Distribution Of Chromophoric Dissolved Organic Matter Research Articles

Regulated vs. unregulated rivers: Impacts on CDOM dynamics in the eastern James Bay

The eastern James Bay (EJB) coast harbors numerous rivers, but there is a dearth of knowledge concerning dissolved organic matter (DOM) in the downstream coastal water. Here we report a four-year (2018–2021) and multi-seasons field study on the mixing behavior and characteristics of the chromophoric DOM (CDOM) in the nearshore EJB. Freshwater discharged from the extensively regulated La Grande River (LGR) was constantly depleted in DOM compared with the unregulated rivers (URRs), being on average 3.35 times lower in CDOM absorption coefficient at 440 nm (aCDOM(440)) and 2.50 times lower in dissolved organic carbon (DOC). In contrast, the absorption spectral slope between 275 and 295 nm (S275–295, a proxy of molecular weight) and the specific absorption coefficient at 254 nm (a*CDOM(254), an indicator of aromaticity) of the LGR CDOM were only 10.6% lower and 11.7% higher than those of the URRs CDOM, respectively. Riverine input was found to be the dominant source of CDOM in the study area, with little influence from sea ice formation or melting. CDOM distribution fell into two distinct regimes: the LGR-influenced low-CDOM area in the north and the URRs-influenced high-CDOM area in the south. The two areas showed strong conservative but separate aCDOM(440)–salinity relationships converging at a common marine endmember (salinity ∼25) with little seasonality. The composite data combing both areas and all seasons and years exhibited non-linear relationships between S275–295, a*CDOM(254) and aCDOM(440) and a robust simple linear correlation of DOC to aCDOM(440). This study suggests a strong impact of river regulation on CDOM input into the EJB, reveals low seasonal variability of CDOM mixing dynamics and characteristics, and demonstrates the feasibility of using remote sensing from space for real-time and synoptical assessment of DOM dynamics and the associated biogeochemical cycles in the EJB.

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.

Estimation of chromophoric dissolved organic matter and its controlling factors in Beaufort Sea using mixture density network and Sentinel-3 data

With the warming of the high-latitude regional climate, melting of permafrost, and acceleration of hydrological cycles, the Arctic Ocean (AO) has undergone a series of rapid changes in the past decades. As a dominant optical component of the AO, the variations in chromophoric dissolved organic matter (CDOM) concentration affect the physiological state marine organisms. In this study, machine learning retrieval model based on in situ data and mixture density network (MDN) was developed. Compared to other models, MDN model performed better on test data (R2 = 0.83, and root mean squared error = 0.22 m−1) and was applied to Sentinel-3 OLCI data. Afterward, the spatiotemporal distribution of CDOM during the ice-free (June–September) from 2016 to 2020 in the Beaufort Sea was obtained. CDOM concentration generally exhibited an upward trend. The maximum monthly average CDOM concentration appeared in June and gradually decreased thereafter, reaching its lowest value in September of each year. The maximum value appeared in June 2020 (0.91 m−1), and the minimum value was observed in September 2017 (0.81 m−1). The CDOM concentration nearshore was higher than that in other areas; and gradually decreased from offshore to the open sea. CDOM was highly correlated with salinity (R2 = 0.49) and discharge (R2 = 0.53), and the tight correlation between salinity and CDOM further suggested that terrestrial inputs were the main source of CDOM in the Beaufort Sea. However, sea level pressure contributed to the spatial variations in CDOM. When southerly wind prevailed and wind direction was aligned with the CDOM diffusion direction, the wind accelerated the diffusion of CDOM into the open sea. Meanwhile, seawater was diluted by the sea ice melting, resulting in decrease in CDOM concentration. Herein, this paper proposed a robust and near real-time method for CDOM monitoring and influence factor analysis, which would promote the understanding of AO CDOM budgets.

