Chromophoric Dissolved Organic Matter Absorption Research Articles

Are Extracted Materials Truly Representative of Original Samples? Impact of C18 Extraction on CDOM Optical and Chemical Properties.

Some properties of dissolved organic matter (DOM) and chromophoric dissolved organic matter (CDOM) can be easily measured directly on whole waters, while others require sample concentration and removal of natural salts. To increase CDOM content and eliminate salts, solid phase extraction (SPE) is often employed. Biases following extraction and elution are inevitable, thus raising the question of how truly representative the extracted material is of the original. In this context, we investigated the wavelength dependence of extraction efficiency for C18 cartridges with respect to CDOM optical properties using samples obtained from the Middle Atlantic Bight (MAB) and the Equatorial Atlantic Ocean (EAO). Further, we compared the optical changes of C18 extracts and the corresponding whole water following chemical reduction with sodium borohydride (NaBH4). C18 cartridges preferentially extracted long-wavelength absorbing/emitting material for samples impacted by riverine input. Extraction efficiency overall decreased with offshore distance away from riverine input. Spectral slopes of C18-OM samples were also almost always lower than those of their corresponding CDOM samples supporting the preferential extraction of higher molecular weight absorbing material. The wavelength dependence of the optical properties (absorption, fluorescence emission, and quantum yield) of the original water samples and their corresponding extracted material were very similar. C18 extracts and corresponding water samples further exhibited comparable optical changes following NaBH4 reduction, thus suggesting a similarity in nature (structure) of the optically active extracted material, independent of geographical locale. Altogether, these data suggested a strong similarity between C18 extracts and corresponding whole waters, thus indicating that extracts are representative of the CDOM content of original waters.

Transport and transformation of dissolved organic matter in the Neuse River estuarine system, NC, USA, following Hurricane Irene (2011)

Rivers are major conduits for the transport of allochthonous dissolved organic matter (DOM) to the ocean in coupled land–coastal systems. DOM can regulate biogeochemical processes and affect water quality, depending on the concentration and quality of DOM. By using spectral parameters calculated from chromophoric dissolved organic matter (CDOM) ultraviolet-visible absorption spectra, along with dissolved organic carbon (DOC) concentrations, we examined the input and change in the amount and quality of DOM in surface waters of the lower Neuse River and upper–middle regions of the Neuse Estuary following a major rainfall (30cm in 1 day) associated with Hurricane Irene (2011). CDOM and DOC nearly tripled in the 3 days following the storm. Although a strong linear relationship was observed between DOC and CDOM absorption coefficient at 350nm (R2=0.85), a higher fraction of non-chromophoric DOC to CDOM was observed during the rising river discharge. The spectral slope at 275–295nm and the slope ratio (275–295:350–400nm) indicated a shift from higher to lower molecular-weight DOM as it transited through the estuary, probably as a result of photodegradation. The present study demonstrated the utility of using CDOM spectral parameters for examining the flux and transformation of DOM in coastal waters following major rain events.

Influence of macrophyte integrity on zooplankton habitat preference, emphasizing the released phenolic compounds and chromophoric dissolved organic matter

Macrophyte health status can influence the composition of their exudates causing different effects on zooplankton behavior and distribution in nature. We hypothesize that: (1) the release of phenolic compounds and chromophoric dissolved organic matter (CDOM) depends on macrophyte species and its health status (broken macrophytes: BM, or healthy macrophytes: HM); (2) the repellency effect depends on zooplankton species, macrophyte species and its health status; and (3) higher concentrations of phenolic compounds and CDOM produce a stronger repellency effect. Phenolic compounds and CDOM were analyzed in exudates of BM and HM of Salvinia sp., Eichhornia crassipes, Pistia stratiotes, Azolla sp. and Ludwigia peploides. Through a flow-through experiment, the repellency produced by these exudates was assessed in two copepods (Notodiaptomus conifer and Argyrodiaptomus falcifer) and one cladoceran (Ceriodaphnia dubia). Our hypotheses were partially validated. The quantity of exudated phenolic compounds and CDOM depended on macrophyte species and, to a lesser extent, on the plant health status. The repellency effect was affected by macrophyte and zooplankton species but not by the health status of plants. Only C. dubia and A. falcifer increased their evasion behavior when phenolic compound and CDOM concentrations increased. In brief, the structuring effect of repellent substances depends on different factors. Under a certain threshold concentration, zooplankton behavior might depend on the information associated with the plant odor (e.g., predation risk, structural complexity) more than on the quantity of the released chemical compounds. Above this threshold, evasion would be the only possible option to avoid damaging effects.

