CDOM Absorption Coefficient Research Articles

Spatio-temporal analysis of colored dissolved organic matter over Ebinur Lake in Xinjiang, China

A lack of water quality monitoring stations in arid regions makes it difficult to estimate the exact level of water pollution. Satellite technology is crucial for monitoring the water quality of salt lakes, but due to the diversity of terrestrial water bodies, it is challenging to establish relationships between lake water color parameters and remote sensing reflectance. This study aims to evaluate the reliability of the colored Quasi-Analytical Algorithm Dissolved Organic Matter (QAA-CDOM) model over Ebinur Lake. The absorption coefficient measured on May 19, 2021 was used to analyze the optical properties of Ebinur Lake. The findings show that the QAA-CDOM model performed well in estimating the absorption coefficient of CDOM at 443 nm, with the r2 and RMSE values of 0.69 and 0.12 m−1, respectively. The Landsat 8 OLI data and QAA-CDOM model were then used to derive the spatial-temporal characteristics of the CDOM concentration over Ebinur Lake from 2014 to 2021. The concentration of CDOM changes significantly across time and space, ranging from 0 to 2.5 m−1, with the lowest concentration in May and the highest in August. The central region of the lake has a lower concentration of CDOM than the other regions due to the reflectance of the shallow parts of the lake. This study clarified the inherent optical characteristics of saline lakes and proved the applicability of using the QAA-CDOM model to monitor surface water quality in arid regions of China. This research may be useful for future analyses of the carbon cycle calculations in salt lakes in arid regions.

Regional Algorithm for Estimating High Coccolithophore Concentration in the Northeastern Part of the Black Sea

A modified regional algorithm to quantify the coccolithophore concentration in the northeastern part of the Black Sea under conditions of intense bloom is presented. To modify the algorithm, the data of in situ measurements of coccolithophore Emiliania huxleyi abundance performed in June 2017 and 2022 (when the maximum values were 9 × 106 and 13 × 106 Cells L−1, respectively), as well as the data from hydro-optical and satellite measurements, were used. In addition, the ratio between the number of detached coccoliths and coccolithophore cells was taken into account. Based on the expanded array of in situ data, the optimal values of the regional algorithm parameters were obtained. The modified algorithm makes it possible to obtain more accurate results in areas of high coccolithophore concentrations and takes into account the contribution of coccoliths. To test the sensitivity of the algorithm to variations in bio-optical characteristics, model calculations were performed using Hydrolight software. The updated algorithm is significantly less sensitive to variations in chlorophyll concentration and CDOM absorption coefficient than its previous version.

Distinct Peaks of UV-Absorbing Compounds in CDOM and Particulate Absorption Spectra of Near-Surface Great Barrier Reef Coastal Waters, Associated with the Presence of Trichodesmium spp. (NE Australia)

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 aCDOM(328) and 13–36 times greater for ap(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.

Remote estimates of CDOM using Sentinel-2 remote sensing data in reservoirs with different trophic states across China

Chromophoric dissolved organic matter (DOM) is called as CDOM which could affect the optical properties of surface waters, and is a useful parameter for monitoring complex inland aquatic systems. Large-scale monitoring of CDOM using remote-sensing has been a challenge due to the poor transferability of CDOM retrieval models across regions. To overcome these difficulties, a study is conducted using Sentinel-2 images, in situ reflectance spectral data, and water chemical parameters at 93 water reservoirs across China classified by trophic state. Empirical algorithms are established between CDOM absorption coefficient aCDOM(355) and reflectance band ratio (B5/B2,vegetation Red Edge/Blue) acquired in situ and via Sentinel-2 MSI sensors. Relationships are stronger (r2 > 0.7, p < 0.05) when analysis is conducted separately by trophic states. Validation models show that, by accounting for trophic state of reservoirs and using B5/B2 band ratios, it is possible to expand the geographical range of remote sensing-based models to determine CDOM. However, the accuracy of model validation decreased from oligotrophic (r2: 0.86) to eutrophic reservoirs (r2: 0.82), likely due to increased complexity of CDOM sources in nutrient-rich systems. This study provides a strategy for using local and remote-sensing data to monitor the spatial variations of CDOM in reservoirs based on different trophic states, and will contribute to water resources management.

