and developing land-use policies in coastal regions undergoing intense anthropogenic alterations.

The characteristics of CDOM structural composition and the effect on indirect photodegradation of sulfamerazine

Sulfamerazine (SM) is a commonly used antibiotic and have been widely used to control various bacterial infectious diseases. The structural composition of colored dissolved organic matter (CDOM) is known to be a major factor that influences the indirect photodegradation of SM, yet the influence mechanism remains unknown. In order to understand this mechanism, CDOM from different sources was fractionated using ultrafiltration and XAD resin, and characterized using UV–vis absorption and fluorescence spectroscopy. The indirect photodegradation of SM in these CDOM fractions was then investigated. Humic acid (JKHA) and Suwannee River natural organic matter (SRNOM) were used in this study. The results showed that CDOM could be divided into four components (three humic-like components and one protein-like component), and terrestrial humic-like components C1 and C2 were found to be the main components that promote SM indirect photodegradation due to their high aromaticity. The indirect photodegradation of SM was much faster in low molecular weight (MW) solutions, whose structures were dominated by greater aromaticity and terrestrial fluorophores in JKHA and higher terrestrial fluorophores in SRNOM. The HIA and HIB fractions of SRNOM contained large aromaticity and high fluorescence intensities of C1 and C2, resulting in a greater indirect photodegradation rate of SM. The HOA and HIB fractions of JKHA had abundant terrestrial humic-like components and contributed more to SM indirect photodegradation.

Tidal exchange of dissolved metal(loid)s and organic matters across the sediment–water interface in a salt marsh-mangrove ecotone

Coastal wetlands are considered as efficient barriers for pollutants such as dissolved metal(loid)s between land and sea. However, there remains a challenge in understanding how tidal hydrology regulates the source-sink patterns of metal(loid)s across the sediment–water interface, especially in the salt marsh-mangrove ecotone that commonly exists in subtropical coastlines. In this study, we conducted detailed time-series observations of dissolved metal(loid)s (Cr, Cu, Zn, Ba, Pb, Hg, and As) and organic matter (DOM) characteristics (bulk concentrations, aromaticity, and molecular size) in southeast China over a spring-neap tidal cycle. The results demonstrated that both DOM and metal(loid)s fluctuated with tide levels, and higher concentrations exited in groundwater than that in the surface water. The concentrations of bulk DOM and chromophoric DOM both in surface water varied oppositely with tide levels, suggesting the potential role of groundwater discharge at low tides. DOM aromaticity also had tidal time scale fluctuations, but the molecular weight of DOM was insensitive to tide changes. The dispersion fluxes were one to four orders of magnitude lower than advective fluxes with the ranges of 0.14–24.8 μg m−2 d-1 for Cr, 6.2–22.4 μg m−2 d-1 for Cu, 14.7–387.4 μg m−2 d-1 for Zn, 0.7–25.4 μg m−2 d-1 for As, 259.2–2308 μg m−2 d-1 for Ba, 0.1–6.0 μg m−2 d-1 for Pb, 0–1.7 μg m−2 d-1 for Hg and 3.1–116 mg m−2 d-1 for DOM, respectively. Mangrove sediments tended to be the sources of DOM and all dissolved metal(loid)s, but the salt marsh was only the source of Cu, As, and DOM. These results have implications on the geochemical cycling of DOM and dissolved metal(loid)s in the salt marsh-mangrove ecotone which has contrasting pattern of surface water-groundwater exchange.