Photomineralization and photomethanification of dissolved organic matter in Saguenay River surface water

Abstract. Rates and apparent quantum yields of photomineralization (AQYDOC) and photomethanification (AQYCH4) of chromophoric dissolved organic matter (CDOM) in Saguenay River surface water were determined at three widely differing dissolved oxygen concentrations ([O2]) (suboxic, air saturation, and oxygenated) using simulated-solar radiation. Photomineralization increased linearly with CDOM absorbance photobleaching for all three O2 treatments. Whereas the rate of photochemical dissolved organic carbon (DOC) loss increased with increasing [O2], the ratio of fractional DOC loss to fractional absorbance loss showed an inverse trend. CDOM photodegradation led to a higher degree of mineralization under suboxic conditions than under oxic conditions. AQYDOC determined under oxygenated, suboxic, and air-saturated conditions increased, decreased, and remained largely constant with photobleaching, respectively; AQYDOC obtained under air saturation with short-term irradiations could thus be applied to longer exposures. AQYDOC decreased successively from ultraviolet B (UVB) to ultraviolet A (UVA) to visible (VIS), which, alongside the solar irradiance spectrum, points to VIS and UVA being the primary drivers for photomineralization in the water column. The photomineralization rate in the Saguenay River was estimated to be 2.31 × 108 mol C yr−1, accounting for only 1 % of the annual DOC input into this system. Photoproduction of CH4 occurred under both suboxic and oxic conditions and increased with decreasing [O2], with the rate under suboxic conditions ~ 7–8 times that under oxic conditions. Photoproduction of CH4 under oxic conditions increased linearly with photomineralization and photobleaching. Under air saturation, 0.00057 % of the photochemical DOC loss was diverted to CH4, giving a photochemical CH4 production rate of 4.36 × 10−6 mol m−2 yr−1 in the Saguenay River and, by extrapolation, of (1.9–8.1) × 108 mol yr−1 in the global ocean. AQYCH4 changed little with photobleaching under air saturation but increased exponentially under suboxic conditions. Spectrally, AQYCH4 decreased sequentially from UVB to UVA to VIS, with UVB being more efficient under suboxic conditions than under oxic conditions. On a depth-integrated basis, VIS prevailed over UVB in controlling CH4 photoproduction under air saturation while the opposite held true under O2-deficiency. An addition of micromolar levels of dissolved dimethyl sulfide (DMS) substantially increased CH4 photoproduction, particularly under O2-deficiency; DMS at nanomolar ambient concentrations in surface oceans is, however, unlikely a significant CH4 precursor. Results from this study suggest that CDOM-based CH4 photoproduction only marginally contributes to the CH4 supersaturation in modern surface oceans and to both the modern and Archean atmospheric CH4 budgets, but that the photochemical term can be comparable to microbial CH4 oxidation in modern oxic oceans. Our results also suggest that anoxic microniches in particulate organic matter and phytoplankton cells containing elevated concentrations of precursors of the methyl radical such as DMS may provide potential hotspots for CH4 photoproduction.

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.