Empirically derived Coloured Dissolved Organic Matter absorption coefficient using in-situ and Sentinel 3/OLCI in coastal waters of India

ABSTRACT Chromophoric Dissolved Organic Matter (CDOM) is the fraction of dissolved organic matter that absorbs sun light in ultra-violet and visible region of the electromagnetic radiation. CDOM is abundantly found in marine and coastal waters and is very important to studying optical properties of water columns and biogeochemistry of the water. It is a proxy to study dissolved organic carbon pools in marine and freshwater pools. This research aims to establish a relation between CDOM absorption and remote sensing reflectance in coastal waters of India using a large pool of in-situ data set. This study is very significant to studying CDOM distribution, its variability and the coastal processes that influence its distribution on larger time scales (seasonal to decadal changes) through ocean colour sensors. This study presents a new empirical algorithm for estimation of CDOM absorption (412nm) in Indian coastal waters using remote sensing reflectance that has not been reported so far. The study also presents its slope and variability in different regions of coastal waters of India using in-situ data. A large pool of 409 datasets has been generated using NASA bio-Optical Marine Algorithm Dataset/SeaWiFS Bio-Optical Archive and Storage System (NOMAD/SeaBASS) datasets and Indian coastal waters datasets collected during 2014-18 exclusively for coastal waters of India. A power equation is obtained using the ratio of remote sensing reflectance (Rrs412/Rrs555) to derive CDOM absorption with R2=0.74. The retrieved absorption coefficient is further validated with independent in-situ data set using another 58 station points with R2=0.84. The root mean square error is 0.03 m−1 (with 20% standard error in modelled data). CDOM slope (S412-443) is also estimated from in-situ data and it varies from 0.010-0.020 (units nm−1) with a mean value of 0.015 (nm−1) for Indian coastal waters. CDOM absorption coefficient (412 nm) varies between 0.01-0.8 m−1 for 90% of the stations whereas remaining 10% of the stations show high CDOM absorption coefficient between 1.2- 2.0 m−1 in the study area. This empirically derived algorithm is then applied on Sentinel-3/OLCI derived remote sensing reflectance time series data of whole year 2019 and CDOM absorption coefficient has been retrieved in different regions of coastal waters off Gujarat (India). The seasonal and spatial variability in these regions show that this algorithm is having great potential to retrieve CDOM absorption in coastal areas using high-resolution ocean colour monitors like Sentinel-3/OLCI. This algorithm is highly useful in retrieving CDOM absorption in coastal waters through existing and future ocean colour monitors from space-borne platform.

Seasonal Dynamic of CDOM in a Shelf Site of the South-Eastern Ligurian Sea (Western Mediterranean)

Chromophoric dissolved organic matter (CDOM) is the fraction of the Dissolved Organic Carbon (DOC) mainly absorbing UV and blue radiation, influencing water optical properties, light availability for primary production, and water-leaving radiance. In open seas, phytoplankton is the main source of organic carbon and CDOM. Despite this, the direct or indirect phytoplankton role in CDOM production is not yet fully clarified. From studies about the relationship between CDOM and phytoplankton biomass as Chlorophyll a (Chl) in the epipelagic layer, positive correlations have been highlighted with regional differences and high levels of variability. Seven years of seasonal dynamic and vertical distribution of CDOM in the Ligurian Sea continental shelf waters have been analyzed in order to evidence the main environmental and/or biological factors determining CDOM dynamic, focusing on the CDOM/Chl relationship. CDOM optical properties (absorption at 440 nm, aCDOM (440), and spectral slope, S) allowed to distinguish different pools and to debate their origin. Four different pools were characterized and two of them were directly or indirectly related to phytoplankton biomass and taxonomic composition. Nevertheless, CDOM/Chl confirm a high level of variability These findings suggest some inputs to improve Mediterranean satellite estimates of Chl and CDOM, such as the seasonal differentiation of optical properties, especially S and CDOM/Chl relationships.

[Spatial Distribution and Degradation of CDOM in the Minjiang River in Summer].

