Search Results (18)

Dissolved organic matter (DOM) is a crucial component of continental aquatic ecosystems. It plays a vital role in the carbon cycle by serving as a significant source and reservoir of carbon in water. DOM provides energy and nutrients to organisms, affecting primary productivity, organic composition, and the food chain. This study presents empirical bio-optical models for estimating the absorption of colored dissolved organic matter (aCDOM) in the Negro River using in situ remote sensing reflectance (Rrs) data. Physical–chemical data (TSS, DOC, and POC) and optical data (aCDOM and Rrs) were collected from the Negro River, its tributaries, and lakes and empirical relationships between aCDOM at 440 nm, single band, and the ratio bands of Rrs were assessed. The analysis of spectral slope shows no statistically significant correlations with DOC concentration or aCDOM absorption coefficient. However, strong relationships were observed between DOC and aCDOM (R² = 0.72), aCDOM and Rrs at 650 nm (R² > 0.80 and RMSE < 1.75 m⁻¹), as well as aCDOM and the green/red band ratio (R² > 0.80 and RMSE < 2.30 m⁻¹). aCDOM displayed large spatial and temporal variations, varying from 1.9 up to 20.1 m⁻¹, with higher values in rivers of the upper course of the Negro basin and lower values in rivers with total solids suspended > 10 mg·L⁻¹. Environmental factors that influence the production of dissolved organic matter include soil type, dense forest cover, high precipitation, and low erosion rates. This study demonstrated that aCDOM can serve as an indicator of DOC, and Rrs can serve as an indicator of aCDOM in the Negro basin. Our findings offer a starting point for future research on the optical properties of Amazonian black-water rivers. Full article

Chromophoric dissolved organic matter (CDOM) is the main sunlight absorber in surface waters and a very important photosensitiser towards the generation of photochemically produced reactive intermediates (PPRIs), which take part in pollutant degradation. The absorption spectrum of CDOM (A_CDOM(λ), unitless) can be described by an exponential function that decays with increasing wavelength: A_CDOM(λ) = 100 d DOC A_o e^− Sλ, where d [m] is water depth, DOC [mg_C L⁻¹] is dissolved organic carbon, A_o [L mg_C⁻¹ cm⁻¹] is a pre-exponential factor, and S [nm⁻¹] is the spectral slope. Sunlight absorption by CDOM is higher when A_o and DOC are higher and S is lower, and vice versa. By the use of models, here we investigate the impact of changes in CDOM spectral parameters (A_o and S) on the steady-state concentrations of three PPRIs: the hydroxyl radical (^•OH), the carbonate radical (CO₃^•−), and CDOM excited triplet states (³CDOM*). A first finding is that variations in both A_o and S have impacts comparable to DOC variations on the photochemistry of CDOM, when reasonable parameter values are considered. Therefore, natural variability of the spectral parameters or their modifications cannot be neglected. In the natural environment, spectral parameters could, for instance, change because of photobleaching (prolonged exposure of CDOM to sunlight, which decreases A_o and increases S) or of the complex and still poorly predictable effects of climate change. A second finding is that, while the steady-state [³CDOM*] would increase with increasing A_CDOM (increasing A_o, decreasing S), the effect of spectral parameters on [^•OH] and [CO₃^•−] depends on the relative roles of CDOM vs. NO₃⁻ and NO₂⁻ as photochemical ^•OH sources. Full article