Direct and indirect effects of additions of chromophoric dissolved organic matter on zooplankton during large‐scale mesocosm experiments in an oligotrophic lake

Summary The effects of changes in chromophoric dissolved organic matter (CDOM) on zooplankton grazers in pelagic food webs can be difficult to predict due to the potential for conflicting direct and indirect effects of CDOM on water column optics and food‐web dynamics. We compared the responses of two dominant zooplankton groups, Daphnia spp. and calanoid copepods, to elevated CDOM in two mesocosm experiments conducted in a transparent, oligotrophic lake. In the first experiment, low, medium and high additions of CDOM [dissolved organic carbon (DOC) = 2.7, 3.0 and 4.3 mg L−1, respectively] and non‐manipulated controls (DOC = 1.6 mg L−1) were used to test the hypothesis that food‐web stimulation will increase with CDOM concentration. In the second experiment, we manipulated both CDOM (control and addition) and ultraviolet radiation (UV; ambient and shielded) to determine the importance of CDOM as a UV screen, as well as to examine the role of UV in food‐web stimulation by CDOM. We also conducted short‐term bioassays alongside this second experiment to assess the food resources for Daphnia and calanoids in treatments with UV‐exposed and non‐UV‐exposed CDOM. Daphnia abundance increased in the low and medium CDOM additions in the first experiment, and when food resources were simultaneously stimulated by CDOM and protected from the direct negative effects of UV in the second experiment. In contrast, calanoid copepod abundance declined or remained unchanged when CDOM was added, and this response was not mediated by food resources. Overall, our findings suggest that increased CDOM in clear lakes could alter the relative abundance of zooplankton groups through a combination of direct and indirect effects.

Retrieval of CDOM and DOC Using In Situ Hyperspectral Data: A Case Study for Potable Waters in Northeast China

Chromophoric dissolved organic matter (CDOM), the light absorbing fraction of dissolved organic carbon (DOC), together with phytoplankton and total suspended matter are the main optically active components could be retrieved by remote sensing data. Generally, different composition of DOC and CDOM corresponds to different water surface reflectance. Absorption properties of CDOM and retrieval models for CDOM and DOC were investigated with data from potable reservoirs located in the central of Jilin Province. Water sampling field surveys were conducted on 15, 16 and 19 of September 2012 across the Shitoukoumen, Erlonghu and Xilicheng reservoirs, respectively. Both empirical regression (single band model and band ratio model) and partial least squares coupled with back-propagation artificial neural models (PLSBPNN) were established to estimate CDOM absorption coefficient at 355 nm [aCDOM(355)] and DOC concentration with in situ measured remote sensing reflectance. It was found that the band ratio models and PLSBPNN model performed well for estimating DOC concentration while the band ratio models yielded the best result in retrieval CDOM. Moreover, all the three models performed better on the DOC concentration estimation than the performance on aCDOM(355). Band ratio models outperformed (R 2 = 0.55) other models for estimating CDOM absorption coefficient, while PLSBPNN model outperformed other models with respect to DOC estimation (R 2 = 0.93). High spectral slope values indicated that CDOM in the potable waters primarily comprised low molecular weight organic substances; while sources of DOC were mainly coming from exogenous input, which was the main reason lead to the difference of model performances on DOC and aCDOM(355) estimation. The algorithms developed in this study is needed to be tested and refined with more in situ spectral data, also future work is still needed to be undertaken for characterizing the dynamic of the potable water quality with remotely sensed imagery.

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 (>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).

Chromophoric dissolved organic matter of black waters in a highly eutrophic Chinese lake: Freshly produced from algal scums?

Field campaigns and an incubation experiment were conducted to evaluate the sources of chromophoric dissolved organic matter (CDOM) in black water spots in highly polluted regions of the Chinese Lake Taihu. A significant positive correlation (p<0.0001) was found between chlorophyll a (Chl-a) and the CDOM absorption coefficient a(350), indicating that algae degradation was likely the primary source of CDOM in black waters. This is supported by our field results that Chl-a, a(350) and the spectral slope ratio (SR) were significantly higher in the black water samples than in the regular samples (p<0.001). Our incubation experiment further substantiated the primary significance of biological CDOM source where a(350) increased with decreasing Chl-a concentrations. After seven days’ incubation, a 72.2% decrease and a 74.9% increase were recorded for Chl-a and a(350), respectively, relative to the initial values. Parallel factor analysis identified five fluorescent components. The maximal fluorescence intensity (Fmax) of tryptophan-like C1 and microbial humic-like C3 of black water samples was significantly higher than in the regular water samples (p<0.0005). This is consistent with incubation experiment results showing a rapid increase in Fmax of the two components, emphasizing the priority of the in situ biological CDOM source in black water spots.