The composition, distribution, and degradation of chromophoric dissolved organic matter (CDOM) in the lower reach and estuary of the Minjiang River were analyzed using fluorescence excitation-emission matrices-parallel factor analysis (EEMs-PARAFAC) and microbial and photochemical degradation experiments. The results show that there are three types of fluorescence components in the study area:humic-like, tyrosine-like, and tryptophan-like. The humic-like components are the main components in the river zone, while the protein-like components become dominant with increasing salinity in the estuary. The change of the CDOM abundance shows a notable spatial distribution pattern. The absorption coefficient a(280) of CDOM is lower in the river, increases after entering the urban area, shows a decreasing trend in the suburbs, and rapidly declines in the estuary. A conservative estimate of the contribution of the Fuzhou urban area to the CDOM of the Minjiang River is 8%. The a(280) in the river is susceptible to microbial and photochemical degradation and the degradation percentages are (28±8)% and (44±7)%, respectively. The bioavailability and photochemical reactivity of a(280) are much higher in the river than in the estuary. The humic-like, tyrosine-like, and tryptophan-like components show a higher photochemical reactivity in the river, with degradation percentages of (75±0.5)%, (58±21)%, and (73±3)%, respectively. The fluorescent components are not labile with respect to microbial degradation and humic-like components are accumulated after 28 days of microbial culture.

Variability of chromophoric dissolved organic matter in three freshwater-influenced systems along central-southern Chile

Chromophoric dissolved organic matter (CDOM) is a biologically active component of dissolved organic matter that influences the optical properties of aquatic environments, thereby playing an important role in the photochemical and photobiology processes occurring in the euphotic layer. Three cruises were carried out at three contrasting oceanographic areas off central and southern Chile (36.8°S, 44.6°S, 54.9°S) between November 2013 and July 2017 in order to assess the variability of the CDOM pool's optical properties (a325, SCDOM, SR). Marked differences, including vertical variability, were found for a325, SCDOM, and SR within and among each of the three zones, although surface DOC concentrations remained constant at all sites (87.59 ± 26.04 µmol L−1). At the coastal area close to the Biobio River (36.8°S), a325 values varied between 0.26–1.93 m−1 with maximum measured in surface waters. For the Puyuhuapi fjord in northern Patagonia (44.6°S), a325 ranged between 0.69 and 3.86 m−1, while in the Beagle Channel (54.9°S) it oscillated between 0.59 and 1.73 m−1. We identified the terrestrial organic matter input to surface waters as the main factor influencing surface CDOM concentrations in central Chile. In the Puyuhuapi fjord, terrestrial influence was heterogeneous because of its seasonal variability of the Channel but it was overall less significant than in situ produced CDOM. We also observed the lowest contribution of terrestrial CDOM inputs compared to the in situ production in southern Patagonia (Beagle Channel). We observed that the distribution of CDOM is potentially dominated by biological in situ production below the surface freshwater-influenced layer in the southern area while highly influenced by terrestrial input in the coastal upwelling area at 36°S.Overall, this study shows that the optical properties of dissolved organic matter (mainly SR) can be suitable for identifying different CDOM pools in coastal areas of contrasting oceanographic conditions. In stratified systems, freshwater inputs determine the distribution of surface CDOM while in well-mixed conditions, the in situ production determine the general CDOM concentration.

Dissolved organic matter dynamics in Mediterranean lagoons: The relationship between DOC and CDOM

