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

Distribution, migration and transformation of chromophoric dissolved organic matter (CDOM) in coastal waters are closely related to marine biogeochemical cycle. Ocean color remote sensing retrieval of CDOM absorption coefficient (ag(λ)) can be used as an indicator to trace the distribution and variation characteristics of the Changjiang diluted water, and further to help understand estuarine and coastal biogeochemical processes in large spatial and temporal scales. The quasi-analytical algorithm (QAA) has been widely applied to remote sensing inversions of optical and biogeochemical parameters in water bodies such as oceanic and coastal waters, however, whether the algorithm can be applicable to highly turbid waters (i.e., Changjiang estuarine and coastal waters) is still unknown. In this study, large amounts of in situ data accumulated in the Changjiang estuarine and coastal waters from 9 cruise campaigns during 2011 and 2015 are used to verify and calibrate the QAA. Furthermore, the QAA is remodified for CDOM retrieval by employing a CDOM algorithm (QAA_CDOM). Consequently, based on the QAA and the QAA_CDOM, we developed a new version of algorithm, named QAA_cj, which is more suitable for highly turbid waters, e.g., Changjiang estuarine and coastal waters, to decompose ag from adg (CDOM and non-pigmented particles absorption coefficient). By comparison of matchups between Geostationary Ocean Color Imager (GOCI) retrievals and in situ data, it reveals that the accuracy of retrievals from calibrated QAA is significantly improved. The root mean square error (RMSE), mean absolute relative error (MARE) and bias of total absorption coefficients (a(λ)) are lower than 1.17, 0.52 and 0.66 m−1, and ag(λ) at 443 nm are lower than 0.07, 0.42 and 0.018 m−1. These results indicate that the calibrated algorithm has a better applicability and prospect for highly turbid coastal waters with extremely complicated optical properties. Thus, reliable CDOM products from the improved QAA_cj can advance our understanding of the land-ocean interaction process by earth observations in monitoring spatial-temporal distribution of the river plume into sea.