Distribution, seasonality, and fluxes of dissolved organic matter in the Pearl River (Zhujiang) estuary, China

Abstract. Dissolved organic carbon (DOC) concentration in the Pearl River estuary (PRE) of China was measured in May, August, and October 2015 and January 2016. Chromophoric and fluorescent dissolved organic matter (CDOM and FDOM) in the latter three seasons were characterized by absorption and fluorescence spectroscopy. CDOM and FDOM exhibited negligible seasonal variations, while DOC displayed a significant seasonality, with the average concentration being highest in May (156 µmol L−1), lowest in November (87 µmol L−1), and comparable between January (118 µmol L−1) and August (112 µmol L−1). Although DOC, CDOM, and FDOM in surface water were generally higher than in bottom water, the difference between the two layers was statistically insignificant. DOC showed little cross-estuary variations in all seasons, while CDOM and FDOM in January were higher on the west side of the estuary than on the east side. All three variables showed rapid drawdowns in the head region of the estuary (salinity &lt;5); their dynamics in the main estuary were primarily controlled by conservative mixing, leading to linearly declining or relatively constant (for DOC in May and November only) contents with increasing salinity. The decrease in FDOM with salinity was 5 %–35 % faster than that of CDOM, which in turn was 2–3 times quicker than that of DOC. Salinity and CDOM absorption coefficients could serve as indicators of DOC in August and January. Freshwater endmembers in all seasons mainly contained fresh, protein-rich DOM of microbial origin, a large part of it likely being pollution-derived. Protein-like materials were preferentially consumed in the head region but the dominance of the protein signature was maintained throughout the estuary. Exports of DOC and CDOM (in terms of the absorption coefficient at 330 nm) into the South China Sea were estimated as 195×109 g and 266×109 m2 for the PRE and 362×109 g and 493×109 m2 for the entire Pearl River Delta. The PRE presents the lowest concentrations and export fluxes of DOC and CDOM among the world's major estuaries. DOM delivered from the PRE is, however, protein-rich and thus may enhance heterotrophs in the adjacent coastal waters. Overall, the PRE manifests lower abundance and smaller spatiotemporal variability of DOM than expected for a sizable estuary with a marked seasonality of river runoff due supposedly to the poorly forested watershed of the Pearl River, the rapid degradation of the pollution-derived DOM in the upper reach, and the short residence time of freshwater.

Multi-decadal trends and influences on dissolved organic carbon distribution in the Barataria Basin, Louisiana from in-situ and Landsat/MODIS observations

Barataria Basin, a deltaic marsh-estuary system in the northern Gulf of Mexico (nGOM), is an important source of dissolved organic matter (DOM) to the coastal ecosystem. To assess DOM dynamics, monthly measurements of dissolved organic carbon (DOC) concentrations and colored DOM (CDOM; an optical proxy for DOC) absorption were obtained over a three-year period (2008–2011) along a transect encompassing the upper, middle and lower regions of the Barataria Basin estuary. While CDOM absorption coefficient at 355 nm (ag355) generally decreased and spectral slope at 275–295 nm (S275–295) increased along the transect from the upper to lower basin, deviations were observed associated with hydrologic and meteorological influences. An application of wavelet analysis to the time-series of ag355 and various freshwater sources revealed a spatiotemporal resonance of ag355 with freshwater input from the Mississippi River (MR), the controlled freshwater discharge from the Davis Pond freshwater diversion, and rainfall in the lower, middle and upper basins, respectively. Field measurements and Landsat/MODIS data spanning across different seasons between 2008 and 2011 were used to develop optimized ag355 empirical algorithms for the high river flow, cold front, and normal conditions. Power law models relating a band ratio (green/red) of a satellite sensor to ag355 showed good performance for the MODIS-Aqua (R2~0.81–0.83, N = 581) and Landsat-5 TM (R2~0.72–0.79, N = 304) imagery. ag355-DOC relationships (R2~0.77–0.84, N = 306) applied to historical Landsat-5 TM (1985–2011) and MODIS-Aqua imagery (2002−2012) showed strong environmental influences on multi-decadal CDOM and DOC spatiotemporal trends. Satellite-derived DOC agreed reasonably well with in-situ measured DOC (averaged R2 = 0.71; RMSE = 2.13). While the time-series DOC showed seasonality and decreasing gradients from the upper to lower basin, an overall increase in DOC was observed in the upper and middle basins from 1985 to 2012. However, in the lower basin, DOC increased from 1985 to 2006 but decreased from 2007 to 2012, likely due to the impact of two major hurricanes in 2005. Furthermore, relationships between satellite-derived DOC and land cover variations (1985–2011) along the wetland-estuary interfaces derived from Landsat-5 TM supervised classification showed an increase in DOC with corresponding increase in developed area in the upper basin, while in the lower basin, DOC increased by 41% (R2 = 0.54) between 1985 and 2006 corresponding to a 17% decrease in salt marsh area, suggesting strong land use/land loss impact on the long-term DOC trends in Barataria Basin.