Coastal lagoons are highly vulnerable to climate change-related pressures, such as floods and increasing temperatures, which lead to higher oxygen consumption, anaerobic metabolism and dystrophic events. Although these factors have a significant impact on the carbon cycle, the dynamics of dissolved organic matter (DOM) in these systems have not been extensively investigated. DOM can be analytically determined from the concentration of dissolved organic carbon (DOC) and/or from the spectral properties of chromophoric dissolved organic matter (CDOM), which is the light-absorbing fraction of DOM. In the present study, we investigated the spatio-temporal distribution of surface water trophic variables (Chl a and DOC) and CDOM in two Mediterranean lagoon systems, the Oristano Lagoon-Gulf system (OLG) and the Varano Lagoon (VL), in order to provide quantitative information on the dynamics of DOM in these systems. Furthermore, we assessed the value of CDOM-related indices (i.e. absorption coefficients, spectral slopes and Specific UV Absorbance at 254 nm [SUVA254]) as tools for describing the dynamics of DOM in coastal lagoons, irrespective of geographical settings, environmental conditions and anthropogenic pressures. In OLG, spatial heterogeneity and compartmentalization, with salinity varying from <1 (riverine sites) to >50 (Mistras Lagoon), affected the distribution of DOC and CDOM, with the lowest values on the south side and at sites far from riverine input. In OLG, the highest DOC and CDOM values were found in the sediment pore-water of the organic-rich Cabras Lagoon, where they were nearly double those of the water column. In VL, salinity was homogeneously distributed throughout the lagoon, which indicated a mixing of freshwater with marine waters. DOC and CDOM values were on average lower in VL than in OLG. However, in VL, DOC and CDOM showed strong peaks following a flood (September 2014) and a dystrophic event (July 2015), demonstrating the quick response of the system to environmental perturbation. In OLG, absorption coefficients at 280 nm and 350 nm were slightly negatively correlated with salinity, which indicated the influence of terrigenous inputs at riverine sites. In contrast, in VL, CDOM varied linearly and positively with salinity as a result of the in situ input of organic matter from phytoplankton during the dry season. Segment analysis showed that besides the differences between the two investigated systems, the trophic variables and optical parameters analyzed in the present study shared a common relationship. These results suggest that CDOM indices can be good predictors for the estimation of DOM. Overall, the present study provides insight into the dynamics of DOC and CDOM in little-studied Mediterranean lagoons and demonstrates that the CDOM indices can be a valuable, cost-effective and simple tool for describing the trophic conditions of these systems.

The contribution of component variation and phytoplankton growth to the distribution variation of chromophoric dissolved organic matter content in a mid-latitude subtropical drinking water source reservoir for two different seasons.

The distribution variation in chromophoric dissolved organic matter (CDOM) content in mid-latitude subtropical drinking water source reservoirs (MDWSRs) has great significance in the security of aquatic environments and human health. CDOM distribution is heavily influenced by biogeochemical processes and anthropogenic activity. However, little is known regarding the impact of component variation and phytoplankton growth on CDOM distribution variation in MDWSR. Therefore, samples were collected from a representative MDWSR (the Shanzai Reservoir) for analysis. CDOM absorption and fluorescence coupling with parallel factor analysis were measured and calculated. The results indicated that only two CDOM components were found in the surface water of Shanzai Reservoir, fulvic acid, and high-excitation tryptophan, originating from terrestrial and autochthonous sources, respectively. The types of components did not change with the season. The average molecular weight of CDOM increased in proportion to its fulvic acid content. The distribution variation in CDOM content mainly resulted from the variation in two CDOM components in summer and from high-excitation tryptophan in winter. Phytoplankton growth strongly influenced the distribution variation of CDOM content in summer; the metabolic processes of Cyanobacteria and Bacillariophyta consumed fulvic acid, while that of Cryptophyta produced high-excitation tryptophan.