Dissolved black carbon as a potential driver of surface water heating dynamics in wildfire-impacted regions: A case study from Pyramid Lake, NV, USA

Black carbon (BC), pyrogenic residues resulting from the incomplete combustion of organics, are liberated from wildfires at high rates. Subsequent introduction to aqueous environments via atmospheric deposition or overland flow results in the formation of a dissolved fraction, called dissolved black carbon (DBC). As wildfire frequency and intensity increases along with a changing climate, it becomes imperative to understand the impact a concurrent increase in DBC load might have to aquatic ecosystems. In the atmosphere BC stimulates warming by absorbing solar radiation, and similar processes may occur with surface waters that contain DBC. In this work we investigated whether the addition of environmentally relevant levels of DBC could impact surface water heating dynamics in experimental settings. DBC was quantified at multiple locations and depths in Pyramid Lake (NV, USA) during peak fire season while two large, proximal wildfires burned. DBC was detected in Pyramid Lake water at all sampled locations at concentrations (3.6–18 ppb) significantly higher than those reported for other large inland lakes. DBC was positively correlated (R2 = 0.84) with chromophoric dissolved organic matter (CDOM) but not bulk dissolved or total organic carbon (DOC, TOC), suggesting that DBC is a significant component of the optically active organics in the lake. Subsequent lab-based experiments were conducted by adding environmentally relevant levels of DBC standards to pure water, exposing the system to solar spectrum radiation, and creating a numerical model of heat transfer based on observed temperatures. The addition of DBC at environmentally relevant orders of magnitude caused reductions to shortwave albedo when exposed to the solar spectrum, which resulted in 5–8 % more incident radiation being absorbed by water and changes to water heating dynamics. In environmental settings, this increase in energy absorption could translate to increased heating of the epilimnion in Pyramid Lake and other wildfire-impacted surface waters.

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.

Urbanization influenced the interactions between dissolved organic matter and bacterial communities in rivers

Increased urbanization and anthropogenic activities can alter dissolved organic matter (DOM) and complicate its interaction with bacteria in rivers' ecosystems, however, there is limited information about how bacterial communities respond to DOM components in rivers with different urbanization levels. Here, we combined spectroscopy-based DOM analysis and 16S rRNA gene amplicon sequencing to investigate the associations of bacterial taxa and DOM properties as well as the impacts of DOM on bacterial niche breadth in North River (NR) and West River (WR) of Jiulong River watershed, southern China, which had low and high urbanization levels, respectively. Spectroscopy analysis showed that hydrophilic DOM was predominant in both rivers whereas chromophoric DOM was higher in WR. Network analysis indicated that only seven bacterial genera (i.e., hg clade, chthoniobacter, Geobacter, Acidibacter, Alphal Cluster, Fluviicola, and Lacunisphaera) showed strong associations with DOM optical variables in both rivers, whereas more than 85% of DOM-bacterial genera associations were different between rivers. These results suggest that the relationship between DOM and bacterial communities had different responses in rivers with different urbanization levels. The partial least square path model indicated that the total standardized effect of physico-chemicals on bacterial niche breadth was higher in NR (0.62) than in WR (0.35), whereas humic substances showed an opposite pattern (NR: −0.42; WR: 1.67). The distinct effects of physico-chemicals and DOM on bacterial niche breadths between rivers could be due to the different effects of urbanization and human activities on the environmental conditions of riverine ecosystems. Our findings revealed a huge dissimilarity in the bacteria-DOM co-occurrence networks between rivers with different urbanization levels and provide a novel insight that urbanization may enhance DOM's importance to bacterial niche breadths.

Estimation of chromophoric dissolved organic matter and non-algal particulate absorption coefficients of seawater in the ultraviolet by extrapolation from the visible spectral region.