Study on Different Fractions of Organic Molecules in the Baltic Sea Surface Microlayer by Spectrophoto- and Spectrofluorimetric Methods

The sea surface microlayer (SML), created by surface active organic molecules (called: surfactants), is a highly active interface between the sea and the atmosphere. In this study we used the dissolved organic matter absorption and fluorescence properties for description of changes in molecular size and weight distribution of organic molecules as well as in their composition in the SML compared to subsurface water layer (SS), of 1 m depth. Data were collected during three research cruises in the Baltic Sea. Spectral values of the CDOM absorption coefficient were higher in the SML about 29 % (in the UV light) to 17 % (in a blue spectral range), while spectral slope coefficients at different spectral ranges, S_Δλ, were lower in SML by 2.3 and 10.5% compared to SS in 275-295 nm and 350-400 nm, respectively. The fluorescence intensities of main Excitation Emission Matrix spectra peaks belonging to the main fluorescing components of marine organic matter, called: A, C, M, T, were higher in SML by 41, 43, 41 and 14% compared to SS. The ratio of fluorescence intensities, ((M+T))⁄((A+C) ) and humification index, HIX, values in SML were, respectively higher by 17.9% and lower by 10.7 % compared to SS. These two parameters where coupled with inverse non-linear relationship. This relationship indicated faster removal of humic components of high MW in the SML compared to SS. We have observed decreasing trend of spectral slope ratio, S_R, with increasing salinity. The S_R decreased 33.5 and 23.6 % in the SML and SS, respectively in the salinity gradient from 4.5 to 7.94 (of practical salinity ). The fluorescence intensity of all FDOM peaks decreased by in the same salinity gradient. The decrease rate was higher in SML for all fluorescing peaks by 34, 36 and 26 % for A, C and M, respectively than in the SS. Decrease rate indicated the susceptibility to photochemical degradation of respective peaks. This effect was strongest for C while T peak was almost unbleached. The fluorescence intensity decrease rate was smaller in SS what indicated shielding effect of the SML.

Seasonal and spatial variability in the optical characteristics of DOM in a temperate shelf sea

The Celtic Sea is a productive temperate sea located on the Northwest European Shelf. It is an important pathway for the delivery of land-derived material to the North Atlantic Ocean, including dissolved organic matter (DOM). The aim of this study was to determine the seasonal and spatial variability in the magnitude, source and composition of DOM at three sites representing on shelf, central shelf and shelf edge regions in the Celtic Sea, using observations collected during the UK Shelf Sea Biogeochemistry (SSB) research programme (November 2014 – August 2015). The concentration of dissolved organic carbon (DOC) alongside DOM absorbance and fluorescence indices were measured and fluorescence Excitation and Emission Matrices (EEMs) combined with Parallel Factor Analysis (PARAFAC) were used to assess DOM composition and lability. The PARAFAC model identified four unique fluorescent components for autumn (November 2014), winter (March 2015), spring (April 2015) and summer (July 2015) consisting of two humic-like components attributed to terrestrial (C1) and marine sources (C2), and two protein components identified as tyrosine-like (C3) and tryptophan-like (C4) attributed to in situ production. DOC varied seasonally and there were strong cross shelf trends. The protein components (C3 and C4) exhibited large seasonal and within season variability particularly during productive periods. In contrast, there were persistent cross shelf gradients in the CDOM absorption coefficient at 305 nm (a305), the UV specific absorbance at 280 nm (SUVA280), the humification index (HIX), and the humic-like fluorescent components (C1 and C2), which were higher in the on shelf region and decreased towards the shelf edge. The humic-like components and the slope ratio (SR) were significantly correlated with salinity throughout all seasons, indicating a strong influence of terrestrially-derived organic matter in the Celtic Sea, with potentially up to 35% of DOC in the central shelf during winter originating from terrestrial inputs. Results from this study illustrate the importance of monitoring DOM quantitatively and qualitatively for a better understanding of the supply, production, cycling and export of this dynamic organic carbon pool in shelf seas.