Optical properties and spatial distribution of chromophoric dissolved organic matter (CDOM) in Poyang Lake, China

Abstract Chromophoric dissolved organic matter (CDOM) is a key component in aquatic ecosystem. CDOM plays a critical role in water optical attenuation and in biogeochemical processing. In this study the optical absorption properties and spatial distribution of CDOM were investigated during the wet season in Poyang Lake. Results showed that the absorption coefficients of CDOM varied significantly in four sections that contain different levels of turbidity. The absorption coefficients of CDOM in the section dominated by wetland water were higher than that in other sections. The study found that the absorption coefficients of CDOM were ranged from 1.59 to 4.71 m − 1 with an average value of 2.77 ± 0.76 m − 1 at 355 nm, and from 0.39 to 1.14 m − 1 with an average value of 0.70 ± 0.19 m − 1 at 440 nm, respectively. Spectral analysis with specific ultraviolet absorbance at 254 nm, absorption ratio between 250 and 365 nm and spectral slope indicated that CDOM in Poyang Lake was relatively high in molecule weight and aromaticity from terrestrial allochthonous inputs. Comparison of water samples revealed that CDOM in wetland contributed water had lower molecule weight and aromaticity influenced by autochthonous sources. Concentrations of dissolved organic carbon (DOC) and coefficients of CDOM absorption were significantly correlated at the range from 280 to 440 nm, especially at shorter wavelengths, which suggests that CDOM can be used as an effective DOC indicator in Poyang Lake. This study established a field-based knowledge for remote sensing measurements of CDOM in the largest fresh water lake in China.

Sources and mixing behavior of chromophoric dissolved organic matter in the Taiwan Strait

The distribution and dynamics of chromophoric dissolved organic matter (CDOM) in the Taiwan Strait were investigated through an examination of the seasonal and geographic variations of its optical properties. The absorption coefficient a280 showed higher values in the onshore area occupied by the low salinity Min-Zhe Coastal Current and river plumes, and decreased toward offshore waters dominated by the South China Sea seawater and Kuroshio Current. The spectral slope over wavelengths of 275–295nm (s275–295) also increased with increasing distance from the coastline. The seasonal variations of a280 and s275–295 were closely related to the salinity, where high a280 and low s275–295 were associated with low salinity. Three fluorescent components, including two protein-like and one humic-like, were further identified with the application of parallel factor analysis to excitation-emission matrix spectra. The seasonal variations of fluorescent components were similar to a280, and the marine autochthonous fluorescent component increased from 38 to 44% along with the increasing fraction of South China Sea seawater and the Kuroshio Current in the Taiwan Strait. The spatial and seasonal distribution of bulk CDOM and fluorescent components suggested the important contribution of terrestrial input and the hydrological control of CDOM dynamics in the Taiwan Strait. A mixing model of three end-members, coastal water, river plumes and offshore seawater, was applied to estimate the CDOM removal or addition in the Taiwan Strait. The results revealed moderate addition of bulk CDOM to the Taiwan Strait in summer, likely contributed by the primary production and input from ungauged small coastal rivers. During winter, CDOM was conservatively mixed in the northern Taiwan Strait but showed up to 50% removal in the southern Taiwan Strait.

The Variability of CDOM Along the Salinity Gradients of the Seomjin River Estuary During Dry and Wet Seasons

The distribution patterns of Chromophoric Dissolved Organic Matter (CDOM) and the chemical characteristics of CDOM in the Seomjin river estuary were investigated in March, June and July 2012 in order to determine the spatial and temporal variability of CDOM along the salinity gradient considering the effects of mixing, nutrients and Chl a. The average CDOM values were 1.0 ± 0.3 m -1 , 1.3 ± 0.4 m -1 , and 1.4 ± 0.3 m -1 in March, June and July, respectively. A high concentration of CDOM (greater than 1.5 m -1 ) was found at the head of the river which decreased towards the river mouth to as low as less than 0.5 m -1 . The average concentrations of CDOM increased from the dry season (March and June) to the wet season (July), and the average slope values (S 300-500 ), which were used as indicators of CDOM characteristics and sources, were in the range of 0.013 - 0.018 ㎚ -1 . The CDOM and S 300-50 values showed that not only the concentration of CDOM but also the chemical properties of DOM clearly changed between upstream and downstream in the Seomjin river. CDOM and FDOM showed a negative correlation with salinity (R² ＞ 0.8), and CDOM was positively correlated with FDOM. Furthermore, the mixing pattern of CDOM was confirmed as conservative for all seasons. The main environmental factors influencing the concentration of CDOM was confirmed as conservative for all seasons. The main environmental factors influencing the concentration of CDOM were salinity (mixing) and water temperature, which meant the dilution of low CDOM seawater, was the controlling factor for the spatial distribution of CDOM. Increases in water temperature seemed to induce the production of CDOM during summer (June and July) through the biological degradation of DOM either by microbial activity or photo-degradation.