Extending the capabilities of optical remote sensing and inverse optical algorithms, which have been commonly focused on the visible (VIS) range of the electromagnetic spectrum, to derive the optical properties of seawater in the ultraviolet (UV) range is important to advancing the understanding of various optical, biological, and photochemical processes in the ocean. In particular, existing remote-sensing reflectance models that derive the total spectral absorption coefficient of seawater, a(λ), and absorption partitioning models that partition a(λ) into the component absorption coefficients of phytoplankton, aph(λ), non-algal (depigmented) particles, ad(λ), and chromophoric dissolved organic matter (CDOM), ag(λ), are restricted to the VIS range. We assembled a quality-controlled development dataset of hyperspectral measurements of ag(λ) (N = 1294) and ad(λ) (N = 409) spanning a wide range of values across various ocean basins, and evaluated several extrapolation methods to extend ag(λ), ad(λ), and adg(λ) ≡ ag(λ) + ad(λ) into the near-UV spectral region by examining different sections of the VIS as a basis for extrapolation, different extrapolation functions, and different spectral sampling intervals of input data in the VIS. Our analysis determined the optimal method to estimate ag(λ) and adg(λ) at near-UV wavelengths (350 to 400 nm) which relies on an exponential extrapolation of data from the 400-450 nm range. The initial ad(λ) is obtained as a difference between the extrapolated estimates of adg(λ) and ag(λ). Additional correction functions based on the analysis of differences between the extrapolated and measured values in the near-UV were defined to obtain improved final estimates of ag(λ) and ad(λ) and then the final estimates of adg(λ) as a sum of final ag(λ) and ad(λ). The extrapolation model provides very good agreement between the extrapolated and measured data in the near-UV when the input data in the blue spectral region are available at 1 or 5 nm spectral sampling intervals. There is negligible bias between the modeled and measured values of all three absorption coefficients and the median absolute percent difference (MdAPD) is small, e.g., < 5.2% for ag(λ) and < 10.5% for ad(λ) at all near-UV wavelengths when evaluated with the development dataset. Assessment of the model on an independent dataset of concurrent ag(λ) and ad(λ) measurements (N = 149) yielded similar findings with only slight reduction of performance and MdAPD remaining below 6.7% for ag(λ) and 11% for ad(λ). These results are promising for integration of the extrapolation method with absorption partitioning models operating in the VIS.

Indirect photodegradation of sulfadimidine and sulfapyridine: Influence of CDOM components and main seawater factors

This study investigated the indirect photodegradation of sulfadimidine (SM2) and sulfapyridine (SP) in the presence of chromophoric dissolved organic matter (CDOM), and studied the influences of main marine factors (salinity, pH, NO3− and HCO3−). Reactive intermediate (RI) trapping experiments demonstrated that triplet CDOM (3CDOM*) played a major role in the photodegradation of SM2 with a 58% photolysis contribution, and the contributions to the photolysis of SP were 32%, 34% and 34% for 3CDOM*, hydroxyl radical (HO·) and singlet oxygen (1O2), respectively. Among the four CDOMs, JKHA, with the highest fluorescence efficiency, exhibited the fastest rate of SM2 and SP photolysis. The CDOMs were composed of one autochthonous humus (C1) and two allochthonous humus (C2 and C3). C3, with the strongest fluorescence intensity, had the strongest capacity to generate RIs and accounted for approximately 22%, 11%, 9% and 38% of the total fluorescence intensity of SRHA, SRFA, SRNOM and JKHA, respectively, indicating the predominance of CDOM fluorescent components in the indirect photodegradation of SM2 and SP. These results demonstrated the photolysis mechanism: The photosensitization of CDOM occurred after its fluorescence intensity decreased, and a large number of RIs (3CDOM*, HO· and 1O2, etc.) were generated by energy and electron transfer, then these RIs reacted with SM2 and SP to cause photolysis. The increase in salinity stimulated the photolysis of SM2 and SP consecutively. The photodegradation rate of SM2 first increased and then decreased with increasing pH, whereas the photolysis of SP was remarkably promoted by high pH but remained stable at low pH. NO3− and HCO3− had little effect on the indirect photodegradation of SM2 and SP. This research may contribute to a better understanding of the fate of SM2 and SP in the ocean and provide new insights into the transformation of other sulfonamides (SAs) in marine ecological environments.