Climate affects the characteristics of the Southern Ocean ecosystem, including bio-optical properties. Remote sensing is a suitable approach for monitoring a rapidly changing ecosystem. Correct remote assessment can be implemented based on a regional satellite algorithm, which requires parameterization of light absorption by all optically active components. The aim of this study is to analyse variability in total chlorophyll a concentration (TChl-a), light absorption by phytoplankton, non-algal particles (NAP), coloured dissolved organic matter (CDOM), and coloured detrital matter (CDM = CDOM+NAP), to parameterize absorption by all components. Bio-optical properties were measured in the austral summer of 2020 according to NASA Protocols (2018). High variability (1–2 orders of magnitude) in TChl-a, absorption of phytoplankton, NAP, CDOM, and CDM was revealed. High variability in both CDOM absorption (uncorrelated with TChl-a) and CDOM share in total non-water absorption, resulting in a shift from phytoplankton to CDOM dominance, caused approximately twofold chlorophyll underestimation by global bio-optical algorithms. The light absorption of phytoplankton (for the visible domain in 1 nm steps), NAP, CDOM, and CDM were parametrized. Relationships between the spectral slope coefficient (S_CDOM/S_CDM) and CDOM (CDM) absorption were revealed. These results can be useful for the development of regional algorithms for Chl-a, CDM, and CDOM monitoring in the Southern Ocean. Full article

Distinct absorption peaks, with maxima at around 328 nm and a shoulder at 360 nm, were observed in the UV region of the absorption spectra for both the particulate and dissolved fractions of water samples collected in Keppel Bay (NE Australia) during the presence of sporadic Trichodesmium colonies. The largest absorption coefficients for these peaks were observed in samples collected in the near-surface waters (top 2–3 cm). Values approximately 3.5–6 times greater for a_CDOM(328) and 13–36 times greater for a_p(328) were observed in the near-surface samples compared to those collected from the top 20 cm of the water column at the same sites. Similar UV-absorption peaks observed in other studies have been attributed to the presence of mycosporine-like amino acids (MAAs). Increased UV absorption can affect both the magnitude of the absorption coefficients in the blue end of the visible region and the spectral slope of the exponential model commonly used to describe the CDOM absorption coefficient. This, in turn, can significantly affect the accuracy of satellite retrieved estimates of ocean colour products related to CDOM and particulate absorption coefficients. In tropical waters where Trichodesmium blooms are prevalent, regional ocean colour algorithms need to be developed using in situ bio-optical measurements from both the UV and visible regions of the spectra. Full article

Optical properties of colored dissolved organic matter (CDOM) were investigated along a latitudinal transect (67°–77° N) in upper water (<80 m depth) of the western Arctic Ocean. The absorption coefficient at 280 nm was 0.48–1.25 m⁻¹, with the average for the oligotrophic basin area (1.04 ± 0.08 m⁻¹) being slightly higher than that of the productive shelf area (0.95 ± 0.16 m⁻¹), implying a decoupling effect between CDOM concentration and biological productivity in the western Arctic Ocean. The spectral slope coefficient S_270–350 was negatively correlated with salinity, indicating that DOM molecular weight increases with salinity, and may be affected by melt-water input. Four fluorescent components were identified by excitation emission matrices elaborated by parallel factor analysis, including three humic-like (C1, C3, and C4) components and one protein-like (C2) component. Significant increases in concentrations of terrestrially derived humic-like C3 and C4 components with salinity were observed in the basin, mainly controlled by the physical mixing of surface fresh water and subsurface inflowing Pacific Ocean water. Terrestrial material carried by Pacific inflow is thus an important factor affecting the distribution of CDOM fluorescence components. The C3 and C4 fluorescence components may be useful as tracers of Pacific water in the western Arctic Ocean. Full article