2004年3月对太湖梅梁湾有色可溶性有机物(CDOM))的吸收和荧光等光学行为进行研究,并由此探讨了CDOM的空间分布．结果表明,溶解性有机碳(DOC)的浓度在10．48-19．72 mg/L间变化,其均值为13．20±2．79 mg/L)CDOM在280 nm,355 nm和440 nm的吸收系数分别为18．73-31．91 m^-1(平均值23．19±4．36 m^-1)、4．63-7．14 m^-1(平均值5．76±0．91 m^-1)、1．45-2．99 m^-1(平均值1．92±0．40 m^-1))355 nm波长处CDOM的比吸收系数为0．34-0．57 L/(mg·m),平均值0．44±0．06 L/(mg·m))表征CDOM分子大小的比值a(250)/a(365)变化范围为5．05-7．55)355 nm的激发波长、450 nm的发射波长处的荧光值的变化范围0．79-3 04 nm^-1(平均值1．69±0．77 nm^-1)．CDOM吸收系数、DOC浓度、荧光强度的分析显示CDOM浓度呈现从河口往湾内、湾口递减的趋势．CDOM吸收与DOC浓度的相关性随波长的降低而增加,在短波部分存在明显的正相关．355 nm处的荧光值、DOC浓度与CDOM吸收系分别存在如下显著性正相关关系:F_n(355)=0．692(±0．135)a(355)-2．297(±0．786),a(355)=0．233(±0．061)DOC+2．690(±0．816)．280-500 nm、280-360 nm、360-440 nm指数函数斜率S值分别为13．86±0．91、18．54±1．11、12．93±0．92μm^-1,S值与比吸收系数之间存在显著的负线性相关关系,而与a(25)/a(365)值则存在显著的正线性关系．比吸收系数越大,a(250)/a(365)值和S值就越小,对应的CDOM分子量就越大,腐质酸的比例就越高．;Chromophoric dissolved organic matter (CDOM) is the light absorption fraction of dissolved organic carbon (DOC) that absorbs light in both the ultraviolet and visible ranges. The spatial and temporal distribution of CDOM in aquatic ecosystems can have important effects on ecosystem productivity, negatively impacting primary productivity due to CDOM absorbs light at both ultraviolet and visible wavelengths while positively impacting secondary productivity by fueling of microbial respiration by photo-degraded CDOM. In water color remote sensing, overlaps of pigment absorption spectra with CDOM absorption at blue wavelength generally complicate the use of chlorophyll a retrieval algorithms that are based on remotely sensed ocean color and lead to overestimated chlorophyll a concentration. In addition, high concentrations of CDOM can also act as a photoprotectant against UV damage for aquatic organisms. However, the protection against UV radiation provided by dissolved humic material for aquatic biota may be diminished if photodegradation of CDOM by UV radiation and acidification increase the UV transparency in lakes. CDOM absorption measurements and their relationship with DOC, and fluorescence are presented in Meiliang Bay of Lake Taihu based on a field investigation and lab analysis to show the spatial distribution of CDOM. Absorption spectral of CDOM was measured from 240 to 800 nm. Concentrations of DOC ranged from 10.48 to 19.72 mg/L with an average of 13.20 ±2.78 mg/L. CDOM absorption coefficients at 280 nm and 355 nm were in the range 18. 73-31. 91 m^-1 ( average 23.19±4.36mm^-1) and 4. 63-7. 14 mm^-1 (average 5. 76±0. 90 mm^-1)，respectively. The values of the DOC-specific absorption coefficient at 355 nm ranged from 0. 34 to 0. 57 L/( mg · m). Fluorescence emission at 450 nm, excited at 355 nm, had a mean value of 1. 69 ±0. 77 nmm^-1. A significant lake zone difference is found in DOC concentration, CDOM absorption coefficient and fluorescence. This regional distribution pattern was in agreement with the location of sources of yellow substance ： highest concentrations close to river mouth under the influence of river inflow, lower values in outlet of Meiliang bay. CDOM fluorescence and absorption coefficient were significantly and positively correlated. The results show a good correlation between CDOM absorption and DOC coefficients during 280-500 nm short wavelength intervals. The coefficient of variation between CDOM absorption and DOC concentration decreased with the increase of wavelength from 280 to 500 nm. The linear regression equations between fluorescence, DOC concentration and absorption coefficients at 355 nm are: F_n ( 355 ) = 0.692( ±0. 135)a(355)-2.291 (±0.786)，a(355) =0.233(±0.061)DOC +2.690(±0.816)，respectively. The exponential slope coefficient ranged from 11.0 to 14.9 um^-1 with a mean value 13. 86 ± 0. 91um^-1， 15. 8-20. 7 um^-1 with a mean value 18. 54 ± 1. 11 um^-1 and 9.9-13. 9 um^-1 with a mean value 12. 93 ± 0. 92um^-1 over the 280-500 nm, 280-360 nm and 360-440 nm intervals. A significant negative linear correlation was found between spectral slope coefficient and DOC specific absorption coefficient, but a significant positive linear correlation for spectral slope coefficient and the ratio of a( 250)/a( 365 ). Larger specific absorption coefficient corresponds to smaller a (250)/a (365) and spectral slope coefficient, which shows higher ratio humic in CDOM.

Effects of land use on the characteristics and composition of fluvial chromophoric dissolved organic matter (CDOM) in the Yiluo River watershed, China

The optical properties of chromophoric dissolved organic matter (CDOM) in shallow lakes in the middle and lower reaches of the Yangtze River were investigated. Concentrations of dissolved organic carbon (DOC) ranged from 2.7 to 10.1 mg/L with an average of 5.65±1.70 mg/L. All spectra of CDOM had a common shape with maximal absorption in shorter wavelength and minimal absorption in longer wavelength. CDOM absorption coefficients at 280 nm and 355 nm were in the range 3.52–24.22 m−1 (average 8.69±4.76 m−1) and 0.53–8.08 m−1 (average 2.03±1.65 m−1), respectively. The values of the DOC-specific absorption coefficient at 355 nm ranged from 0.09 to 0.80 L(mg org.C)−1m−1. The exponential slope coefficient ranged from 0.015 to 0.023 nm−1 with a mean value 0.020±0.002 nm−1 over the 280–400 nm interval. Fluorescence emission at 450 nm, excited at 355 nm, had a mean value of 0.86 nm−1. CDOM fluorescence and absorption were significantly and positively correlated. The coefficient of variation between CDOM absorption and DOC concentration decreased with the increase of wavelength from 280 to 800nm. The CDOM absorption measurements are of use to study regional distribution of CDOM, contribution of CDOM to light attenuation, and development of regional models for the optical properties of CDOM which allow higher precision in remote sensing applications of water quality monitoring.

