Chromophoric Dissolved Organic Matter Photobleaching Research Articles

High photoreactivity of chromophoric dissolved organic matter derived from Ulva prolifera and Sargassum

The epipelagic macroalgae of Ulva prolifera and Sargassum are the primary contributors to widespread seaweed tides globally. Both ocean plants release large amounts of chromophoric dissolved organic matter (CDOM) into the surrounding seawater. The photochemical reactivity of this CDOM, however, has not been adequately addressed. In this study, we extracted CDOM from Ulva prolifera and Sargassum, examined their ultraviolet (UV)-visible absorption characteristics, and quantified their broadband apparent quantum yields (AQY) of absorbance photobleaching and photomineralization (in terms of CO2, CO, and CH4 photoproduction). On a per-unit-weight basis, Sargassum leached 3.5 times more CDOM than did Ulva prolifera in terms of the absorption coefficient averaged over 254–500 nm. Both Ulva prolifera and Sargassum CDOM were characterized by quasi-exponential decay absorption spectra, with Sargassum CDOM exhibiting a distinct shoulder over 310–350 nm suggestive of mycosporine amino acids. The Sargassum CDOM had a higher photobleaching AQY but lower photomineralization AQYs compared to Ulva prolifera CDOM. The photobleaching and photomineralization AQYs of both macroalgal CDOM are, however, orders of magnitude higher than those of CDOM in various natural waters. Potential photoproduction rates of CO2 and CO from the Ulva prolifera CDOM and Sargassum CDOM during the bloom periods are several times to orders of magnitude higher than the air-sea fluxes of these gases in the absence of the macroalgae. This study demonstrates that CDOM released by Ulva prolifera and Sargassum is extremely prone to photobleaching and photomineralization, rendering floating mats of these plants in oceans as potential “hotspots” of greenhouse gas emissions to the atmosphere. This photochemical feedback should be considered when assessing ocean afforestation as a CO2 removal approach to mitigate climate warming.

The apparent quantum yield matrix (AQY-M) of CDOM photobleaching in estuarine, coastal, and oceanic surface waters

The photochemical degradation of chromophoric dissolved organic matter (CDOM) upon solar exposure, known as photobleaching, can significantly alter the optical properties of the surface ocean. By leading to the breakdown of UV- and visible-radiation-absorbing moieties within dissolved organic matter, photobleaching regulates solar heating, the vertical distribution of photochemical processes, and UV exposure and light availability to the biota in surface waters. Despite its biogeochemical and ecological relevance, this sink of CDOM remains poorly quantified. Efforts to quantify photobleaching globally have long been hampered by the inherent challenge of determining representative apparent quantum yields (AQYs) for this process, and by the resulting lack of understanding of their variability in natural waters. Measuring photobleaching AQY is made challenging by the need to determine AQY matrices (AQY-M) that capture the dual spectral dependency of this process (i.e., magnitude varies with both excitation wavelength and response wavelength). A new experimental approach now greatly facilitates the quantification of AQY-M for natural waters, and can help address this problem. Here, we conducted controlled photochemical experiments and applied this new approach to determine the AQY-M of 27 contrasting water samples collected globally along the land-ocean aquatic continuum (i.e., rivers, estuaries, coastal ocean, and open ocean). The experiments and analyses revealed considerable variability in the magnitude and spectral characteristics of the AQY-M among samples, with strong dependencies on CDOM composition/origin (as indicated by the CDOM 275–295-nm spectral slope coefficient, S275–295), solar exposure duration, and water temperature. The experimental data facilitated the development and validation of a statistical model capable of accurately predicting the AQY-M from three simple predictor variables: 1) S275–295, 2) water temperature, and 3) a standardized measure of solar exposure. The model will help constrain the variability of the AQY-M when modeling photobleaching rates on regional and global scales.