Colored dissolved organic matter (CDOM) is an important part of aquatic ecosystems and plays a key role in the biogeochemical cycle. In this study, CDOM absorption spectrum curves and water quality parameters from 30 sampling sites in the wetlands of Poyang Lake, Jiangxi Province, China, were collected in October 2016. The optical absorption characteristics and spatial distribution of CDOM, the correlation between the absorption coefficient of CDOM at a wavelength of 355 nm (a_g(355)), and the concentration of dissolved organic carbon (DOC) were analyzed. Spectral characteristic parameters—namely, E₂/E₃ (the ratio of the CDOM absorption coefficient at a wavelength of 250 nm to the CDOM absorption coefficient at a wavelength of 365 nm), SUVA₂₅₄ (the ratio of the CDOM absorption coefficient at a wavelength of 254 nm to the DOC concentration), and spectral slopes—were used to infer the composition and sources of CDOM in the Poyang Lake wetlands. The results showed the following: (1) the CDOM absorption spectrum of water of the Poyang Lake wetlands presented significant spatial variation, showing a trend of south > west > north > east; (2) there was a strong linear correlation between the CDOM absorption coefficient and the DOC concentration in the water of the Poyang Lake wetlands (a_g(355) = 1.075DOC–0.659 (r² = 0.723, p < 0.001, n = 30)); (3) the analysis of the spectral characteristic parameters E₂/E₃, SUVA₂₅₄, and spectral slopes showed that the CDOM in the Poyang Lake wetlands has relatively high aromaticity and molecular weight, which were shown to be mainly affected by terrestrial inputs. The results showed that the molecular weight and aromaticity of CDOM were higher in the south of the lake than in other parts. Full article

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, a_CDOM (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. Full article

Dissolved organic matter (DOM) is a critical part of the global carbon cycle. Currently, it is understood that at least a portion of the chromophoric DOM (CDOM) character can be described through an electronic interaction of charge transfer (CT) complexes. While much work has been done to understand the influence of CT on soil and aquatic reference standard DOM, little is known about the influence of CT in fresh terrestrially derived DOM. In this study, leaf litter leachates from three tree species were treated (reduced) with sodium borohydride to determine the contribution of CT on a source of fresh terrestrial DOM. Leaf litter was sampled four times through decomposition under natural (field) conditions to determine the influence of degradation on response to borohydride treatment. Leaf litter CDOM displayed a unique loss of UVB absorption following borohydride treatment, as well as a homogenizing effect on fluorescence emission character. Humification index (HIX) differentiated Elliot Soil Humic Acid and Suwannee River Fulvic Acid from leaf litter leachates. However, biological index (BIX), and spectral slope metrics were not able to differentiate leaf leachates from these reference standards. Apparent quantum yields were similar in magnitude between leaf leachates and reference standards, although leaf leachate spectra displayed features not evident in reference standards. These results help understand the origins of DOM optical properties and associated quantitative indices in freshly sourced terrestrial material. Overall, these results suggest that even at the initial stages of decomposition, terrestrial CDOM exhibits optical characteristics and responses to removal of electron accepting ketones and aldehydes, through borohydride treatment, similar to more processed CDOM. Full article

We present absorption properties of colored dissolved organic matter (CDOM) sampled in six different water bodies along extreme altitudinal, latitudinal, and trophic state gradients. Three sites are in Norway: the mesotrophic Lysefjord (LF), Samnangerfjord (SF), and Røst Coastal Water (RCW); two sites are in China: the oligotrophic Lake Namtso (LN) and the eutrophic Bohai Sea (BS); and one site is in Uganda: the eutrophic Lake Victoria (LV). The site locations ranged from equatorial to subarctic regions, and they included water types from oligotrophic to eutrophic and altitudes from 0 m to 4700 m. The mean CDOM absorption coefficients at 440 nm [ $a_{\mathrm{CDOM}}\left(440\right)$ ] and 320 nm [ $a_{\mathrm{CDOM}}\left(320\right)$ ] varied in the ranges 0.063–0.35 m ${}^{-1}$ and 0.34–2.28 m ${}^{-1}$ , respectively, with highest values in LV, Uganda and the lowest in the high-altitude LN, Tibet. The mean spectral slopes $S_{280-500}$ and $S_{350-500}$ were found to vary in the ranges of 0.017–0.032 nm ${}^{-1}$ and 0.013–0.015 nm ${}^{-1}$ , respectively. The highest mean value for $S_{280-500}$ as well as the lowest mean value for $S_{350-500}$ were found in LN. Scatter plots of $S_{280-500}$ versus $a_{\mathrm{CDOM}}\left(440\right)$ and $a_{\mathrm{CDOM}}\left(320\right)$ values ranges revealed a close connection between RCW, LF, and SF on one side, and BS and LV on the other side. CDOM seems to originate from terrestrial sources in LF, SF, BS, and LV, while RCW is characterized by autochthonous-oceanic CDOM, and LN by autochthonous CDOM. Photobleaching of CDOM is prominent in LN, demonstrated by absorption towards lower wavelengths in the UV spectrum. We conclude that high altitudes, implying high levels of UV radiation and oligotrophic water conditions are most important for making a significant change in CDOM absorption properties. Full article