Tracking the early signals of crude oil in seawater and plankton after a major oil spill in the Red Sea

Understanding the immediate impacts of oil spills is essential to recognizing their long-term consequences on the marine environment. In this study, we traced the early (within one week) signals of crude oil in seawater and plankton after a major oil spill in October 2019 in the Red Sea. At the time of sampling, the plume had moved eastward, but we detected significant signs of incorporation of oil carbon into the dissolved organic carbon pool, resulting in a 10–20% increase in the ultraviolet (UV) absorption coefficient (a254) of chromophoric dissolved organic matter (CDOM), elevated oil fluorescence emissions, and depletion of the carbon isotope composition (δ13C) of the seawater. The abundance of the picophytoplankton Synechococcus was not affected, but the proportion of low nucleic acid (LNA) bacteria was significantly higher. Moreover, specific bacterial genera (Alcanivorax, Salinisphaera, and Oleibacter) were enriched in the seawater microbiome. Metagenome-assembled genomes (MAGs) suggested that such bacteria presented pathways for growing on oil hydrocarbons. Traces of polycyclic aromatic hydrocarbons (PAHs) were also detected in zooplankton tissues, revealing the rapid entry of oil pollutants into the pelagic food web. Our study emphasizes the early signs of short-lived spills as an important aspect of the prediction of long-term impacts of marine oil spills.

Multi‐year high‐frequency sampling provides new runoff and biogeochemical insights in a discontinuous permafrost watershed

AbstractPermafrost‐underlain watersheds in the subarctic are sensitive to warming as small changes in ground thermal status will alter all components of the hydrological cycle. Globally, observed increases in winter flows and shifting water chemistry have most often been ascribed to permafrost thaw and deepening runoff pathways. However, there remain few studies in headwater catchments that examine coupled flow‐chemistry relations at high frequency over multiple years and seasons to evaluate the implications of environmental change. In this study, we use multi‐year high‐frequency measurement of discharge, specific conductance (SpC) and chromophoric dissolved organic matter (CDOM) along with traditional grab sampling of major ions to understand the sources and pathways of water and evaluate how distinct solutes are mobilized in a well‐studied subarctic basin in Yukon, Canada. Seasonally, the catchment exhibited considerable hysteresis in flow‐solute relations and had both chemostatic and dilution SpC–Q patterns with respect to major ions depending upon season and mobilization CDOM–Q signals. Storm events were extracted from high‐frequency data and normalized C–Q indices were determined and related to flow, catchment and meteorological variables. CDOM–Q events predominantly had an anti‐clockwise hysteresis and increases in DOC concentrations during storms, with some exception in the spring and fall. Conversely, SpC–Q events exhibited clockwise hysteresis and a dilution behaviour during events with less seasonal or inter‐annual variability. Information from this study supports previous conceptual models of thermally regulated runoff generation in a layered soil profile, yet also points to the importance of lateral connectivity and distal sources of solutes.

Carbon Monoxide Cycling in the Eastern Indian Ocean

AbstractCarbon monoxide (CO) is an indirect greenhouse gas and a crucial component of the global carbon cycle. The ocean is an important source of atmospheric CO, but the factors controlling the sources and sinks of CO in the marine environment remain uncertain. For the first time, the spatial distribution, production, and removal pathways of seawater CO (including photoproduction, dark production, microbial consumption, and sea‐to‐air exchange) were systematically investigated in the Eastern Indian Ocean. The abundance and component source of chromophoric dissolved organic matter (CDOM) dominated the photoproduction and dark production rates of CO, and the net CO production rate was 2.19 ± 0.71 nmol L−1 d−1 (mean ± SD). The ratio of CO photoproduction efficiencies (normalized by solar radiation) for ultraviolet B radiation (UVB): ultraviolet A radiation (UVA): photosynthetically active radiation (PAR) was 854: 39: 1, while UVA was the most significant contributor to surface CO photoproduction due to the stronger radiation intensity. In the mixed layer, UVB, UVA, and PAR were estimated to contribute 12% ± 5%, 38% ± 8%, and 50% ± 8% (mean ± SD) to CO photoproduction, respectively. The total removal of CO by microbial consumption (96%) and sea‐to‐air exchange (1%) accounted for 97% of the total production. Overall, the sources and sinks of CO kept a dynamic balance in the mixed layer. Our comprehensive approach provides in‐detail insights into the understanding of CO cycling processes in the Eastern Indian Ocean, which is scientifically important for understanding the bio‐geochemical cycling and climate effects of CO in the Eastern Indian Ocean, and also provides additional data support for the CO global cycle modeling studies.