Impacts of haze on the photobleaching of chromophoric dissolved organic matter in surface water

Sunlight plays an important role in the photochemical processes of chromophoric dissolved organic matter (CDOM), which is closely related to water self-purification and primary productivity of healthy aquatic ecosystem health. The fine particles of haze, a widespread air pollutant, absorb natural ultraviolet (UV) irradiation and have an unknown degree of influence on the photochemical transformation of CDOM. Here, an in-situ experiment investigating how the amount and composition of CDOM changes under hazy conditions was conducted in Ningbo, southeastern China, a city that frequently suffers from seasonal haze pollution. The results indicated that haze attenuated UV light under different weather conditions. The UV intensities were reduced from 1124.90 ± 91.58 to 510.26 ± 40.26 μW cm-2 and 748.54 ± 101.68 to 316.32 ± 40.48 μW cm-2 on sunny and cloudy days, respectively; these values approached those on rainy days (186.97 ± 28.58 μW cm-2). Consequently, the loss of dissolved organic carbon during the irradiation test was reduced on hazy days (e.g., from 5.63% to 2.59% on sunny/hazy days). The impact of haze on CDOM photobleaching was further assessed by an excitation-emission matrix (EEM) combined with parallel factor (PARAFAC) analysis. On hazy days, the EEM-PARAFAC components were saved from photobleaching to different degrees; and humic-like substances showed a stronger protective effect from haze than protein-like substances because of their higher photosensitivity. Consequently, haze could cause more terrestrial CDOM to remain in surface water. UV intensity played a critical role in the composition characteristics of CDOM. This study identifies the linkage between atmospheric pollution and water quality and demonstrates that long-term and large-scale haze may adversely influence aquatic ecology through pollutant/nutrient accumulation.

Simple Method to Determine the Apparent Quantum Yield Matrix of CDOM Photobleaching in Natural Waters.

The photobleaching of chromophoric dissolved organic matter (CDOM) is considered an important loss process for CDOM absorption in sunlit natural waters, where it can regulate the biota's exposure to sunlight, surface solar heating, and dissolved organic matter dynamics. Despite its importance, this sink remains poorly quantified, primarily because of the difficulty of determining photobleaching apparent quantum yields (AQYs) that capture the dual spectral dependency of this process and are applicable to polychromatic sunlight. Here, we present a simple method to determine a CDOM photobleaching AQY matrix (AQY-M) for natural water samples that does not require any a priori assumptions about the spectral dependency of photobleaching. It combines controlled irradiation experiments, a partial least-square regression, and an optimization procedure to produce AQY-Ms that are spectrally coherent and optimized for modeling accurate photobleaching rates in natural waters. Water temperature and the solar exposure history of CDOM had a major influence on the magnitude and spectral characteristics of the AQY-M. These factors should be considered when determining the AQY-M of samples and provide constraints when modeling photobleaching rates in natural waters. We expect that this effective method will provide future studies with a robust means to characterize and understand the variability of AQY-M in natural waters.

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.

Characteristics, sources, and photobleaching of chromophoric dissolved organic matter (CDOM) in large and shallow Hongze Lake, China

Abstract Hongze Lake, the fourth largest freshwater lake in China, is located on the eastern route of the South-to-North Water Diversion Project. The current study provides a first look at the source and composition of chromophoric dissolved organic matter (CDOM) in Hongze Lake through the combination of excitation emission matrix fluorescence spectra and parallel factor (PARAFAC) analysis. Six fluorescence components were identified by the PARAFAC model: three humic-like components (C1, C2, and C3), two autochthonous protein-like components (C4 and C6), and one photoproduced humic-like component (C5). The results of fluorescence indices and principal component analyses of fluorescence components demonstrate that CDOM is primarily of autochthonous origin, specifically comprising C4 and C5 in surface and bottom waters and C6 in sediment pore water. Sediment pore water contained CDOM of relatively lower aromaticity and molecular weight than that of surface and bottom waters, suggesting various biogeochemical processes involving CDOM in the lake. Moreover, the decrease in fluorescence intensities (primarily humic-like), aromaticity, and the molecular weight suggested photobleaching of CDOM. However, the hydrological conditions in Hongze Lake were strongly affected by human activities due to the urgent demands for water resources. Therefore, the hydrological controls may affect the CDOM characteristics and biogeochemical processes (e.g., photochemical), which should be investigated in further studies.

Photobleaching of chromophoric dissolved organic matter (CDOM) in the Yangtze River estuary: kinetics and effects of temperature, pH, and salinity.