In this study, absorption variation of chromophoric dissolved organic matter (CDOM) was investigated based on spectroscopic measurements of the water surface and bottom during a cruise survey on 2–12 May 2014 in the Pearl River Estuary (PRE). Multiple spectral signatures were utilized, including the absorption ratios E2/E3 (a(250)/a(365)) and E2/E4 (a(254)/a(436))) as well as the spectral slopes over multiple wavelength ranges. The horizontal variations of a(300), E2/E3, spectral slope (S) of Ultraviolet C (S_UVC, 250–280 nm), Ultraviolet B (S_UVB, 280–315 nm), and S_275–295 (275–295 nm) were highly correlated, revealing that CDOM of terrigenous origin in the upper estuary contained chromophores of larger molecular size and weight, while the marine CDOM in the lower estuary comprised organic compounds of smaller molecular size and weight; the molecular size of surface CDOM was generally larger than that at the bottom. Results of Gaussian decomposition methods showed that CDOM in the middle estuary of terrigenous origin produced more Gaussian components per spectrum than those of marine origin in the lower estuary and the adjacent Hong Kong waters. The surface CDOM composition was more diverse than at the bottom, inferred by the finding that the average number of Gaussian components yielded per surface sample (5.44) was more than that of the bottom sample (4.8). A majority of components was centered below 350 nm, indicating that organic compounds with relatively simple structures are ubiquitous in the estuary. Components centered above 350 nm only showed high peaks at the head of the estuary, suggesting that terrigenous CDOM with chromophores in complex structures rapidly lose visible light absorptivity during its transport in the PRE. The relatively low and homogenous peak heights of the components in Hong Kong waters imply higher light stability and composition consistency of the marine CDOM compared with the terrigenous CDOM. Full article

A Global Ocean Carbon Algorithm Database (GOCAD) has been developed from over 500 oceanographic field campaigns conducted worldwide over the past 30 years including in situ reflectances and coincident satellite imagery, multi- and hyperspectral Chromophoric Dissolved Organic Matter (CDOM) absorption coefficients from 245–715 nm, CDOM spectral slopes in eight visible and ultraviolet wavebands, dissolved and particulate organic carbon (DOC and POC, respectively), and inherent optical, physical, and biogeochemical properties. From field optical and radiometric data and satellite measurements, several semi-analytical, empirical, and machine learning algorithms for retrieving global DOC, CDOM, and CDOM slope were developed, optimized for global retrieval, and validated. Global climatologies of satellite-retrieved CDOM absorption coefficient and spectral slope based on the most robust of these algorithms lag seasonal patterns of phytoplankton biomass belying Case 1 assumptions, and track terrestrial runoff on ocean basin scales. Variability in satellite retrievals of CDOM absorption and spectral slope anomalies are tightly coupled to changes in atmospheric and oceanographic conditions associated with El Niño Southern Oscillation (ENSO), strongly covary with the multivariate ENSO index in a large region of the tropical Pacific, and provide insights into the potential evolution and feedbacks related to sea surface dissolved carbon in a warming climate. Further validation of the DOC algorithm developed here is warranted to better characterize its limitations, particularly in mid-ocean gyres and the southern oceans. Full article