Chromophoric dissolved organic matter (CDOM) in first-year sea ice in the western Canadian Arctic

We monitored the spatiotemporal progression of chromophoric dissolved organic matter (CDOM) in first-year sea ice in the western Canadian Arctic between mid-March and early July 2008. CDOM abundance in bottom ice, as quantified by absorption coefficient at 325nm, aCDOM(325), showed a positive, linear relationship with the concentration of chlorophyll a, being low at the start of ice algal accumulation, highly enriched during the peak bloom and early post-bloom, and depleted again during sea ice melting. Vertical profiles of CDOM in early and late spring were typically characterized by slight to moderate elevations at both the surface and bottom and rather constancy within the interior ice. In the ice algae-thriving mid-spring, L-type profiles prevailed due to extremely high CDOM in the lowermost 10-cm layer. Bottom-layer CDOM in landfast ice (aCDOM(325): 15.8m−1) more than doubled that in drift ice (aCDOM(325): 6.6m−1). CDOM in the ice cover, except the bottom layer, was generally lower than or similar to that in the under-ice surface water. Salinity accounted for 58% of the CDOM variability in drift ice having minimal terrestrial and ice algal signatures. CDOM absorption spectra of algae-rich bottom ice samples exhibited ultraviolet (UV) absorption shoulders attributable to mycosporine-like amino acids (MAAs). These compounds were readily photodegradable by solar UV radiation. Our results suggest that 1) inclusion of organic solutes and in situ biological production are the dominant processes controlling the distribution of CDOM in sea ice in the study area, 2) biological production in bottom ice is a minor source of CDOM to the underlying surface water; 3) CDOM plays a critical role in shielding sympagic organisms in bottom ice against UV radiation; 4) the MAAs are effective photoprotectants only under low-UV conditions.

Absorption and fluorescence characteristics of chromophoric dissolved organic matter in the Yangtze Estuary

The Yangtze Estuary is heavily influenced by coast-continent geochemical processes and anthropogenic activity; thus, the source and distribution of chromophoric dissolved organic matter (CDOM) in the estuary are strongly impacted by these processes. Here, a series of samples were collected from across the Yangtze Estuary to investigate the source and spatial dynamics of CDOM and its components throughout the system. Three indices (a(355), spectral slope, and fluorescence) were then calculated and interpreted. The results indicated that the distribution of CDOM was dominated by allochthonous input, conservative mixing, and phase transfer. The contribution of biogenic CDOM to total CDOM increased with salinity, and three individual CDOM components were identified upon fluorescence excitation emission matrix spectroscopy and parallel factor analysis of the water samples: C1, corresponding to humic substance-like CDOM, C2, corresponding to tryptophan-like CDOM, and C3, corresponding to tyrosine-like CDOM. C1 primarily originated from a terrestrial source, C2 had widespread origins, none of which played a dominant role, and C3 mainly originated from allochthonous input in the medium salinity area. Unexpectedly, no marine humic-like component was found in the surface water of the Yangtze Estuary, possibly because turbidity decreased the depth of sunlight penetration, limiting production of this component.