Dynamics of chromophoric dissolved organic matter influenced by hydrological conditions in a large, shallow, and eutrophic lake in China.

High concentrations of chromophoric dissolved organic matter (CDOM) are terrestrially derived from upstream tributaries to Lake Taihu, China, and are influenced by hydrological conditions of the upstream watershed. To investigate how the dynamics of CDOM in Lake Taihu are influenced by upstream inflow runoff, four sampling cruises, differing in hydrological conditions, were undertaken in the lake and its three major tributaries, rivers Yincun, Dapu, and Changdou. CDOM absorption, fluorescence spectroscopy, chemical oxygen demand (COD), and stable isotope δ(13)C and δ(15)N measurements were conducted to characterize the dynamics of CDOM. The mean absorption coefficient a(350) collected from the three river profiles (5.15 ± 1.92m(-1)) was significantly higher than that of the lake (2.95 ± 1.88m(-1)), indicating that the upstream rivers carried a substantial load of CDOM to the lake. This finding was substantiated by the exclusively terrestrial signal exhibited by the level of δ(13)C (-26.23 ± 0.49‰) of CDOM samples collected from the rivers. Mean a(350) and COD in Lake Taihu were significantly higher in the wet season than in the dry season (t test, p < 0.0001), suggesting that the abundance of CDOM in the lake is strongly influenced by hydrological conditions of the watershed. Four components were identified by parallel factor analysis, including two protein-like components (C1 and C2), a terrestrial humic-like component (C3), and a microbial humic-like (C4) component. The contribution percentage of the two humic-like components relative to the summed fluorescence intensity of the four components (C humic) increased significantly from the dry to the wet season. This seasonal difference in contribution further substantiated that an enhanced rainfall followed by an elevated inflow runoff in the lake watershed in the wet season may result in an increase in humic-like substances being discharged into the lake compared to that in the dry season. This finding was further supported by an elevated a(250)/a(365) of CDOM samples collected in the lake in the wet season than in the dry season. Significantly higher mean levels of C3 and a(350) were recorded for CDOM samples collected from River Yincun than those from rivers Dapu and Changdou, differing in seasons, suggesting the significance of terrestrial CDOM input from River Yincun.

Lake Taihu, a large, shallow and eutrophic aquatic ecosystem in China serves as a sink for chromophoric dissolved organic matter

Abstract Photochemical reactions and microbial activity often result in chromophoric dissolved organic matter (CDOM) being lost to downstream cycles. The role of a specific lake ecosystem serving as a sink for CDOM was elucidated in Lake Taihu, China. The lake was divided into CDOM-enriched and CDOM-depleted regions based on principal component analysis (PCA) results. The spatial distribution of molecular size ( M ), spectral slope ratio ( S R ) and humification index (HIX) of CDOM suggested that CDOM in Lake Taihu was mainly from exogenous sources. Parallel factor analysis (PARAFAC) identified four components, including a microbially humic-like component (C1), two protein-like components (C2 and C3) and a terrestrial humic-like component (C4). Strong conservative mixing behaviors of C4 and C1 were observed in the lake, and River Yincun is likely responsible for high CDOM concentration in Zhushan Bay. Compared with C2, C3 seemed to be more independent of the terrestrial humic-like CDOM. HIX decreased and S R increased significantly from the CDOM-enriched region to the CDOM-depleted region ( t -test, p a (350), dissolved organic carbon (DOC) and summed fluorescence intensity of the four components in the CDOM-enriched region were significantly higher than in the CDOM-depleted region ( t -test, p 9 m 3 m − 1 yr − 1 and 16.91 × 10 3 t yr − 1 , respectively.

