Production Of Chromophoric Dissolved Organic Matter Research Articles

Chromophoric dissolved organic matter (CDOM) release by Dictyocha fibula in the central Bohai Sea

Phytoplankton blooms can be an important source of autochthonous chromophoric dissolved organic matter (CDOM) in surface oceans. Here we report the first detection and optical characterization of CDOM produced during a Dictyocha fibula bloom occurring in the western central Bohai Sea in mid-summer 2019. The mean CDOM absorption coefficient at 330 nm (aCDOM(330)) in the surface water of the bloom area (3.16 ± 0.48 m−1) was 2.5 times higher than that of the adjacent bloom-absent area (1.26 ± 0.34 m−1). The aCDOM(330) increased exponentially to a maximum with increasing chlorophyll a and cell abundance of D. fibula, indicating a steady state was reached between production and consumption of CDOM. The biological index (BIX) of the fluorescent DOM (FDOM) showed little variation between bloom and background areas. The combination of these observations points to an extensive production of autochthonous CDOM by D. fibula in the bloom area. Mass budgeting indicates that the CDOM source from D. fibula (aCDOM(330): 1.98 ± 0.48 m−1) accounted for 63% of the CDOM stock in the bloom area. The spectral slope coefficients over 275–295 nm (S275–295) in the bloom area (range: 0.0228–0.0241 nm−1; mean: 0.0235 nm−1) were substantially lower than those in the bloom-absent area (range: 0.0240–0.0311 nm−1; mean: 0.0271 nm−1), suggesting a high-molecular-weight nature of the D. fibula-derived CDOM. The freshly produced CDOM contained two humic-like FDOM components and one protein-like FDOM component, and the three FDOM components were depleted in fresh CDOM relative to their counterparts in CDOM within the background area. Owing to the episodic nature of algal bloom-driven CDOM production, future attention should be paid to accelerated biogeochemical cycles (e.g., spiked oceanic trace gas emission) associated with CDOM photochemistry during algal blooms which can be easily missed for pre-scheduled field surveys.

Microplastics Contamination versus Inorganic Particles: Effects on the Dynamics of Marine Dissolved Organic Matter

Microplastic contamination has been linked to a range of impacts on aquatic environments. One important area that is only beginning to be addressed is the effect of microplastics on marine carbon cycling and how these compare to the effects related to inorganic particles typically present in ocean waters. The present study explores these impacts on dissolved organic matter dynamics by comparing three scenarios: a particle-free environment, a particle-enriched system with polystyrene microplastics, and a particle-enriched system with inorganic particles (water insoluble SiO2). Natural marine organic matter was obtained by culturing a non-axenic strain of Chaetoceros socialis in 2 L flasks under each of three scenarios. Following the diatom growth phase, filtered samples from the three flasks containing dissolved organic matter and bacteria were incubated separately in the dark for 5 days to monitor changes in dissolved organic matter. Chromophoric dissolved organic matter (CDOM), a bulk optical property, was monitored daily to examine changes in its quality and quantity and to compare degradation dynamics in the three systems. CDOM absorbance (quantity) remained higher in the control with respect to particle-enriched systems, suggesting that the presence of particles led to different rates of CDOM production and degradation. Using indicators for CDOM that could be related to microbial activity, results showed a higher CDOM alteration in the particle-enriched systems. These results indicate that microplastics have a potential role in modifying marine organic matter dynamics, on a similar magnitude to that of biogenic inorganic particles. Given their increasing concentrations of marine ecosystems, their role in marine microbial processing of organic matter needs to be better understood.