Inversion of Chromophoric Dissolved Organic Matter From EO-1 Hyperion Imagery for Turbid Estuarine and Coastal Waters

The significant implication of chromophoric dissolved organic matter (CDOM) for water quality and biogeochemical cycle leads to an increasing need of CDOM monitoring in coastal regions. Current ocean-color algorithms are mostly limited to open-sea water and have high uncertainty when directly applied to turbid coastal waters. This paper presents a semianalytical algorithm, quasi-analytical CDOM algorithm (QAA-CDOM), to invert CDOM absorption from Earth Observing-1 (EO-1) Hyperion satellite images. This algorithm was developed from a widely used ocean-color algorithm QAA and our earlier extension of QAA. The main goal is to improve the algorithm performance for a wide range of water conditions, particularly turbid waters in estuarine and coastal regions. The algorithm development, calibration, and validation were based on our intensive high-resolution underwater measurements, International Ocean Color Coordinating Group synthetic data, and global National Aeronautics and Space Administration Bio-Optical Marine Algorithm Data Set data. The result shows that retrieved CDOM absorption achieved accuracy (root mean square error (RMSE) = 0.115 m-1andR2= 0.73) in the Atchafalaya River plume area. QAA-CDOM is also evaluated for scenarios in three additional study sites, namely, the Mississippi River, Amazon River, and Moreton Bay, whereag(440) was in the wide range of 0.01-15 m-1. It resulted in expected CDOM distribution patterns along the river salinity gradient. This study improves the high-resolution observation of CDOM dynamics in river-dominated coastal margins and other coastal environments for the study of land-ocean interactive processes.

The Global Distribution and Dynamics of Chromophoric Dissolved Organic Matter

Chromophoric dissolved organic matter (CDOM) is a ubiquitous component of the open ocean dissolved matter pool, and is important owing to its influence on the optical properties of the water column, its role in photochemistry and photobiology, and its utility as a tracer of deep ocean biogeochemical processes and circulation. In this review, we discuss the global distribution and dynamics of CDOM in the ocean, concentrating on developments in the past 10 years and restricting our discussion to open ocean and deep ocean (below the main thermocline) environments. CDOM has been demonstrated to exert primary control on ocean color by its absorption of light energy, which matches or exceeds that of phytoplankton pigments in most cases. This has important implications for assessing the ocean biosphere via ocean color-based remote sensing and the evaluation of ocean photochemical and photobiological processes. The general distribution of CDOM in the global ocean is controlled by a balance between production (primarily microbial remineralization of organic matter) and photolysis, with vertical ventilation circulation playing an important role in transporting CDOM to and from intermediate water masses. Significant decadal-scale fluctuations in the abundance of global surface ocean CDOM have been observed using remote sensing, indicating a potentially important role for CDOM in ocean-climate connections through its impact on photochemistry and photobiology.

Competition for spectral irradiance between epilimnetic optically active dissolved and suspended matter and phytoplankton in the metalimnion. Consequences for limnology and chemistry

In deep lakes, water column stratification isolates the surface water from the deeper bottom layers, creating a three dimensional differentiation of the chemical, physical, biological and optical characteristics of the waters. Chromophoric dissolved organic matter (CDOM) and total suspended solids (TSS) play an important role in the attenuation of ultraviolet and photosynthetically active radiation. In the present analysis of spectral irradiance, we show that the wavelength composition of the metalimnetic visible irradiance was influenced by epilimnetic spatial distribution of CDOM. We found a low occurrence of blue-green photons in the metalimnion where epilimnetic concentrations of CDOM are high. In this field study, the spatial variation of the spectral irradiance in the metalimnion correlates with the observed metalimnetic concentrations of chlorophyll a as well as chlorophyll a : chlorophyll b/c ratios. Dissolved oxygen, pH, and nutrients trends suggest that chlorophyll a concentrations were representative of the phytoplankton biomass and primary production. Thus, metalimnetic changes of spectral irradiance may have a direct impact on primary production and an indirect effect on the spatial trends of pH, dissolved oxygen, and inorganic nutrients in the metalimnion.