The kinetics and temperature-, pH- and salinity-dependences of photobleaching of chromophoric dissolved organic matter (CDOM) in the Yangtze River estuary (YRE) were evaluated using laboratory solar-simulated irradiation and compared to those of Suwannee River humic substances (SRHSs). Nearly all CDOM in water at the head of the estuary (headwater herein) was photobleachable in both summer and winter, while significant fractions of CDOM (13-29%) were resistant to photobleaching in saltier waters. The photobleaching rate constant in the headwater was 25% higher in summer than that in winter. The absorbed photon-based photobleaching efficiency (PE) increased with temperature following the linear Arrhenius equation. For a 20 °C increase in temperature, PE increased by ∼45% in the headwater and by 70-81% in the saltier waters. PE for YRE samples exhibited minima at pH from 6 to 7 and increased with both lower and higher pH values, contrasting the consistent increase in PE with pH shown by SRHSs. No consistent effect of salinity on PE was observed for both SRHSs and YRE samples. Photobleaching increased the spectral slope coefficient between 275 nm and 295 nm in summer, consistent with the behavior of SRHSs, but decreased it in winter, implying a difference in the molecular composition of chromophores between the two seasons. Temperature, salinity, and pH modified the photoalteration of the spectral shape but their effects varied spatially and seasonally. This study demonstrates that CDOM quality, temperature, and pH should be incorporated into models involving quantification of photobleaching.

Distribution and spectral characteristics of chromophoric dissolved organic matter in a coastal bay in northern China

The absorption spectra of chromophoric dissolved organic matter (CDOM), along with general physical, chemical and biological variables, were determined in the Bohai Bay, China, in the springs of 2011 and 2012. The absorption coefficient of CDOM at 350nm (a350) in surface water ranged from 1.00 to 1.83m−1 (mean: 1.35m−1) in May 2011 and from 0.78 to 1.92m−1 (mean: 1.19m−1) in April 2012. Little surface-bottom difference was observed due to strong vertical mixing. The a350 was weakly anti-correlated to salinity but positively correlated to chlorophyll a (Chl-a) concentration. A shoulder over 260–290nm, suggestive of biogenic molecules, superimposed the overall pattern of exponentially decreasing CDOM absorption with wavelength. The wavelength distribution of the absorption spectral slope manifested a pronounced peak at ca. 300nm characteristic of algal-derived CDOM. All a250/a365 ratios exceeded 6, corresponding to CDOM molecular weights (Mw) of less than 1kDa. Spectroscopically, CDOM in the Bohai Bay differed substantively from that in the Haihe River, the bay's dominant source of land runoff; photobleaching of the riverine CDOM enlarged the difference. Results point to marine biological production being the principal source of CDOM in the Bohai Bay during the sampling seasons. Relatively low runoff, fast dilution, and selective photodegradation are postulated to be among the overarching elements responsible for the lack of terrigenous CDOM signature in the bay water.

Photobleaching as a factor controlling spectral characteristics of chromophoric dissolved organic matter in open ocean

Abstract. Chromophoric dissolved organic matter (CDOM) ubiquitously occurs in marine environments and plays a significant role in the marine biogeochemical cycles. Basin scale distributions of CDOM have recently been surveyed in the global ocean and indicate that quantity and quality of oceanic CDOM are mainly controlled by in situ production and photobleaching. However, factors controlling the spectral parameters of CDOM in the UV region, i.e., spectral slope of CDOM determined at 275–295 nm (S275–295) and the ratio of two spectral slope parameters (SR); the ratio of S275–295 to S350–400, have not been well documented. To evaluate the factor controlling the spectral characteristics of CDOM in the UV region in the open ocean, we determined the quantitative and qualitative characteristics of CDOM in the subarctic and subtropical surface waters (5–300 m) of the western North Pacific. Absorption coefficients at 320 nm in the subarctic region were higher than those in the subtropical region throughout surface waters, suggesting that magnitudes of photobleaching were different between the two regions. The values of S275–295 and SR were also higher in the subtropical region than the subarctic region. The dark microbial incubation showed biodegradation of DOM little affected S275–295, but slightly decreased SR. On the other hand, increases in S275–295 and relative stableness of SR were observed during photo-irradiation incubations respectively. These experimental results indicated that photobleaching of CDOM mainly induced qualitative differences in CDOM at UV region between the subarctic and subtropical surface waters. The results of this study imply that S275–295 can be used as a tracer of photochemical history of CDOM in the open ocean.