Biodegradable dissolved organic carbon shapes bacterial community structures and co-occurrence patterns in large eutrophic Lake Taihu

Interactions between dissolved organic matter (DOM) and bacteria are central in the biogeochemical cycles of aquatic ecosystems; however, the relative importance of biodegradable dissolved organic carbon (BDOC) compared with other environmental variables in structuring the bacterial communities needs further investigation. Here, we investigated bacterial communities, chromophoric DOM (CDOM) characteristics and physico-chemical parameters as well as examined BDOC via bioassay incubations in large eutrophic Lake Taihu, China, to explore the importance of BDOC for shaping bacterial community structures and co-occurrence patterns. We found that the proportion of BDOC (%BDOC) correlated significantly and positively with the DOC concentration and the index of the contribution of recent produced autochthonous CDOM (BIX). %BDOC, further correlated positively with the relative abundance of the tryptophan-like component and negatively with CDOM aromaticity, indicating that autochthonous production of protein-like CDOM was an important source of BDOC. The richness of the bacterial communities correlated negatively with %BDOC, indicating an enhanced number of species in the refractory DOC environments. %BDOC was identified as a significant stronger factor than DOC in shaping bacterial community composition and the co-occurrence network, suggesting that substrate biodegradability is more significant than DOC quantity determining the bacterial communities in a eutrophic lake. Environmental factors explained a larger proportion of the variation in the conditionally rare and abundant subcommunity than for the abundant and the rare bacterial subcommunities. Our findings emphasize the importance of considering bacteria with different abundance patterns and DOC biodegradability when studying the interactions between DOM and bacteria in eutrophic lakes.

Chromophoric dissolved organic matter composition and load from a coastal river system under variable flow regimes

Chromophoric dissolved organic matter (CDOM) exported from riverine catchments can influence biogeochemical processes in coastal environments with implications for water quality and carbon budget. Despite recent efforts to quantify C fluxes during high flow events, knowledge gaps exist regarding the fluxes and yield of terrestrial, reactive vs. recalcitrant CDOM under episodic to base-flow conditions from uplands to downstream estuaries. We used stream dissolved organic carbon (DOC) concentrations and CDOM optical properties using parallel factor analysis to characterize composition and fluxes under variable flow conditions for a coastal river basin in the SE USA. Our findings showed that episodic flows (>75th percentile) were marked by the elevated flux of humic acid-like CDOM and lower in-stream autochthonous production, or microbial degradation. Further, 70% of the terrestrial CDOM was exported during high flows, with a 3-fold increase in CDOM flux during episodic events, including Hurricane Irene in 2011. While, low flows (<25th percentile) were marked by an increased abundance of microbial, humic CDOM that can be easily processed within the estuary. Due to greater wetland coverage in the Neuse, the annual CDOM yield was 5–6 times higher than the larger rivers, such as the Mississippi, USA, and Changjiang, China. We suggest that similar coastal watersheds in SE USA or elsewhere may contribute substantial amounts of reactive CDOM to the estuaries during high flow conditions and can have negative water quality implications for the coastal C dynamics. These findings can help predict the evolution of coastal C cycling under projected climate change and inform the development of appropriate management strategies.

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.

Strong linkages between dissolved organic matter and the aquatic bacterial community in an urban river

Aquatic bacterial communities play an important role in biogeochemical cycling in river ecosystems; however, knowledge of the linkages between bacterial communities and dissolved organic matter (DOM) in urban rivers is limited. Here, 16S rRNA amplicon sequencing and parallel factor (PARAFAC) modeling of excitation-emission fluorescence spectroscopy were used to analyze the compositions, co-occurrence patterns, and interactions with chromophoric DOM (CDOM) of bacterial communities in urban river water samples influenced by different human activities. The results revealed that two protein-like components accounted for 65.2 ± 9.56% of the total variability in all three fluorescence components, which suggests that CDOM in urban rivers is mainly a microbial source. In addition to pH and DO, CDOM is also an important factor affecting bacterial community structure, and the main classes (Gammaproteobacteria and Clostridia) and genera (Limnohabitans and Alpinimonas) showed strong positive correlations with terrestrial humic-like C1 and tryptophan-like C2, respectively. When autotrophic and heterotrophic bacteria coexist in urban rivers, the production and degradation of CDOM will occur simultaneously. Furthermore, the riverine bacterial co-occurrence network had a nonrandom modular structure, which was mainly driven by classification correlation and bacterial function. The high abundance of genes related to xenobiotic metabolism, carbon metabolism and nitrogen metabolism in the urban river indicated that anthropogenic activity may be the dominant selective force altering the bacterial communities. Overall, our results provide a novel view for the assembly of bacterial communities in urban river ecosystems under the influence of different human activities.