Source characterization of dissolved organic matter in the eastern Beagle Channel from a spring situation

Dissolved organic matter (DOM) was analyzed by its optical and fluorescent properties in order to study its distribution, source and transformation in relation to microbial abundances, chlorophyll and physicochemical gradients along a transect in the eastern Beagle Channel (BC) during austral spring 2019. Moreover, DOM evolution was followed during three tidal cycles in two fixed stations (F1, F2) separated by the Mackinlay Strait, a hydrographic frontier between water masses under stronger continental influence (inner sector) and that of less modified Sub-Antarctic waters (outer sector). The fluorescence signals of protein- and humic-like compounds were used as a proxy for labile and non-labile material, respectively, while the concentration of nano-, pico- and virioplankton populations, chlorophyll-a (Chl-a) and dissolved organic carbon (DOC) were used as a proxy for biological activity. Moreover, several spectroscopic indices, such as the indicator of microbially produced DOM (BIX), the Freshness index (FIX), an indicator of molecular weight, the Slope Ratio (SR) and an indicator of chromophoric DOM (CDOM), were used to trace the quality and origin of DOM. Chl-a concentration was generally low in the sampling area (< 1 μg L−1) but increased towards the end of the campaign and especially during the visit of F1 along with the concentration of picoeukaryotes and the protein-like fluorophores evidencing the onset of a bloom event. Protein-like fluorophores peaked in the euphotic zone of the inner sector of the BC while humic-like material of terrestrial origin was homogeneously distributed throughout the water column and showed a decreasing gradient towards the outer sector and Atlantic waters. Furthermore, the inner sector was characterized by higher amounts of CDOM, high molecular weight material and more recalcitrant DOM. Accumulation of autochthonous humic-like material in bottom waters of the inner sector along with enhanced values of FIX and BIX preceded by colored organic material suggest an efficient functioning of the microbial carbon pump fueled by terrestrial DOM sources in these waters.

Validation of standard ocean-color chlorophyll-a products in turbid coastal waters: A case study on statistical evaluation and quality control tests in the Persian Gulf

Selecting the appropriate in-situ and satellite matchups is a critical task for evaluating and application of ocean color products in optically complex coastal waters. This article investigates the performance of five single-sensor and two merged multi-sensors Chl-a products in the Persian Gulf. In this study, bio-optical and Chl-a measurements collected from 531 stations in the northern Persian Gulf from 2008 to 2019 were used. In-situ samples were initially controlled to filter out inappropriate datasets. Results indicated that surface measurements may not be representative of satellite-derived Chl-a, and satellite estimations were mostly consistent with Chl-a concentrations in the first optical depth (Chlopt). Statistical analysis showed that all satellite-derived Chl-a products overestimated the Chlopt by 48%–170%, in which Chl-a from Neural-Network and OC5 algorithms yielded the best agreement. Afterwards, the QC tests were designed based on the remote sensing reflectance (Rrs) at 555 nm as a proxy of SPM, Rrs(412)/Rrs(443) as a proxy of CDOM, and Rrs(560)/Rrs(490) as a proxy of Chl-a, to select the matchups with the least interference of Colored Dissolved Organic Matter (CDOM) and Suspended Particulate Matter (SPM) on Chl-a. The correlation between Chlopt and satellite-derived Chl-a improved significantly after applying the QC tests (R2 = 0.80–0.89). The valid pixels were determined using matching the results of QC tests and standard level-2 quality flags. Finally, statistical calibration was used to create the final quality controlled calibrated Chl-a maps, using regression coefficients obtained from training/validation exercises and bootstrapping-like techniques applied to the quality-controlled datasets. The findings showed that in turbid coastal waters, standard ocean color Chl-a products cannot be validated only by performing statistical methods, and quality control experiments are necessary for their applications.