Assessment of ocean color data records from MODIS-Aqua in the western Arctic Ocean

A broad suite of bio-optical and biogeochemical observations collected during the NASA-funded ICESCAPE expeditions to the western Arctic Ocean in 2010 and 2011 was used to validate ocean color satellite data products in this region, which is undergoing fast ecological changes in the context of a changing climate. Satellite-to-in situ match-ups for the MODIS instrument onboard Aqua (MODISA) were evaluated using standard NASA empirical and semi-analytical algorithms to estimate chlorophyll-a (Ca), spectral marine inherent optical properties, and particulate organic carbon (POC). Results for the empirical algorithms were compared with those from the semi-analytical Generalized Inherent Optical Property (GIOP) algorithm. The findings presented here showed that MODISA Ca estimates were positively biased relative to in situ measurements, in agreement with previous studies that have evaluated ocean color retrievals in the Arctic Ocean. These biases were reproduced using both satellite and in situ measured remote sensing reflectances, Rrs(λ), indicating that estimation errors are derived from the application of the empirical algorithm and not by the observed radiometry. This disparity appears to be caused by contributions of high spectral absorption from chromophoric dissolved organic matter (CDOM), which is a well-documented feature of Arctic Ocean waters. The current MODISA empirical algorithm (OC3M) appears to attribute CDOM absorption in the blue region of the spectrum to phytoplankton absorption. In contrast, GIOP showed significant improvement over OC3M Ca estimates by effectively discriminating between phytoplankton and CDOM absorption. Additionally, executing GIOP with an expanded set of spectral bands derived from in situ radiometry, instead of just six MODISA bands, further improved the performance of absorption estimates. These findings reinforce previous suggestions that semi-analytical approaches will provide more reliable data records for Arctic studies than existing empirical methods. POC estimates showed no clear bias relative to in situ measurements, suggesting that the empirical algorithm better represents high latitude oceans with regards to the bio-optical signature of suspended particulate stocks.

Seasonal Variations of CDOM Optical Properties in Rivers Across the Liaohe Delta

Absorption and fluorescence properties of chromophoric dissolved organic matter (CDOM) in rivers across the Liaohe River Delta, a large estuary located in the southern region of northeast China, were investigated using spectroscopy and fluorescence to analyze CDOM characteristics, composition and sources in winter (January), spring (April and May) and autumn (September) 2013. Results indicated that CDOM absorption of ice samples was lower compared to water samples. CDOM absorption also showed significant spatial variation but not temporal variation. In contrast, dissolved organic carbon (DOC) concentrations showed obvious temporal characteristics. A stronger linear relationship was found between CDOM absorption and DOC concentration in winter (water, R 2 = 0.95, p < 0.001; ice, R 2 = 0.85, p < 0.001) compared to samples from other seasons (April, R 2 = 0.51, p < 0.01; May, R 2 = 0.34, p < 0.05; September, R 2 = 0.45, p < 0.01). CDOM fluorescence varied over a large range across seasons, with the highest levels observed in January at the Xisha River (XSR). Moreover, strong linear relationships were also observed between CDOM absorption and fluorescence intensity at 355 nm [Fn(355)] in January (R 2 = 0.87, p < 0.001), May (R 2 = 0.76, p < 0.001) and September (R 2 = 0.94, p < 0.001). Also based on study findings, CDOM fluorophores identified by 3-D excitation-emission matrices (EEMs) illustrated that the rivers across the Liaohe Delta were seriously polluted by anthropogenic disturbances, exhibiting strong protein-like fluorescence of CDOM in the water column. In addition, the results also confirm that absorption and EEMs would be useful tools for tracing the sources and characteristics of CDOM and monitoring riverine water quality.