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

Chromophoric Dissolved Organic Matter (CDOM) plays a critical role in the carbon and biogeochemical cycles within aquatic ecosystems. Satellite imagery can be employed to determine aquatic CDOM concentrations, highlighting the need for effective and precise algorithms for this task. In this study, a cruise survey dataset containing CDOM absorption coefficients and water-leaving radiances in the Pearl River estuary (PRE) was utilized to develop machine learning algorithms for CDOM retrieval from Landsat-8 Operational Land Imager (OLI) observations. Based on OLI wavelength bands, five bands and six band-ratios were chosen as input parameters for the machine learning models. Six machine learning models were trained to develop CDOM algorithms, including Support Vector Regression (SVR), Random Forest (RF), Extreme Gradient Boosting (XGBoost), Multi-Layer Perceptron (MLP), and Convolutional Neural Network (CNN). The results indicated that, among the six machine learning models, the XGBoost algorithm performed best, with the highest R2 value of 0.9 and the lowest CDOM root mean square error (RMSE) of 0.37 m−1, outperforming empirical algorithms. The XGBoost algorithm identified B4/B1 as the most critical input parameter, contributing 71%, followed by B3/B2 with a 16% contribution, where B1, B2, B3, and B4 are the wavelength bands of the OLI. These two band-ratios accounted for most of the contributions, suggesting their significant role in CDOM retrieval from Landsat OLI images. By employing the developed XGBoost algorithm, CDOM spatial patterns at six instances were derived from Landsat-8 OLI image reflectance, illustrating CDOM variations in the PRE influenced by various factors. Further analysis revealed that, in the PRE, tides and winds are the primary driving forces behind the spatial and temporal variability of CDOM. At present, the exploration of employing machine learning algorithms to infer CDOM concentrations in this region remains relatively limited; therefore, with a higher R2 value, the machine learning model we established unveils fresh and novel results.

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

Open-ocean carbon monoxide photoproduction

Estimation of terrestrial humic-like substances in inland lakes based on the optical and fluorescence characteristics of chromophoric dissolved organic matter (CDOM) using OLCI images

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.

Optical models for remote sensing of chromophoric dissolved organic matter (CDOM) absorption in Poyang Lake

Chromophoric dissolved organic matter and dissolved organic carbon in Chesapeake Bay

Changes in the concentration of chromophoric dissolved organic matter (CDOM) were determined over short (hours to days) and medium (weeks) time scales during summer in a shallow, polymictic lake in subarctic Quebec, Canada. CDOM fluorescence (F CDOM ) decreased linearly from mid-June until early August, consistent with the zero-order kinetics expected for photodegradation of organic matter in a regularly mixed lake in which most of the photochemically-active radiation is absorbed by CDOM. A major rainfall event then resulted in a sharp increase in F CDOM back to levels recorded at the start of the study. These changes showed that CDOM concentrations were controlled by the balance between catchment inputs and subsequent photodegradation processes within the lake. Shorter term changes in CDOM concentration were monitored by measuring the penetration of ultraviolet radiation (UVR) using a Satlantic TACCS system moored near the center of the lake and set to monitor down-welling 380 nm irradiance at 10 min intervals. The diffuse attenuation coefficient K d (380) decreased by 12 % during the deployment indicating increased penetration of UVR and the change paralleled the drop in F CDOM over the same period. Many of the daily records showed a significant decline in K d (380) in the near-surface waters over the course of the morning and early afternoon indicating diurnal photobleaching of CDOM, and an increase later in the day consistent with the breakdown of the diurnal thermocline and entrainment of deeper waters. The results illustrate the short and medium term dynamics of CDOM, the importance of diurnal stratification and mixing for photochemical processes, and the variability that aquatic organisms must experience in UV exposure in their natural environment.

The dynamics of absorption coefficients of CDOM and particles in the St. Lawrence estuarine system: Biogeochemical and physical implications

Bio-optical algorithm evaluation for MODIS for western Canada coastal waters: An exploratory approach using in situ reflectance