Depth‐Resolved Photochemical Lability of Dissolved Organic Matter in the Western Tropical Pacific Ocean

AbstractWater samples collected from various depths of the offshore South China and Philippine Seas were exposed to solar‐simulated radiation. Photomineralization of dissolved organic carbon (DOC) and photobleaching of chromophoric dissolved organic matter (CDOM) and its humic‐like fluorescent constituent (FDOM) were observed in all samples. Protein‐like FDOM was, however, either photo‐decomposed or photo‐produced, depending on the sample's depth. The photobleaching of CDOM and humic‐like FDOM was much faster in deep than in shallow water samples while photomineralization displayed a weaker vertical zonation. Prior‐irradiated deep water inoculated with surface‐water bacteria showed enhanced microbial DOC removal but CDOM production. Results from this study suggest that deep‐ocean CDOM and FDOM can barely survive photobleaching during one ocean mixing cycle, but photochemical turnover of the bio‐refractory deep DOC is considerably longer than its average radiocarbon age. Coupled photochemical‐microbial processes can not only remove part of the bio‐refractory deep DOM but also regenerate part of it during ocean overturning circulation.

Optical properties of chromophoric dissolved organic matter (CDOM) and dissolved organic carbon (DOC) levels in constructed water treatment wetland systems in southern California, USA

Many removal mechanisms in treatment wetlands involve absorption to organic matter. Optical properties and DOC levels of waters entering and exiting 4 treatment wetland systems in Orange County, Southern California, were measured to characterize the dissolved organic matter pool. Average DOC levels decreased between the inlets and outlets, except for Forge Street (FS), which increased. For 3 wetlands, absorption coefficients decreased between inlet and outlet; the exception was FS, which increased. Average spectral slopes for the inlets and outlets were similar. Average intensities of terrestrial humic peaks A and C from 3D EEM fluorescence spectra decreased between the inlets and outlets for most wetlands. No EEM protein peaks were observed. Average flu/abs ratios for inlets and outlets were similar (high point for FS inlet excluded), suggesting chromophoric dissolved organic matter (CDOM) of a similar composition was present. The average FI value for the inlets and outlets was ∼1.5, consistent with terrestrial sources of CDOM. Average BIX values for the inlets and outlets were ∼0.8, suggesting limited contributions from autochthonous production of CDOM. Dominant plant species in the wetlands were cattail and bulrush. Humic peaks A and C, along with protein peaks, were observed in plant leachates. Protein peaks rapidly degraded with solar simulator irradiation. Results indicate that most of the wetlands are a net sink for CDOM, possibly due to absorption to sediments. The FS wetland appears to have a source of non-CDOM optically active organic carbon, possibly from a pollutant.

Production of Chromophoric Dissolved Organic Matter (CDOM) in Laboratory Cultures of Arctic Sea Ice Algae