A new algorithm to retrieve chromophoric dissolved organic matter (CDOM) absorption spectra in the UV from ocean color

AbstractAccurate estimation of the absorption coefficient (ag) for chromophoric dissolved organic matter (CDOM) over ultraviolet (UV) and short visible radiation wavelengths (with λ = 275–450 nm) is crucial to provide a robust assessment of the biogeochemical significance of UV in the global ocean. Using a training data set spanning a variety of water types from the clearest open ocean to dynamic inshore waters, a novel algorithm to accurately resolve CDOM absorption spectra from ocean color is presented. Employing a suite of multivariate statistical approaches (principal component analysis, cluster analysis, and multiple linear regression), this new algorithm was developed with matched field data for CDOM spectra and remote sensing reflectance (Rrs) at Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) bands. Freed from any presupposition about CDOM spectral shape or conventional spectral extrapolations from visible data, our algorithm allows direct retrieval of a fully resolved CDOM absorption spectrum over UV wavelengths from visible Rrs and further enables a global scale view of the dynamics of CDOM over different water types. Accuracy of ag retrieval is good, with a mean absolute percent difference for ag in the UV of ∼25%. With fully resolved spectra, maps of calculated CDOM spectral slopes (S275–295, S350–400) and slope ratios (SR) are presented with the potential to provide new information about the chemical composition (e.g., molecular weight and aromaticity), sources, transformation, and cycling pathways of CDOM on global as well as regional scales. The new algorithm will contribute to improved accuracy for photochemical and photobiological rate calculations from ocean color.

The relationship between dissolved organic matter absorption and dissolved organic carbon in reservoirs along a temperate to tropical gradient

Recent and upcoming launches of new satellite sensors will provide the spatial, spectral and radiometric resolution to globally assess freshwater chromophoric dissolved organic matter (CDOM), and thus estimate dissolved organic carbon (DOC) concentration. However, estimating DOC from optical remote sensing requires a robust relationship between CDOM and DOC. This is particularly problematic for reservoirs because they have variable dissolved organic matter composition that complicates the CDOM–DOC relationship. We investigated six manmade reservoirs along a temperate to tropical gradient that represent a range of reservoir types and watershed conditions to determine whether a linear relation between CDOM and DOC could be established. We measured CDOM absorption and DOC concentration during the wet and dry seasons in the six reservoirs.We found the CDOM absorption coefficient and CDOM spectral slope were uncorrelated due to exogenous DOC inputs from multiple sources. Alone, the absorption coefficient of CDOM was a poor predictor of DOC concentration. Including both CDOM absorption coefficient and spectral slope in a multiple regression accounted for both composition and concentration, significantly improving the regression r2. By using both CDOM absorption coefficient and spectral slope, we identify a framework for a potential solution to overcome the influence of dissolved organic matter source and transformation history on the CDOM–DOC relationship. We conclude that local variability, seasonality and optical complexity should be considered in remote sensing based approaches for global freshwater DOC estimation.

Biophysical processes affecting DOM dynamics at the Arno river mouth (Tyrrhenian Sea)

Dissolved organic carbon (DOC) and optical properties (absorption and fluorescence) of chromophoric dissolved organic matter (CDOM) were measured in October 2012, at the Arno river mouth and in a coastal station close to it. The data reported indicates that the Arno river represents an important source of DOC and CDOM to this coastal area, with a total DOC flux of 11.23–12.04·109g C·y−1.Moving from the river to the sea, CDOM absorption and fluorescence decreased, while the spectral slope increased, suggesting a change in the molecular properties of CDOM.Mineralization experiments were carried out in order to investigate the main processes of DOM removal and/or transformation in riverine and coastal water. DOC removal rates were 20μM·month−1 in the river and 3μM·month−1 in the seawater, while CDOM was released during the first 30days and removed in the following 40days.

Hourly, daily, and seasonal variability in the absorption spectra of chromophoric dissolved organic matter in a eutrophic, humic lake

AbstractThe short‐term (hourly and daily) variation in chromophoric dissolved organic matter (CDOM) in lakes is largely unknown. We assessed the spectral characteristics of light absorption by CDOM in a eutrophic, humic shallow mixed lake of temperate Sweden at a high‐frequency (30 min) interval and during a full growing season (May to October). Physical time series, such as solar radiation, temperature, wind, and partial pressures of carbon dioxide in water and air, were measured synchronously. We identified a strong radiation‐induced summer CDOM loss (25 to 50%) that developed over 4 months, which was accompanied by strong changes in CDOM absorption spectral shape. The magnitude of the CDOM loss exceeded subhourly to daily variability by an order of magnitude. Applying Fourier analysis, we demonstrate that variation in CDOM remained largely unaffected by rapid shifts in weather, and no apparent response to in‐lake dissolved organic carbon production was found. In autumn, CDOM occasionally showed variation at hourly to daily time scales, reaching a maximum daily coefficient of variation of 15%. We suggest that lake‐internal effects on CDOM are quenched in humic lake waters by dominating effects associated with imported CDOM and solar exposure. Since humic lake waters belong to one of the most abundant lake types on Earth, our results have important implications for the understanding of global CDOM cycling.