Absorption and Reflection Characteristics of Black Water Blooms in the Eutrophic Water

Studying on optical properties of black water blooms, is the precondition for using remote sensing technology to monitor and evaluate the black water blooms event. Black water blooms occurred in Taihu Lake in July 2015. A total of 36 water samples were observed in the three water regions of Taihu Lake, the region 1 with black water blooms characteristics, region 2 with cyanobacterial bloom characteristics, and regional 3 with characteristics of clean lake water. The reflectance spectra and absorption coefficient of these three regions were analyzed, and the results show that:1 The absorption coefficients of the total particles, the pigment particles and the non-pigment particlesin black water blooms are 1 to 2 times higher than the other two areas. The absorption coefficient of CDOM between 400-500 nm in region 1 is about 2 times higher than the other two areas, which lead the black water area with a very lower reflectance, and presents as black color. 2 The range of M value in black water blooms is lower than Dianchi Lake and Chaohu Lake, which means the humic acid content of CDOM with black water characteristics is higher. A significant positive correlation is found between chlorophyll a (Chl-a) and the CDOM absorption coefficient at 350 nm, indicating that algae degradation is likely to be the primary source of CDOM in black waters. 3 The contribution of each optically active component indicates that the water absorption of region 1 strongly controlled by CDOM below 380 nm, but by Chla absorption between 380 nm and 700 nm.

Light absorption properties of CDOM in the Changjiang (Yangtze) estuarine and coastal waters: An alternative approach for DOC estimation

Field measurements of CDOM absorption properties and DOC concentrations were collected in the Changjiang estuarine and coastal waters from 2011 to 2013. CDOM absorption coefficient at 355 nm (ag (355)) was found to be inversely correlated with salinity, with Pearson's coefficients r of −0.901 and −0.826 for summer and winter observations, respectively. Analysis results of the relationships between salinity and CDOM optical properties (i.e., absorption coefficient and spectral slope) suggested that terrigenous inputs dominated CDOM sources in the Changjiang estuary, but the proportion of terrigenous CDOM declined with increasing salinity. The level of CDOM in the Changjiang estuary was lower compared to some of the major estuaries in the world, which could be attributed to several controlling factors such as vegetation cover in the drainage basin, the origin of recharged streams and high sediment load in the Changjiang estuary. We further evaluated the relationships between CDOM and DOC and their mixing behavior among world's major estuaries. An empirical model was finally developed to estimate DOC concentration from ag (355) and spectral slope S275-295 using a non-linear regression. This empirical relationship was calibrated using the Cal dataset, and was validated with the Val dataset, resulting in an acceptable error with the R2 of 0.746, the RMSE of 20.99 μmol/L and the rMAD of 14.46%.