Chromophoric dissolved organic matter (CDOM) is highly enriched in bottom sea ice in the Arctic during ice algal blooms, giving rise to multifaceted ecological implications in both the sea ice and the underlying seawater. We conducted laboratory culture incubations to assess the potential role of ice algae in the accumulation of CDOM in Arctic sea ice. Non-axenic monocultures of Attheya septentrionalis and Nitzschia frigida and a natural ice algal assemblage (NIAA) were grown at 4 °C in an f/2 medium under cool white fluorescent light. Culture samples were collected several days apart throughout the exponential, stationary, and senescent phases, and analyzed for CDOM absorbance, chlorophyll a, and bacterial cell abundance. The cultures displayed apparent specific growth rates of algal and bacterial cells comparable to those in the field. Accumulations of CDOM were observed in all cultures during the time-course incubations, with the senescent phase showing the largest accumulations and the highest production rates. The senescent-phase production rate for NIAA was ~40% higher than that for A. septentrionalis. The chlorophyll a-normalized CDOM production rates in the cultures are comparable to those reported for Arctic first-year sea ice. The absorption spectra of CDOM in the cultures exhibited characteristic short-ultraviolet shoulders similar to those previously identified in sea ice. This study demonstrates that ice algal-derived CDOM can account for the springtime accumulation of CDOM in Arctic sea ice.

Polystyrene microplastics increase microbial release of marine Chromophoric Dissolved Organic Matter in microcosm experiments

About 5 trillion plastic particles are present in our oceans, from the macro to the micro size. Like any other aquatic particulate, plastics and microplastics can create a micro-environment, within which microbial and chemical conditions differ significantly from the surrounding water. Despite the high and increasing abundance of microplastics in the ocean, their influence on the transformation and composition of marine organic matter is largely unknown. Chromophoric dissolved organic matter (CDOM) is the photo-reactive fraction of the marine dissolved organic matter (DOM) pool. Changes in CDOM quality and quantity have impacts on marine microbial dynamics and the underwater light environment. One major source of CDOM is produced by marine bacteria through their alteration of pre-existing DOM substrates. In a series of microcosm experiments in controlled marine conditions, we explored the impact of microplastics on the quality and quantity of microbial CDOM. In the presence of microplastics we observed an increased production of CDOM with changes in its molecular weight, which resulted from either an increased microbial CDOM production or an enhanced transformation of DOM from lower to higher molecular weight CDOM. Our results point to the possibility that marine microplastics act as localized hot spots for microbial activity, with the potential to influence marine carbon dynamics.

Characterization of light absorption by chromophoric dissolved organic matter (CDOM) in the upper layer of the Red Sea

The absorption coefficient of chromophoric dissolved organic matter (CDOM) is a major variable used in developing robust bio‐optical models and understanding biogeochemical processes. Over the last decade, the optical properties of CDOM in the open sea have been intensely studied. However, their variations in clear water are poorly documented, particularly in the Red Sea, owing to the absence of in situ measurements. We performed several cruises in the Red Sea to investigate the spatial distribution of the absorption coefficient of CDOM. The spectral absorption coefficients were determined from 400 nm to 740 nm using a WETLabs ac-s hyper-spectral spectrophotometer. In general, we found a latitudinal gradient in the CDOM absorption coefficient at 443 nm (aCDOM(443)) from south to north that is likely influenced by the exchange of water through the strait of Bab-el-Mandeb and the thermohaline circulation of the Red Sea. However, high aCDOM(443) values were observed in the northern Red Sea due to the existence of a sub-mesoscale feature that may induce an increase in phytoplankton production and lead to CDOM production. The aCDOM(443) covaried with the chlorophyll a concentration ([Chl a],) despite a high scatter. Furthermore, the aCDOM(443) for a given [Chl a] concentration was higher than those predicted by global ocean bio-optical models. This study advances our understanding of CDOM concentration in the Red Sea and may help improve the accuracy of the algorithms used to obtain CDOM absorption from ocean color.