Influence of severe drought conditions on chromophoric dissolved organic matter dynamics in south Texas coastal waters

We investigated chromophoric dissolved organic matter (CDOM) dynamics in response to the severe drought conditions of 2009 in the Aransas–Copano (AC) Bay complex and adjacent nearshore Gulf of Mexico (GoM) along the southern Texas (USA) coast. Surface water absorption coefficients (a305 in m−1) were measured daily at the University of Texas at Austin Marine Science Institute (UTMSI) pier located at the interface between the AC complex and the GoM. From June to August 2009, a305 averaged 0.81 ± 0.31 m−1 compared to 1.20 ± 0.70 m−1 during summer of 2008 (June–August) and 1.84 ± 0.34 m−1 during summer 2007 (non-drought year). Despite negligible freshwater input to the AC system for most of 2009, mean a305 values for AC Bay sites were similar in 2008 and 2009, ranging from 3.24 ± 0.60 m−1 in the lower estuary to 6.22 ± 0.55 m−1 in the upper estuary during summer 2009, and 3.15 ± 0.38 m−1 to 6.81 ± 0.73 m−1 during summer 2008 (no data for 2007). CDOM photobleaching experiments were performed using a SunTest XLS + solar simulator with an irradiation spectrum that closely matches solar UV at subtropical latitudes. AC Bay samples exhibited a305 photobleaching half-lives of 33–48 h, while GoM samples exhibited a305 photobleaching half-lives of 36–89 h. We estimate that summertime photobleaching may reduce CDOM levels by >50% in the AC bay complex before discharge into the GoM and by up to 25% more throughout the nearshore waters along the western GoM shelf.

Chromophoric dissolved organic matter (CDOM) in the Equatorial Atlantic Ocean: Optical properties and their relation to CDOM structure and source

Extensive data exist on the optical properties of CDOM from terrestrial and coastal environments, yet the open oceans have been historically under-sampled. Consequently, the source and structural basis of marine CDOM optical properties are still debated. To address this issue, detailed optical measurements were acquired for both untreated and sodium borohydride (NaBH4) reduced natural waters and C18 extracts (C18-OM) across the Equatorial Atlantic Ocean. Except in regions of upwelling or in the vicinity of the Congo River outflow, CDOM absorption coefficients and visible emission intensity were far smaller for surface waters (aCDOM(355): 0.057–0.162m−1; λex/λem=350/450nm: 0.396–1.431 qse) than for waters below the mixed layer (aCDOM(355) 0.084–0.344m−1; λex/λem=350/450nm: 0.903–3.226 qse), while spectral slopes were higher (surface: 0.019 to 0.025nm−1; deep: 0.013 to 0.019nm−1), consistent with photobleaching of CDOM in surface waters. Distinct emission bands were observed in the ultraviolet, primarily at excitation/emission wavelengths (λex/λem)=280/320nm, but also at λex/λem=300/340, 300/405 and 320/380nm for some stations and depths. In contrast, visible emission exhibited maxima that continuously redshifted with increasing λex (>330nm), a property characteristic of CDOM from estuarine and coastal environments. Further evidence that CDOM in the offshore waters of this region is composed of a major terrestrial component includes: 1) similar spectral dependencies of the emission maxima and fluorescence quantum yields; 2) a large Stokes shift in the emission maxima with short-wavelength excitation (λex=280nm); 3) correlation of visible emission intensities with absorption at λex=280, 320 and 450nm, with absorption to fluorescence ratios comparable to those found in estuarine and coastal environments; 4) affinity of C18 cartridges for the long wavelength (visible) absorbing and emitting material, but not the UV emitting material; 5) preferential loss of visible absorption and substantially enhanced blue-shifted emission in the visible following borohydride reduction of both the Equatorial Atlantic waters and the C18-OM of these waters. These results support the occurrence in offshore waters of a major terrestrial CDOM component that absorbs in the UV and visible and emits in the visible, as well as marine CDOM components that absorb and emit in the UV. The results further demonstrate that the simultaneous acquisition of complete spectral absorption and emission properties, combined with chemical tests (C-18 extractions, borohydride reduction) can provide a far clearer picture of the sources and cycling of CDOM within the oceans.