Two semi-analytical algorithms, Generalized Inherent Optical Property (GIOP) and Garver-Siegel-Maritorena (GSM), were evaluated in terms of how well they reproduced the absorption coefficient of phytoplankton (a_ph(λ)) and dissolved and detrital organic matter (a_dg(λ)) at three wavelengths (λ of 412, 443, and 488 nm) in a zone with optically complex waters, the Upper Gulf of California (UGC) and the Northern Gulf of California (NGC). In the UGC, detritus determines most of the total light absorption, whereas, in the NGC, chromophoric dissolved organic material (CDOM) and phytoplankton dominate. Upon comparing the results of each model with a database assembled from four cruises done from spring to summer (March through September) between 2011 and 2013, it was found that GIOP is a better estimator for a_ph(λ) than GSM, independently of the region. However, both algorithms underestimate in situ values in the NGC, whereas they overestimate them in the UGC. Errors are associated with the following: (a) the constant a*_ph(λ) value used by GSM and GIOP (0.055 m² mgChla⁻¹) is higher than the most frequent value observed in this study’s data (0.03 m² mgChla⁻¹), and (b) satellite-derived chlorophyll a concentration (Chla) is biased high compared with in situ Chla. GIOP gave also better results for the a_dg(λ) estimation than GSM, especially in the NGC. The spectral slope S_dg was identified as an important parameter for estimating a_dg(λ), and this study’s results indicated that the use of a fixed input value in models was not adequate. The evaluation confirms the lack of generality of algorithms like GIOP and GSM, whose reflectance model is too simplified to capture expected variability. Finally, a greater monitoring effort is suggested in the study area regarding the collection of in situ reflectance data, which would allow explaining the effects that detritus and CDOM may have on the semi-analytical reflectance inversions, as well as isolating the possible influence of the atmosphere on the satellite-derived water reflectance and Chla. Full article

The Eastern Lagoon of New Caledonia (ELNC) is a semi-enclosed system surrounded by an extensive coral reef barrier. The system has been suffering impacts from climate variability and anthropogenic activities, including mining exploitation. Satellite monitoring is thus an essential tool to detect such changes. The present study aimed to assess the bio-optical variability of the ELNC and examine the applicability of ocean colour algorithms, using in situ bio-optical and radiometric data, collected during the March 2014 CALIOPE 2 cruise. The chlorophyll a concentration (Chla) varied from 0.13–0.72 mg·m⁻³, and the coastal stations were spectrally dominated by non-algal particles (NAP) and coloured dissolved organic matter (CDOM) (>80% of the total non-water absorption at 443 nm), due to the contribution of allochthonous sources. The phytoplankton specific absorption was generally lower (mean, 0.049 m²·mg Chla⁻¹) than typical values observed for the corresponding Chla range, as well as the spectral slopes of the absorption of CDOM plus NAP (a_dg) (mean, 0.016 nm⁻¹) and of the particle backscattering coefficient (b_bp) (mean, 0.07 nm⁻¹). The remote sensing reflectance obtained using two in-water approaches and modelled from Inherent Optical Properties (IOPs) showed less than 20% relative percent differences (RPD). Chla estimates were highly biased for the empirical (OC4 and OC3) and semi-analytical (GSM, QAA, GIOP, LMI) algorithms, especially at the coastal stations. Excluding these stations, the GSM01 yielded the best retrievals with 35–40% RPD. a_dg(443) was well retrieved by all algorithms with ~18% RPD, and b_bp(443) with ~40% RPD. Turbidity algorithms also performed reasonably well (30% RPD), showing the capacity and usefulness of the derived products to monitor the water quality of the ELNC, provided accurate atmospheric correction of the satellite data. Regionally tuned algorithms may potentially improve the Chla retrievals, but better parameterization schemes that consider the spatiotemporal variability of the specific IOPs are still needed. Full article