Variations in Spectral Absorption Properties of Phytoplankton, Non-algal Particles and Chromophoric Dissolved Organic Matter in Lake Qiandaohu

Light absorption by phytoplankton, non-algal particles (NAP) and chromophoric dissolved organic matter (CDOM) was investigated at 90 sites of a clear, deep artificial lake (Lake Qiandaohu) to study natural variability of absorption coefficients. Our study shows that CDOM absorption is a major contributor to the total absorption signal in Lake Qiandaohu during all seasons, except autumn when it has an equivalent contribution as total particle absorption. The exponential slope of CDOM absorption varies within a narrow range around a mean value of 0.0164 nm−1 ( s d = 0.00176 nm−1). Our study finds some evidence for thIS autochthonous production of CDOM in winter and spring. Absorption by phytoplankton, and therefore its contribution to total absorption, is generally greatest in spring, suggesting that phytoplankton growth in Lake Qiandaohu occurs predominantly in the spring. Phytoplankton absorption in freshwater lakes generally has a direct relationship with chlorophyll-a concentration, similar to the one established for open ocean waters. The NAP absorption, whose relative contribution to total absorption is highest in summer, has a spectral shape that can be well fitted by an exponential function with an average slope of 0.0065 nm−1 ( s d = 0.00076 nm−1). There is significant spatial variability present in the summer of Lake Qiandaohu, especially in the northwestern and southwestern extremes where the optical properties of the water column are strongly affected by the presence of allochthonous matter. Variations in the properties of the particle absorption spectra with depths provides evidence that the water column was vertically inhomogeneous and can be monitored with an optical measurement program. Moreover, the optical inhomogeneity in winter is less obvious. Our study will support the parameterization of the Bio-optical model for Lake Qiandaohu from in situ or remotely sensing aquatic color signals.

Surface Gradients in Dissolved Organic Matter Absorption and Fluorescence Properties along the New Zealand Sector of the Southern Ocean

The Southern Ocean plays a critical role in global carbon cycling; dissolved organic matter (DOM), a component in the carbon cycle, can be characterized optically. Sea surface chromophoric dissolved organic matter (CDOM) absorption and fluorescence properties were examined in the New Zealand sector of the Southern Ocean (NZSSO) along a transect encompassing various hydrographic fronts associated with the Antarctic Circumpolar Current (ACC) during summer. Phytoplankton chlorophyll, dissolved organic carbon (DOC) and CDOM absorption were observed to be most elevated off the New Zealand shore and then decreased to low values (chlorophyll: 0.21 ± 0.06 mg m-3; DOC: 54.19 ± 4.02 μM; and CDOM absorption coefficient at 325 nm (ag325): 0.097 ± 0.061 m-1) between the Subtropical (STF) and Antarctic Polar (APF) Fronts. Increases in phytoplankton biomass and DOC concentrations between the fronts were associated with meanders or eddies observed in satellite sea surface salinity and chlorophyll imagery. Overall, CDOM absorption was the dominant contributor to total absorption at 443 nm with implications for ocean color. Beyond the APF in the Antarctic Zone, an elevated chlorophyll band likely associated with upwelled waters transitioned to low chlorophyll in the summer ice edge zone that influenced DOM optical properties. A latitudinal increase in ag325 and corresponding decrease in spectral slope S (µm-1) poleward from the STF could be due to the a number of factors including, decreasing CDOM photooxidation, upwelling of high-CDOM waters or bacterial CDOM production in the Antarctic Zone. Parallel factor (PARAFAC) analysis of fluorescence spectra identified two protein-like (C1 and C2) and two humic-like (C3 and C4) components common in the global ocean. ag325 and the humic-like C4 fluorescent component, were positively correlated to chlorophyll indicating biological control. Surface distribution of the protein-like C1 and C2 and the marine humic-like C3 components showed patterns that appeared to be influenced by both physical and biological processes. This study provides insights into surface CDOM optical properties and its transformation along a complex topographically influenced sector of the Southern Ocean that could be used to trace changes linked to the meridional overturning circulation. I

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.