Estimating the CDOM absorption coefficient in tropical inland waters using OLI/Landsat-8 images

ABSTRACTColoured dissolved organic matter (CDOM) is the most abundant dissolved organic matter (DOM) in many natural waters and can affect the water quality, such as the light penetration and the thermal properties of water system. So the objective of this letter was to estimate the CDOM absorption coefficient at 440 nm, aCDOM(440), in Barra Bonita Reservoir (São Paulo State, Brazil) using operational land imager (OLI)/Landsat-8 images. For this two field campaigns were conducted in May and October 2014. During the field campaigns remote sensing reflectance (Rrs) were measured using a TriOS hyperspectral radiometer. Water samples were collected and analysed to obtain the aCDOM(440). To predict the aCDOM(440) from Rrs at two key wavelengths (650 and 480 nm) were regressed against laboratory-derived aCDOM(440) values. The validation using in situ data of aCDOM(440) algorithm indicated a goodness of fit, R2 = 0.70, with a root mean square error (RMSE) of 10.65%. The developed algorithm was applied to the OLI/Lansat-8 images. Distribution maps were created with OLI/Landsat-8 images based on the adjusted algorithm.

Mixing anomaly in deoxygenated Baltic Sea deeps indicates benthic flux and microbial transformation of chromophoric and fluorescent dissolved organic matter

Optical measurements have indicated an anomalous distribution of Colored Dissolved Organic Matter absorption coefficient – aCDOM(λ), and dissolved organic matter fluorescence – FDOM intensity, in the salinity gradient in deep Baltic Sea waters below the permanent pycnocline. The two steps mixing model of the aCDOM(λ) with salinity have been derived based on 3521 CDOM absorption observations done in Baltic Sea, from 1993 until April 2014 and 1011 CDOM absorption observation done in the Danish Straits from August 2006 until February 2008. Disproportionate decrease and even increase of CDOM absorption coefficient at 350 nm, aCDOM(350), with increased salinity was observed in deep waters. There was a significant increase of the aCDOM(350) mixing model residual values in deep water compared to overlaying waters. The aCDOM(350) mixing model residual values tended to increase significantly with increasing salinity in deep waters. The CDOM and FDOM composition has been assessed by spectral slope coefficient, S300–600, the Humification Index (HIX), calculated from measured DOM fluorescence Excitation–Emission Matrix spectra and from ratio of the humic-like and protein-like DOM fluorescence measured in situ. The S300–600 values decreased significantly in deep waters compared with overlaying waters. There was a strong (R2 = 0.705) inverse linear relationship between S300–600 and aCDOM(350) mixing model residuals. The HIX, and the values of the ratio of the humic-like to protein-like DOM fluorescence increased significantly in the deep waters, what indicated increase of humic-like and decrease of protein-like DOM fraction in the deep waters samples compared to control surface waters samples. An exponential relationship between the humic-like to protein-like FDOM intensity ratio and the apparent oxygen utilization – AOU was observed in the Gulf of Gdansk. Presented results suggested that possible DOM emission from sediments into the adjacent near bottom water in anoxic conditions, could explain the aCDOM(λ) mixing anomaly. Microbial DOM reprocessing in anoxic condition, documented by direct in situ measurements, significantly changed the CDOM and FDOM composition.

A model for partitioning the light absorption coefficient of natural waters into phytoplankton, nonalgal particulate, and colored dissolved organic components: A case study for the Chesapeake Bay

AbstractWe present a model, referred to as Generalized Stacked‐Constraints Model (GSCM), for partitioning the total light absorption coefficient of natural water (with pure‐water contribution subtracted), anw(λ), into phytoplankton, aph(λ), nonalgal particulate, ad(λ), and CDOM, ag(λ), components. The formulation of the model is based on the so‐called stacked‐constraints approach, which utilizes a number of inequality constraints that must be satisfied simultaneously by the model outputs of component absorption coefficients. A major advancement is that GSCM provides a capability to separate the ad(λ) and ag(λ) coefficients from each other using only weakly restrictive assumptions about the component absorption coefficients. In contrast to the common assumption of exponential spectral shape of ad(λ) and ag(λ) in previous models, in our model these two coefficients are parameterized in terms of several distinct spectral shapes. These shapes are determined from field data collected in the Chesapeake Bay with an ultimate goal to adequately account for the actual variability in spectral shapes of ad(λ) and ag(λ) in the study area. Another advancement of this model lies in its capability to account for potentially nonnegligible magnitude of ad(λ) in the near‐infrared spectral region. Evaluation of model performance demonstrates good agreement with measurements in the Chesapeake Bay. For example, the median ratio of the model‐derived to measured ad(λ), ag(λ), and aph(λ) at 443 nm is 0.913, 1.064, and 1.056, respectively. Whereas our model in its present form can be a powerful tool for regional studies in the Chesapeake Bay, the overall approach is readily adaptable to other regions or bio‐optical water types.