Photochemical production of formaldehyde, acetaldehyde and acetone from chromophoric dissolved organic matter in coastal waters

Abstract Low molecular weight carbonyl compounds are produced photochemically from chromophoric dissolved organic matter (CDOM) in natural waters. Photoproduction rates for formaldehyde, acetaldehyde and acetone were measured in the laboratory using a pre-column 2,4-dinitrophenylhydrazine derivitization HPLC method as a function of optical properties (absorption coefficient, spectral slope) and irradiation time. Rates decreased linearly with decreasing absorption coefficient at 350 nm ( a (350 nm)), a measure of CDOM levels, with substantial variability in low a (350 nm) beach waters. Apparent quantum yields ( Θ ) were unchanged for a (350 nm) = 2–16 m −1 , but increased rapidly (×5) for beach waters with low a (350 nm) values. Θ increased linearly with increasing spectral slope for beach waters, consistent with enhanced production efficiency with photobleaching of CDOM in coastal waters. Θ trends with oxygen and molecular reaction probes (hydroxyl radical scavengers and producers) suggested a combination of direct photolysis and singlet oxygen quenching as primary production mechanisms.

Distribution and photoreactivity of chromophoric dissolved organic matter in northern Gulf of Mexico shelf waters

The distribution and photoreactivity of chromophoric dissolved organic matter (CDOM) in the northern Gulf of Mexico along the Louisiana coastal shelf were examined during three cruises in summer 2007, fall 2007, and summer 2008. The influence of the Mississippi River plume was clearly evident as CDOM levels (defined as a 305) and dissolved organic carbon (DOC) concentrations were well-correlated with salinity during all cruises. Elevated CDOM and CDOM:DOC ratios of surface samples collected offshore of Atchafalaya Bay and the Breton–Chandeleur Sound complex indicated emanations of organic-rich waters from coastal wetlands are also an important source to nearshore shelf waters. Generally, CDOM and DOC levels were highest in surface waters and decreased with depth, but during summer 2007 and summer 2008, CDOM levels in near-bottom samples were occasionally higher than at mid-depths without concomitant increases in DOC. CDOM photobleaching was measured during 24 irradiations using a SunTest XLS+ solar simulator with photobleaching rate coefficients ( k 305) ranging from 0.011 to 0.32 h −1. For fall 2007 and summer 2008, higher k 305 values were generally observed in samples with higher initial CDOM levels. However, samples collected during summer 2007 did not exhibit a similar pattern nor were there differences in photobleaching rates between surface and bottom samples. Spectral slope coefficients ( S 275–295 or S 350–400) and DOC levels were largely unchanged after 24 h irradiations. Modeled CDOM photobleaching for northern Gulf of Mexico mid-shelf waters predicts that during the summer when solar irradiance is high and the water column becomes stratified, nearly 90% of the CDOM in the upper 1 m may be lost to photobleaching, with losses up to 20% possible even at 10 m depth.

Fluorescence and absorption properties of chromophoric dissolved organic matter (CDOM) in coastal surface waters of the northwestern Mediterranean Sea, influence of the Rhône River

Abstract. Seawater samples were collected monthly in surface waters (2 and 5 m depths) of the Bay of Marseilles (northwestern Mediterranean Sea; 5°17'30" E, 43°14'30" N) during one year from November 2007 to December 2008 and studied for total organic carbon (TOC) as well as chromophoric dissolved organic matter (CDOM) optical properties (absorbance and fluorescence). The annual mean value of surface CDOM absorption coefficient at 350 nm [aCDOM(350)] was very low (0.10 ± 0.02 m−1) in comparison to values usually found in coastal waters, and no significant seasonal trend in aCDOM(350) could be determined. By contrast, the spectral slope of CDOM absorption (SCDOM) was significantly higher (0.023 ± 0.003 nm−1) in summer than in fall and winter periods (0.017 ± 0.002 nm−1), reflecting either CDOM photobleaching or production in surface waters during stratified sunny periods. The CDOM fluorescence, assessed through excitation emission matrices (EEMs), was dominated by protein-like component (peak T; 1.30–21.94 QSU) and marine humic-like component (peak M; 0.55–5.82 QSU), while terrestrial humic-like fluorescence (peak C; 0.34–2.99 QSU) remained very low. This reflected a dominance of relatively fresh material from biological origin within the CDOM fluorescent pool. At the end of summer, surface CDOM fluorescence was very low and strongly blue shifted, reinforcing the hypothesis of CDOM photobleaching. Our results suggested that unusual Rhône River plume eastward intrusion events might reach Marseilles Bay within 2–3 days and induce local phytoplankton blooms and subsequent fluorescent CDOM production (peaks M and T) without adding terrestrial fluorescence signatures (peaks C and A). Besides Rhône River plumes, mixing events of the entire water column injected relative aged (peaks C and M) CDOM from the bottom into the surface and thus appeared also as an important source of CDOM in surface waters of the Marseilles Bay. Therefore, the assessment of CDOM optical properties, within the hydrological context, pointed out several biotic (in situ biological production, biological production within Rhône River plumes) and abiotic (photobleaching, mixing) factors controlling CDOM transport, production and removal in this highly urbanized coastal area.