Distribution and photoreactivity of chromophoric dissolved organic matter in the Antarctic Peninsula (Southern Ocean)

Chromophoric dissolved organic matter (CDOM) plays a key role regulating light attenuation in the ocean. This optically reactive pool of organic matter is driven by several physical and biological processes such as photobleaching, photohumification, and biogeneration, that act as primary sinks and sources of CDOM. In this study, we described the geographical and vertical distribution of CDOM in the Antarctic Peninsula area (Southern Ocean), and assessed its potential driving factors, with special emphasis on CDOM photoreactivity. CDOM values were between the detection limit and 2.17 m − 1 at 325 nm and between the detection limit and 0.76 m − 1 at 443 nm (average a 325 = 0.36 ± 0.02 m − 1 , average a 443 = 0.11 ± 0.01 m − 1 ), with the highest values inside Deception Island in 2004, and the lowest in the Eastern Bransfield Strait. In Bellingshausen Sea, CDOM was higher below the mixed layer suggesting a significant role of photobleaching. By contrast in the Weddell Sea maximum values were found within the mixed layer. In the Weddell Sea, a positive correlation between CDOM and both chlorophyll a and bacterial production and a negative correlation with salinity suggest a biological source of CDOM likely associated to ice melting. Salinity was also negatively related to the spectral slopes from 275 to 295 nm, considered a good proxy for DOM molecular weight. The experimental results demonstrate the photoreactive nature of CDOM, with half lives from 2.1 to 5.1 days due to photobleaching in the upper layer and duplication times from 4.9 to 15.7 days due to photohumification, that highlight the highly dynamic nature of CDOM in this area.

Importance of CDOM Distribution and Photoreactivity in a Shallow Texas Estuary

This study examined freshwater discharge of dissolved organic matter (DOM) to the shallow Lavaca–Matagorda (LM) Bay estuarine system along the central Texas coast and investigated whether chromophoric DOM (CDOM) photochemical reactions have the potential to stimulate microbial activity within LM estuarine waters. Dissolved organic carbon (DOC) concentrations ranged from 3 to 10 mg C l−1 and CDOM levels (reported as a 305) ranged from 8 to 77 m−1 during April and July, 2007, when the LM system was experiencing very high freshwater inputs. DOC and CDOM levels were well-correlated with salinities > 3, but exhibited considerable variability at salinities < 3. CDOM photobleaching rates (i.e., decrease in a 305 resulting from exposure to solar radiation) for estuarine samples ranged from 0.014 to 0.021 h−1, corresponding to photobleaching half-lives of 33–50 h. Our data indicate when Matagorda Bay waters photobleach; dissolved organic carbon utilization is enhanced perhaps due to enhanced microbial respiration of biologically labile photoproducts (BLPs). Net ecosystem metabolism calculations indicate that most of the LM system was net heterotrophic during our study. We estimate that BLP formation could support up to 20% of the daily microbial respiratory C demand in LM surface waters and combined with direct photochemical oxygen consumption could have an important influence on O2 cycles in the LM system.

Distributions and characteristics of dissolved organic matter in temperate coastal waters (Southern North Sea)

The spatial and temporal distributions of chromophoric dissolved organic matter (CDOM) and dissolved organic carbon (DOC) was studied in the East-Frisian Wadden Sea (Southern North Sea) during several cruises between 2002 and 2005. The spatial distribution of CDOM in the German Bight shows a strong gradient towards the coast. Tidal and seasonal variations of dissolved organic matter (DOM) identify freshwater discharge via flood-gates at the coastline and pore water efflux from tidal flat sediments as the most important CDOM sources within the backbarrier area of the Island of Spiekeroog. However, the amount and pattern of CDOM and DOC is strongly affected by various parameters, e.g. changes in the amount of terrestrial run-off, precipitation, evaporation, biological activity and photooxidation. A decoupling of CDOM and DOC, especially during periods of pronounced biological activity (algae blooms and microbial activity), is observed in spring and especially in summer. Mixing of the endmembers freshwater, pore water, and open sea water results in the formation of a coastal transition zone. Whilst an almost conservative behaviour during mixing is observed in winter, summer data point towards non-conservative mixing.