Fluorescence measured using a field-portable laser fluorometer as a proxy for CDOM absorption

The colored fraction of dissolved organic matter, CDOM, directly influences water optical properties and spectral quality, playing an important role in aquatic ecosystems, optical remote sensing and carbon circulation in the sea. Measuring in situ CDOM absorption coefficient, however, is difficult because it requires prefiltration of water samples. A field-portable laser fluorometer for fast acquisition of in situ CDOM fluorescence data without sample filtering was developed. Using the instrument in the Yangtze River estuary, the East China Sea coast, the West Lake, and the Qiandao Lake, good estimates of CDOM absorption were obtained from in situ fluorescence measurements. High correlation (r = 0.94) was observed between CDOM absorption coefficient and fluorescence normalized to Raman scattering in diverse water types. Comparison of different methods for measuring CDOM fluorescence was carried out. Fluorescence measured by in situ laser fluorometer compared very well to the values obtained using more sophisticated laboratory spectrophotofluorometer; a significant relationship (r = 0.95) was identified between the two methods. Spatial distributions of CDOM in the Yangtze River estuary and its adjacent sea compiled from discrete samples analyses were presented. A linear relationship was identified between CDOM fluorescence and salinity in surface water, reflecting the potential of CDOM fluorescence to be used as a passive tracer of freshwater input. The in situ measurement results demonstrated the utility of the laser fluorometer for rapid acquisition of high-resolution profiles, and that the work resulting from sample filtration or storage can be avoided.

Composition of dissolved organic matter along an Atlantic Meridional Transect from fluorescence spectroscopy and Parallel Factor Analysis

Absorption spectra and induced fluorescence excitation emission matrices of colored dissolved organic matter were measured in water samples collected along the Atlantic Meridional Transect in different bio-geographic provinces of the Atlantic Ocean from October to November 2010. The highest values of CDOM absorption coefficient at 305nm (aCDOM(305)), were recorded at the continental margins of the English Channel and Patagonian Shelf. The lowest values of aCDOM(305) were observed in the mixed layer of both North and South Atlantic subtropical oligotrophic gyres. The DOM composition was assessed using fluorescence spectroscopy, Excitation Emission Matrix spectra (EEMs) and the Parallel Factor Analysis (PARAFAC) model in addition to spectral indices calculated from CDOM absorption spectrum and EEMs. Six different components were identified in the EEMs by PARAFAC: Two components were similar to the humic-like fraction of DOM, associated with basin scale microbial mineralization processes. These components represent allochthonous DOM in the biogeographic provinces studied. One component of marine humic-like material of autochthonous origin was associated with DOM production from marine phytoplankton. Three components were associated with protein-like DOM. Two protein-like components had the spectral characteristics of pure tryptophan and tyrosine. There was a significant difference in DOM composition both between bio-geographical provinces and above and below the mixed layer. In the mixed layer in all provinces, except the waters of the Western European Shelf, the DOM was dominated by protein-like components. At the Western European Shelf, it was dominated by humic-like components. Fluorescence intensities of humic-like components were high at the Patagonian Shelf, but were up to 40% lower compared to northern hemisphere shelf waters. Humic-like components made a significant contribution to the DOM composition of the upper mesopelagic layer in all provinces, with the highest values at the Equatorial Upwelling Zone. There was a significant inverse relationship between humic-like components and salinity and temperature and a positive relationship with Apparent Oxygen Utilization. The humification index (HIX) was linearly correlated with the intensity of the humic-like DOM components. These trends suggest that the humic-like components are in dynamic equilibrium between likely microbial production in the deep ocean and photochemical degradation in the mixed layer.