Evolution of dissolved and particulate chromophoric materials during the VAHINE mesocosm experiment in the New Caledonian coral lagoon (south-west Pacific)

Abstract. In the framework of the VAHINE project, we investigated the spectral characteristics and the variability of dissolved and particulate chromophoric materials throughout a 23-day mesocosm experiment conducted in the south-west Pacific at the mouth of the New Caledonian coral lagoon (22°29.073 S–166°26.905 E) from 13 January to 4 February 2013. Samples were collected in a mesocosm fertilized with phosphate at depths of 1, 6 and 12 m and in the surrounding waters. Light absorption coefficients of chromophoric dissolved organic matter (CDOM) [ag(λ)] and particulate matter [ap(λ)] were determined using a point-source integrating-cavity absorption meter (PSICAM), while fluorescent DOM (FDOM) components were determined from excitation-emission matrices (EEMs) combined with parallel factor analysis (PARAFAC). The evolutions of ag(λ) and ap(λ) in the mesocosm were similar to those of total chlorophyll a concentration, Synechococcus spp. and picoeukaryote abundances, bacterial production, particulate organic nitrogen and total organic carbon concentrations, with roughly a decrease from the beginning of the experiment to days 9–10, and an increase from days 9–10 to the end of the experiment. In the surrounding waters, the same trend was observed but the increase was much less pronounced, emphasizing the effect of the phosphate fertilization on the mesocosm's plankton community. Correlations suggested that both Synechococcus cyanobacteria and heterotrophic bacteria were strongly involved in the production of CDOM and absorption of particulate matter. The increase in phytoplankton biomass during the second part of the experiment led to a higher contribution of particulate material in the absorption budget at 442 nm. The three FDOM components identified (tryptophan-, tyrosine- and ultraviolet C (UVC) humic-like fluorophores) did not follow the evolution of CDOM and particulate matter, suggesting they were driven by different production/degradation processes. Finally, the results of this work support the idea there is indirect coupling between the dynamics of N2 fixation and that of chromophoric material via the stimulation of Synechococcus bloom.

Changes in optical characteristics of surface microlayers hint to photochemically and microbially mediated DOM turnover in the upwelling region off the coast of Peru

Abstract. The coastal upwelling system off the coast of Peru is characterized by high biological activity and a pronounced subsurface oxygen minimum zone, as well as associated emissions of atmospheric trace gases such as N2O, CH4 and CO2. From 3 to 23 December 2012, R/V Meteor (M91) cruise took place in the Peruvian upwelling system between 4.59 and 15.4° S, and 82.0 to 77.5° W. During M91 we investigated the composition of the sea-surface microlayer (SML), the oceanic uppermost boundary directly subject to high solar radiation, often enriched in specific organic compounds of biological origin like chromophoric dissolved organic matter (CDOM) and marine gels. In the SML, the continuous photochemical and microbial recycling of organic matter may strongly influence gas exchange between marine systems and the atmosphere. We analyzed SML and underlying water (ULW) samples at 38 stations focusing on CDOM spectral characteristics as indicator of photochemical and microbial alteration processes. CDOM composition was characterized by spectral slope (S) values and excitation–emission matrix fluorescence (EEMs), which allow us to track changes in molecular weight (MW) of DOM, and to determine potential DOM sources and sinks. Spectral slope S varied between 0.012 to 0.043 nm−1 and was quite similar between SML and ULW, with no significant differences between the two compartments. Higher S values were observed in the ULW of the southern stations below 15° S. By EEMs, we identified five fluorescent components (F1–5) of the CDOM pool, of which two had excitation/emission characteristics of amino-acid-like fluorophores (F1, F4) and were highly enriched in the SML, with a median ratio SML : ULW of 1.5 for both fluorophores. In the study region, values for CDOM absorption ranged from 0.07 to 1.47 m−1. CDOM was generally highly concentrated in the SML, with a median enrichment with respect to the ULW of 1.2. CDOM composition and changes in spectral slope properties suggested a local microbial release of DOM directly in the SML as a response to light exposure in this extreme environment. In a conceptual model of the sources and modifications of optically active DOM in the SML and underlying seawater (ULW), we describe processes we think may take place (Fig. 1); the production of CDOM of higher MW by microbial release through growth, exudation and lysis in the euphotic zone, includes the identified fluorophores (F1, F2, F3, F4, F5). Specific amino-acid-like fluorophores (F1, F4) accumulate in the SML with respect to the ULW, as photochemistry may enhance microbial CDOM release by (a) photoprotection mechanisms and (b) cell-lysis processes. Microbial and photochemical degradation are potential sinks of the amino-acid-like fluorophores (F1, F4), and potential sources of reworked and more refractory humic-like components (F2, F3, F5). In the highly productive upwelling region along the Peruvian coast, the interplay of microbial and photochemical processes controls the enrichment of amino-acid-like CDOM in the SML. We discuss potential implications for air–sea gas exchange in this area.