Photobleaching kinetics of chromophoric dissolved organic matter derived from mangrove leaf litter and floating Sargassum colonies

We examined the photoreactivity of chromophoric dissolved organic matter (CDOM) derived from Rhizophora mangle (red mangrove) leaf litter and floating Sargassum colonies as these marine plants can be important contributors to coastal and open ocean CDOM pools, respectively. Mangrove and Sargassum CDOM readily degraded when exposed to simulated solar irradiance (CPS SunTest solar simulator exposures). CDOM produced from brown mangrove leaves (representative of substantial senescence) exhibited shorter photobleaching half-lives ( a 305 t 1/2 < 50 h) than CDOM produced from yellow and orange (early and mid senescence) leaves ( a 305 t 1/2 ∼ 60–90 h). Mangrove CDOM photobleaching rates were higher in the mid-UVA ( a 350) than in the UVB ( a 305) spectral region. Photobleaching half-lives of Sargassum CDOM were mostly < 40 h and more consistent across UVB ( a 305) and UVA ( a 350) wavelengths. Sargassum CDOM photomineralized during simulated solar irradiation producing DIC at rates exceeding 2500 nmol m l – 1 h – 1 , indicating that regions of the surface ocean with large concentrations of this plant may provide a strong CO 2 source to the atmosphere. Sargassum CDOM photoreactions also produced CO more efficiently than terrestrial CDOM and much more efficiently than ambient CDOM in the open ocean. Thus, biological production of CDOM may be the rate-limiting step for photoproduction of DIC from Sargassum and other sources in the open ocean. An examination of CDOM photobleaching in Florida Keys coastal waters indicates that one month of summertime solar radiation may substantially increase UVB and UVA exposure to corals in shallow waters (< 4 m), especially along the offshore reef tract. But, our results also indicate that when ambient CDOM levels are high, the corals are well-buffered against increases in ultraviolet radiation (UV-R) exposure even after periods of extended CDOM photobleaching.

Alteration of chromophoric dissolved organic matter by solar UV radiation causes rapid changes in bacterial community composition

We evaluated the effect of photochemical alterations of chromophoric dissolved organic matter (CDOM) on bacterial abundance, activity and community composition in a coastal lagoon of the Atlantic Ocean with high dissolved organic carbon concentration. On two occasions during the austral summer, bacteria-free water of the lagoon was exposed to different regions of the solar spectrum (full solar radiation, UV-A+PAR, PAR) or kept in the dark. Subsequently, dilution cultures were established with bacterioplankton from the lagoon that were incubated in the pre-exposed water for 5 h in the dark. Cell abundance, activity, and community composition of bacterioplankton were assessed before and after incubation in the different treatments. Changes in absorption, fluorescence, and DOC concentration were used as proxies for CDOM photoalteration. We found a significant CDOM photobleaching signal, DOC loss, as well as a stimulation of bacterial activity in the treatments pre-exposed to UV radiation, suggesting increased bioavailability of DOM. Bacterial community analysis by fluorescence in situ hybridization revealed that this stimulation was mainly accompanied by the specific enrichment of Alpha- and Betaproteobacteria. Thus, our results suggest that CDOM photoalteration not only stimulates bacterioplankton growth, but also induces rapid changes in bacterioplankton composition, which can be of relevance for ecosystem functioning, particularly considering present and future changes in the input of terrestrial CDOM to aquatic systems.