Wind and submerged aquatic vegetation influence bio‐optical properties in large shallow Lake Taihu, China

AbstractWe studied the bio‐optical properties and sediment dynamics of typical phytoplankton‐dominated (PD) and macrophyte‐dominated (MD) regions in shallow Lake Taihu in China, from long‐term site‐specific studies and short‐term high‐frequency observations. The long‐term studies showed that five parameters were significantly lower in the MD region than in the PD region: photosynthetically active radiation (PAR) diffuse attenuation coefficient (Kd(PAR)), concentrations of total suspended matter (CTSM), tripton (CTripton), and phytoplankton pigment (CChla+Pa), and CDOM absorption coefficient (aCDOM(350)). In winter in the MD region, with only scarce submerged aquatic vegetation (SAV) coverage, the Kd(PAR), CTSM, and CTripton were significantly higher than in the PD region with no SAV; in contrast, CChla+Pa and aCDOM(350) were significantly lower in the MD region than in the PD region. In the other three seasons with high SAV coverage, Kd(PAR), CTSM, CTripton, CChla+Pa, and aCDOM(350) were all significantly lower in the MD region than in the PD region. The appearance and growth of SAV decreased CTSM, CTripton, and Kd(PAR). The short‐term high‐frequency study showed that phytoplankton and tripton absorption coefficients were significantly lower in the MD region than in the PD region. In the PD region, there were highly significant exponential relationships between wind speed, wave height, and wave shear stress, and CTripton and Kd(PAR), showing that wind‐driven sediment resuspension could significantly affect both the tripton concentration and PAR attenuation. However, in the MD region, there were only weakly significant correlations, or no significant correlations, between wind speed, wave height, and wave shear stress, and CTripton, and Kd(PAR). The combination of the long‐term site‐specific and short‐term high‐frequency observations is an excellent tool for study of the bio‐optical properties in lake environments.

FY-3A/MERSI, ocean color algorithm, products and demonstrative applications

A medium resolution spectral imager (MERSI) on-board the first spacecraft of the second generation of China’s polar-orbit meteorological satellites FY-3A, is a MODIS-like sensor with 20 bands covering visible to thermal infrared spectral region. FY-3A/MERSI is capable of making continuous global observations, and ocean color application is one of its main targets. The objective is to provide information about the ocean color products of FY-3A/MERSI, including sensor calibration, ocean color algorithms, ocean color products validation and applications. Although there is a visible on-board calibration device, it cannot realize the on-board absolute radiometric calibration in the reflective solar bands. A multisite vicarious calibration method is developed, and used for monitoring the in-flight response change and providing post-launch calibration coefficients updating. FY-3A/MERSI ocean color products consist of the water-leaving reflectance retrieved from an atmospheric correction algorithm, a chlorophyll a concentration (CHL1) and a pigment concentration (PIG1) from global empirical models, the chlorophyll a concentration (CHL2), a total suspended mater concentration (TSM) and the absorption coefficient of CDOM and NAP (YS443) from China’s regional empirical models. The atmospheric correction algorithm based on lookup tables and ocean color components concentration estimation models are described. By comparison with in situ data, the FY-3A/MERSI ocean color products have been validated and preliminary results are presented. Some successful ocean color applications such as algae bloom monitoring and coastal suspended sediment variation have demonstrated the usefulness of FY-3A/MERSI ocean color products.

Monsoon variability of ultraviolet radiation (UVR) attenuation and bio-optical factors in the Asian tropical coral-reef waters

The East coast of Peninsular Malaysia is strongly influenced by the North-East (NE) monsoon, and may significantly influence the optical environment of coral-reef ecosystems. However, our knowledge of temporal variability, including episodic events, of environmental factors in Asian tropical regions is still limited. The objectives of this study were to (1) observe temporal variability in ultraviolet radiation (UVR) and photosynthetically active radiation (PAR) attenuation and (2) determine the bio-optical factors regulating the optical environment in shallow coral-reef waters. Downwelling UVR and PAR irradiance and in situ bio-optical factors were measured monthly near Bidong Island on the East coast of Peninsular Malaysia from June 2010 to June 2011. The NE monsoon was recognized between November 2010 and January 2011. The highest diffuse attenuation coefficient at 305 nm was 2.05 ± 0.03 m−1 in a coral-reef area on December 2010. The most significant bio-optical factor at 305, 380, 440 nm during the NE monsoon season was CDOM (89 ± 8% at 305 nm, 84 ± 9% at 380 nm and 49 ± 17% at 440 nm). All UVR attenuation coefficients showed significant correlations with the CDOM absorption coefficients (aCDOM). CDOM with relatively low S275-295 during the NE monsoon season (0.0177 ± 0.0020 nm−1) suggests terrestrial sources, which is also supported by the correlation between salinity and aCDOM(305). A significant correlation between S275-295 and the carbon specific absorbance coefficient (a*(305)) suggest the potential to measure DOC optically in these waters. The high CDOM during the NE monsoon season may have an important role to reduce harmful UVR exposure reaching benthic communities.