Will enhanced turbulence in inland waters result in elevated production of autochthonous dissolved organic matter?

Biological activity in lakes is strongly influenced by hydrodynamic conditions, not least turbulence intensity; which increases the encounter rate between plankter and nutrient patches. To investigate whether enhanced turbulence in shallow and eutrophic lakes may result in elevated biological production of autochthonous chromophoric dissolved organic matter (CDOM), a combination of field campaigns and mesocosm experiments was used. Parallel factor analysis identified seven components: four protein-like, one microbial humic-like and two terrestrial humic-like components. During our field campaigns, elevated production of autochthonous CDOM was recorded in open water with higher wind speed and wave height than in inner bays, implying that elevated turbulence resulted in increased production of autochthonous CDOM. Confirming the field campaign results, in the mesocosm experiment enhanced turbulence resulted in a remarkably higher microbial humic-like C1 and tryptophan-like C3 (p<0.01), indicating that higher turbulence may have elevated the production of autochthonous CDOM. This is consistent with the significantly higher mean concentrations of chlorophyll-a (Chl-a) and dissolved organic carbon (DOC) and the enhanced phytoplanktonic alkaline phosphatase activity (PAPA) recorded in the experimental turbulence groups than in the control group (p<0.05). The C:N ratio (from 3.34 to 25.72 with a mean of 13.13±4.08) for the mesocosm CDOM samples further suggested their probable autochthonous origin. Our results have implications for the understanding of CDOM cycling in shallow aquatic ecosystems influenced by wind-induced waves, in which the enhanced turbulence associated with extreme weather conditions may be further stimulated by the predicted global climate change.

Taxonomic variability of phytoplankton and relationship with production of CDOM in the polynya of the Amundsen Sea, Antarctica

To evaluate the relationship between the phytoplankton composition and the CDOM production in the Amundsen Sea, we examined the taxonomic variability of phytoplankton, nutrients and chromophoric dissolved organic matter (CDOM) participating in two research cruises, ANA02C (February, 2012) and ANA04B (January, 2014). For both cruises, the peak concentrations of chlorophyll a (Chl-a) were measured in the Amundsen Sea Polynya (ASP) where the average Chl-a concentration was 2.31μgL−1 (±1.01μgL−1) in February 2012 and 3.92μgL−1 (±3.14μgL−1) in January 2014. The major phytoplankton groups were diatoms and prymnesiophytes, while the minor groups were cryptophytes, chlorophytes, dinophytes, chrysophytes and cyanophytes. The phytoplankton compositions in the sea ice zone (SIZ), the ASP and ice shelf (IS) was distinct. The predominance of Phaeocystis antarctica (P. antarctica) (70–90%) was observed at all stations, except in the SIZ in January 2014. While the CDOM concentrations (a355) ranged from 0.07 to 0.98m−1, the average values of a355 of the euphotic layer in the ASP was 0.51m−1 (±0.19m−1), which was much higher than any other region of the Southern Ocean. CDOM showed strong correlation with Chl-a but no relationship with salinity, which implies that biological processes were the main source of CDOM. Among the major phytoplankton taxa, P. antarctica was confirmed as the most important contributor to the production of CDOM, according to the high correlation between marker pigment (hex-fuco) and CDOM. The unusually high CDOM in the ASP is expected to give as impact on the environment in various aspects. As a nature of CDOM, absorption of solar radiation at the surface can result in the increase of sea surface temperature (SSTs), which will accelerate the ice melting. Also the photo-oxidation of CDOM can serve as a feedback to increasing atmospheric CO2 concentrations after peak bloom in austral summer.