Photoreactivity of chromophoric dissolved organic matter transported by the Mackenzie River to the Beaufort Sea

The photoreactivity of chromophoric dissolved organic matter (CDOM) transported to Arctic shelf environments by rivers has only recently been studied and its quantitative role in Arctic shelf biogeochemistry has received little attention. Sunlight exposure experiments were performed on CDOM collected over a three year period (2002 to 2004) from river, estuary, shelf, and gulf regions of the Western Canadian Arctic. Decreases in CDOM absorption, synchronous fluorescence (SF), and dissolved organic carbon (DOC) concentration were followed after 3 days of exposure, and in two experiments, six optical cutoff filters were used to incrementally remove ultraviolet radiation incident on the samples. Apparent quantum yields for CDOM photobleaching (AQY ble) and for DOC photomineralization (AQY min) were computed, as were two AQY spectra (ϕ ble and ϕ min) for the Mackenzie River and a sample from the Mackenzie Shelf. The photoreactivity of Mackenzie River CDOM was highest after break-up and peak discharge and lowest in late summer. The half-lives of CDOM and DOC were estimated at 3.7 days and 4.8 days, respectively, when Mackenzie River water was exposed to full sunlight. Photobleaching of Mackenzie River CDOM fluorescence after most UV-B wavelengths were removed increased the correlation between the river and offshore waters in the Beaufort Sea. When light attenuation from particle- and CDOM-rich river water was considered for the Mackenzie Shelf, our photodegradation models estimated around 10% loss of absorption and < 1% DOC loss, suggesting that sunlight exposure does not substantially degrade CDOM on Arctic shelves.

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.

Photochemical degradation of chromophoric-dissolved organic matter exposed to simulated UV-B and natural solar radiation

Photochemical degradation of chromophoric-dissolved organic matter (CDOM) by UV-B radiation decreases CDOM absorption in the UV region and fluorescence intensity, and alters CDOM composition. CDOM absorption, fluorescence, and the spectral slope indicating the CDOM composition were studied using 0.22-μm-filtered samples of Meiliang Bay water from Lake Taihu that were exposed to short-term (0–12 h) simulated UV-B radiation and long-term (0–12 days) natural solar radiation in summer. CDOM absorption coefficient and fluorescence decreased with increasing exposure time, which relates to the amounts of absorbed light energy. The decreases of CDOM absorption and normalized fluorescence corresponded to first order kinetics reactions. Different decreases of CDOM absorption and fluorescence at different wavelengths suggested that the composition of CDOM changed when it absorbed ultraviolet radiation. Photochemical degradation increased the spectral slope during 275–295 nm region (S 275–295) but decreased the spectral slope during 275–295 nm region (S 350–400). The slope ratio S R (S 275–295:S 350–400) increased in the photochemical process, which could be used as an indicator of photobleaching and composition change of CDOM. Our results show that photochemical degradation is important in the cycling of CDOM, which indicated change in the composition of CDOM.

Carbon monoxide and chromophoric dissolved organic matter cycles in the shelf waters of the northern California upwelling system

An annual program of approximately bimonthly seawater sampling was conducted off the coast of the Bodega Marine Laboratory in the California Current upwelling system (CCUS) to investigate controls on dissolved carbon monoxide (CO) and its photochemical parent, chromophoric dissolved organic matter (CDOM). A diel steady state model was developed to quantify CO sources and sinks throughout the day. Observed midday CO spanned 2.7–17 nmol L−1, and correlated strongly with coincident CDOM absorption; however, rapid microbial consumption rates (0.05–0.80 h−1) were such that ∼95% of the CO generated was metabolized within the ocean surface. Further indication of the dominant influence of the microbial ecosystem was the persistent observation of a strong “dark production,” which contributed ∼25% of the diurnal source strength within the top 17 m of the ocean. The dark source correlated strongly with biological oxidation rates and CDOM, suggesting that it is the result of incomplete respiration of biologically labile DOM. CDOM was lowest in recently upwelled waters, implying that its origin is from the induced primary production and not from shelf sediment resuspension. The estimated CO air‐sea flux was highly variable (average of 5 μmol m−2 d−1), and most likely inconsequential to the atmospheric budget in coastal regions. The photolytic source strength of CO2 from CDOM (2.7 mmol m−2 d−1) was estimated by scaling to modeled CO photoproduction rates. This photolytic remineralization rate represents ∼2.5% of the net primary production in the CCUS, implying that CDOM photobleaching may play a significant role in the carbon cycling of upwelling systems.