The effect of intense hydrodynamic disturbance on chromophoric dissolved organic matter in a shallow eutrophic lake

Chromophoric dissolved organic matter (CDOM) is an important component of aquatic ecosystems and its movement is a complicated process. To investigate the effect of strong wind-induced sediment suspension on CDOM concentration, we carried out an in situ observation in a large, shallow lake, which is seriously affected by eutrophication and cyanobacterial blooms, using a finely stratified sampling device and several automatic monitoring instruments. During the observation, a typhoon-induced strong wind with a mean wind speed of 6.6 m/s swept the Lake Taihu basin. The strong wind-induced hydrodynamic disturbance caused explosive sediment suspension. During the strong winds, the maximum turbidity in the overlying waters reached 175.4 NTU, which was 12.2 times the background value recorded before the strong winds. Sediment suspension resulted in the release of CDOM from within the sediment. The CDOM absorption coefficient a(355) in the overlying water showed that there was no significant difference between any two water layers for all time points during the observation (13 July to 16 July). The CDOM concentration was fully mixed along the water depth. However, the mean a(355) values recorded in all time points during the wind were higher than that before the wind (p < 0.001). The maximum value during the wind event was 2.524 ± 0.097 m−1, which was 1.4 times that of the value before the disturbance. CDOM concentrations were significantly correlated with hourly mean wind speed (p < 0.05) and turbidity (p < 0.01). Because Lake Taihu is frequently affected by strong wind processes, CDOM concentration changes caused by intense hydrodynamic disturbance are important for quantitative remote sensing of water quality and primary productivity.

Absorption and fluorescence characteristics of rainwater CDOM and contribution to Lake Taihu, China

We characterized the composition and sources of chromophoric dissolved organic matter (CDOM) in rainwater, and assessed the relative contribution of rainwater CDOM to lake water in Lake Taihu based on rainwater collected during 35 rainfall events in 2012. Chemical analysis, ultraviolet–visible absorbance, and three-dimensional fluorescence spectroscopy were used to characterize CDOM. The CDOM absorption coefficient at 254 nm (a254) had a significant seasonal variation, with a mean of 3.67 ± 1.69 m−1 in the wet season (from April to early August), which was significantly lower than the means in the two dry seasons (8.26 ± 2.94 m−1 from January to March, and 7.60 ± 3.80 m−1 from late August to December). The mean humification index and the mean index of recent autochthonous contribution were 0.74 ± 0.48 and 1.31 ± 0.35, respectively, indicating that rainwater CDOM was dominated by an atmospheric microbial origin component. We identified four fluorescence components using parallel factor analysis modeling in the rainwater CDOM, i.e., two protein-like components (C1 and C2) and two fulvic-like components (C3 and C4), which had characteristics similar to those of protein and humic-like substances, respectively. The a254 was significantly and positively (p < 0.001) correlated with each of the five nutrient parameters: total dissolved nitrogen (r2 = 0.76), ammonium (r2 = 0.65), nitrate (r2 = 0.36), total dissolved phosphorus (r2 = 0.55), and phosphate (r2 = 0.50) showing the tightly coupling between CDOM and nutrients. Based on the deposition of the rainwater CDOM and the storage of the CDOM in Lake Taihu, the annual relative contribution of rainwater CDOM to the lake water was 11.7% in 2012. The results showed the important effect of wet deposition on CDOM sources in Lake Taihu, which is located in a region with severe air pollution in the Yangtze River Delta.

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.