Characterizing Uncertainty in Pan‐Arctic Land‐Ocean Dissolved Organic Carbon Flux: Insights From the Onega River, Russia

AbstractDissolved organic carbon (DOC) flux from rivers in the pan‐Arctic watershed represents an important connection between major terrestrial carbon stocks and the Arctic Ocean. Previous estimates of Arctic carbon flux and dissolved organic matter (DOM) seasonal dynamics have relied predominantly on measurements from the six major Arctic rivers, yet these may not be representative of northern high‐latitude constrained smaller watersheds. Here, we evaluate DOC concentration and DOM composition in the Onega River, a small Arctic watershed, using optical measurements and ultrahigh resolution mass spectrometry. Compared to the six largest Arctic rivers, DOC, absorbance at a350, and indicators of terrestrial DOM (e.g., specific UV absorbance at 254 nm, modified aromaticity index, relative abundance of condensed aromatics and polyphenolics) were elevated in the Onega throughout the year. Seasonality was also generally muted in comparison to the major Arctic rivers with relatively elevated DOC and terrestrial markers in both spring and fall seasons. The Onega exhibits a strong relationship between a350 and DOC, and its organic‐rich nature is apparent in its high DOC yield (4.85 g m2yr−1), and higher chromophoric DOM per unit DOC than the six largest Arctic rivers. As DOC yield from the Onega may be more representative of smaller northern high‐latitude rivers, we derived a new pan‐Arctic DOC flux scaling estimate which is over 50% higher than previous estimates scaled solely from the six major Arctic rivers. These observations suggest that smaller northern high‐latitude rivers may be underrepresented in Arctic carbon flux models and highlights uncertainty around constraining the export of DOC to the Arctic Ocean.

Contrasting effects of different light regimes on the photoreactivities of allochthonous and autochthonous chromophoric dissolved organic matter

Chromophoric dissolved organic matter (CDOM) plays an important role in ultraviolet (UV) light absorption in the ocean. CDOM is known to originate from either an allochthonous or autochthonous source and has varying compositions and levels of reactivity; however, the effects of individual radiation treatments and the combined effects of UVA and UVB on allochthonous and autochthonous CDOM remain poorly understood. Thus, here, we measured changes in the common optical properties of CDOM collected from China's marginal seas and the Northwest Pacific, using full-spectrum, UVA (315–400 nm), and UVB (280–315 nm) irradiation to induce photodegradation over the same time period (60 h). Excitation–emission matrices (EEMs) combined with parallel factor analysis (PARAFAC) identified four components: marine humic-like C1, terrestrial humic-like C2, soil fulvic-like C3, and tryptophan-like C4. Although the behaviours of these components during full-spectrum irradiation exhibited similar decreasing tendencies, three components (C1, C3, and C4) underwent direct photodegradation under UVB exposure, whereas C2 was more susceptible to UVA degradation. The diverse photoreactivities of the source-dependent components to different light treatments led to differing photochemical behaviours of other optical indices [aCDOM(355), aCDOM(254), SR, HIX, and BIX]. The results indicate that irradiation preferentially reduced the high humification degree or humic substance content of allochthonous DOM, and promoted the transformation from the allochthonous humic DOM components to recently produced components. Although values for the samples from different sources overlapped frequently, principal component analysis (PCA) indicated that the overall optical signatures could be linked to the original CDOM source features. The degradation of CDOM humification, aromaticity, molecular weight, and autochthonous fractions under exposure can drive the CDOM biogeochemical cycle in marine environments. These findings can aid in a better understanding of the effects of different combinations of light treatments and CDOM characteristics on CDOM photochemical processes.