Water mass age and aging driving chromophoric dissolved organic matter in the dark global ocean

AbstractThe omnipresence of chromophoric dissolved organic matter (CDOM) in the open ocean enables its use as a tracer for biochemical processes throughout the global overturning circulation. We made an inventory of CDOM optical properties, ideal water age (τ), and apparent oxygen utilization (AOU) along the Atlantic, Indian, and Pacific Ocean waters sampled during the Malaspina 2010 expedition. A water mass analysis was applied to obtain intrinsic, hereinafter archetypal, values of τ, AOU, oxygen utilization rate (OUR), and CDOM absorption coefficients, spectral slopes and quantum yield for each one of the 22 water types intercepted during this circumnavigation. Archetypal values of AOU and OUR have been used to trace the differential influence of water mass aging and aging rates, respectively, on CDOM variables. Whereas the absorption coefficient at 325 nm (a325) and the fluorescence quantum yield at 340 nm (Φ340) increased, the spectral slope over the wavelength range 275–295 nm (S275–295) and the ratio of spectral slopes over the ranges 275–295 nm and 350–400 nm (SR) decreased significantly with water mass aging (AOU). Combination of the slope of the linear regression between archetypal AOU and a325 with the estimated global OUR allowed us to obtain a CDOM turnover time of 634 ± 120 years, which exceeds the flushing time of the dark ocean (&gt;200 m) by 46%. This positive relationship supports the assumption of in situ production and accumulation of CDOM as a by‐product of microbial metabolism as water masses turn older. Furthermore, our data evidence that global‐scale CDOM quantity (a325) is more dependent on aging (AOU), whereas CDOM quality (S275–295, SR, Φ340) is more dependent on aging rate (OUR).

Photochemical Release of Dissolved Organic Matter from Resuspended Salt Marsh Sediments

Incubation experiments were conducted to determine the influence of natural sunlight irradiation on the release of dissolved organic matter (DOM) from resuspended surface sediments. The photo-release of dissolved organic carbon (DOC), dissolved nitrogen (DN), and chromophoric dissolved organic matter (CDOM) were quantified over time. During the 5–7-day incubations, 3.4–12.4 and 23–41 % of the sedimentary organic carbon (TOC) and 2.8–10.2 and 25–46 % of the solid phase nitrogen (TN) were released into DOC and DN pools from Florida and Massachusetts salt marsh sediments, respectively. The suspended, fractionated, clay-sized particles from Massachusetts released much higher DOC and DN than Florida bulk sediment. Up to 65 % of the DOC and 100 % of the DN photo-released from sediments were respired rapidly by bacteria, indicating that microorganisms play an important role in regulating the concentration of DOC and DN. The bioavailability of photo-released DOC and DN also depends largely on the biochemical nature of the DOM released from different sediments. Optical measurements confirmed that significant CDOM production was associated with the sunlight-induced DOC release. Photo-released CDOM appeared to be marine and humic-like as characterized by fluorescence and excitation–emission matrix (EEM) spectroscopy. It is estimated that photo-production of DOC from resuspended sediments could account for 10–22 % of the DOC measured in the coastal waters adjacent to the marsh systems studied. These results indicate that production of DOM from resuspended sediments due to direct sunlight irradiation could be an important source of DOC, CDOM, and secondary production in salt marshes and marsh-dominated coastal waters.