Considering the influence of river discharge and strong winds, the diurnal variability of ocean optical absorption properties in the Yellow River Estuary (YRE) is quantified, using in-situ measurements. The study finds that terrestrial sources due to the Yellow River discharge can cause high diurnal variation of water absorption because of the movement of river plume in the YRE, but such an influence diminishes far away from the Yellow River plume. The diurnal variability of water absorption, affected by strong winds, is found to be strengthened with a rapid increase of particles and colored dissolved organic matter (CDOM) arising from re-suspended sediment induced by wave forcing. The diurnal variability of particle absorption is controlled by non-algal particle absorption in the YRE, and the ratio of non-algal particle absorption (a_NAP) and total particle absorption for most wavelengths is more than 0.56. The diurnal variation of spectral slope of non-algal particle absorption (S_NAP) is found to vary within a narrow range, although large variability in the a_NAP spectrum is observed. The CDOM is correlated negatively with salinity, and such negative correlation becomes weaker with the decreasing influence of riverine input. The spectral slope of CDOM absorption (S_g) may reflect the formation and constituents of CDOM with weak relationship to its concentration, and its relationship with the absorption of CDOM at 440 nm may be associated with the source of CDOM. The value of S_g, which is affected by re-suspended bottom sediment, is much lower than that derived from CDOM affected by Yellow River runoff. Disregarding the absorption of pure water, the diurnal variability of total water absorption stems principally from changes in non-algal particle matter rather than CDOM and Chl-a. By the observations of hourly GOCI (Geostationary Ocean Color Imager) data, the major diurnal variations of remote sensing reflectance at 680 nm are observed in near-coastal waters and the estuary of the Yellow River, which are mainly influenced by the flow discharge of Yellow River and strong winds. Finally, the seasonal differences of diurnal variations of water absorption caused by strong winds and river discharge are determined. Full article

In order to retrieve geophysical satellite products in coastal waters with high coloured dissolved organic matter (CDOM), models and processors require parameterization with regional specific inherent optical properties (sIOPs). The sIOPs of the Baltic Sea were evaluated and compared to a global NOMAD/COLORS Reference Data Set (RDS), covering a wide range of optical provinces. Ternary plots of relative absorption at 442 nm showed CDOM dominance over phytoplankton and non-algal particle absorption (NAP). At 670 nm, the distribution of Baltic measurements was not different from case 1 waters and the retrieval of Chl a was shown to be improved by red-ratio algorithms. For correct retrieval of CDOM from Medium Resolution Imaging Spectrometer (MERIS) data, a different CDOM slope over the Baltic region is required. The CDOM absorption slope, S_CDOM, was significantly higher in the northwestern Baltic Sea: 0.018 (±0.002) compared to 0.016 (±0.005) for the RDS. Chl a-specific absorption and a_{d [SPM]}*(442) and its spectral slope did not differ significantly. The comparison to the MERIS Reference Model Document (RMD) showed that the S_NAP slope was generally much higher (0.011 ± 0.003) than in the RMD (0.0072 ± 0.00108), and that the SPM scattering slope was also higher (0.547 ± 0.188) vs. 0.4. The SPM-specific scattering was much higher (1.016 ± 0.326 m² g⁻¹) vs. 0.578 m² g⁻¹ in RMD. SPM retrieval could be improved by applying the local specific scattering. A novel method was implemented to derive the phase function (PF) from AC9 and VSF-3 data. $\tilde{b}$ was calculated fitting a Fournier–Forand PF to the normalized VSF data. $\tilde{b}$ was similar to Petzold, but the PF differed in the backwards direction. Some of the sIOPs showed a bimodal distribution, indicating different water types—e.g., coastal vs. open sea. This seems to be partially caused by the distribution of inorganic particles that fall out relatively close to the coast. In order to improve remote sensing retrieval from Baltic Sea data, one should apply different parameterization to these distinct water types, i.e., inner coastal waters that are more influenced by scattering of inorganic particles vs. open sea waters that are optically dominated by CDOM absorption. Full article