Combined processing and mutual interpretation of radiometry and fluorometry from autonomous profiling Bio‐Argo floats: 2. Colored dissolved organic matter absorption retrieval

Eight autonomous profiling “Bio‐Argo” floats were deployed offshore during about 2 years (2008–2010) in Pacific, Atlantic, and Mediterranean zones. They were equipped with miniaturized bio‐optical sensors, namely a radiometer measuring within the upper layer the downward irradiance at 412, 490, and 555 nm, and two fluorometers for detection of chlorophyll‐a (Chla) and colored dissolved organic matter (CDOM; profiles from 400 m to surface). A first study dealt with the interpretation of the Chla fluorescence signal in terms of concentration, using for this purpose the diffuse attenuation coefficient for irradiance at 490 nm, Kd(490), taken as a proxy for the Chla absorption. The present study examines the possibility of similarly using the Kd(412) values combined with retrieved Chla profiles to convert the CDOM fluorometric qualitative information into a CDOM absorption coefficient (ay). The rationale is to take advantage of the fact that Kd is more sensitive to CDOM presence at 412 nm than at 490 nm. A validation of this method is tested through its application to field data, collected from a ship over a wide range of trophic conditions (Biogeochemistry and Optics South Pacific Experiment (BIOSOPE) cruise); these data include both in situ fluorescence profiles and CDOM absorption as measured on discrete samples. In addition, near‐surface ay values retrieved from the floats agree with those derivable from ocean color imagery (Moderate Resolution Imaging Spectroradiometer (MODIS‐A)). The low sensitivity of commercially available CDOM fluorometers presently raises difficulties when applying this technique to open ocean waters. It was nevertheless possible to derive from the floats records meaningful time series of CDOM vertical distribution.

Remote sensing reflectance of Pomeranian lakes and the Baltic

The remote sensing reflectance Rrs, concentrations of chlorophyll a and other pigments Ci, suspended particulate matter concentrations CSPM and coloured dissolved organic matter absorption coefficient aCDOM(λ) were measured in the euphotic zones of 15 Pomeranian lakes in 2007–2010. On the basis of 235 sets of data points obtained from simultaneous estimates of these quantities, we classified the lake waters into three types. The first one, with the lowest aCDOM(440nm) (usually between 0.1 and 1.3m−1 and chlorophyll a concentrations 1.3 < Ca < 33mg m−3), displays a broad peak on the reflectance spectrum at 560–580nm and resembles the shape of the remote sensing reflectance spectra usually observed in the Baltic Proper. A set of Rrs spectra from the Baltic Proper is given for comparison. The second lake water type has a very high CDOM absorption coefficient (usually aCDOM(440nm) > 10m−1, up to 17.4m−1 in Lake Pyszne; it has a relatively low reflectance (Rrs < 0.001sr−1) over the entire spectral range, and two visible reflectance spectra peaks at ca 650 and 690–710nm. The third type of lake water represents waters with a lower CDOM absorption coefficient (usually aCDOM (440nm)< 5m−1) and a high chlorophyll a concentration (usually Ca > 4mg m−3, up to 336mg m−3 in Lake Gardno). The remote sensing reflectance spectra in these waters always exhibit three peaks (Rrs > 0.005sr−1): a broad one at 560–580nm, a smaller one at ca 650nm and a well-pronounced one at 690–720nm. These Rrs(λ) peaks correspond to the relatively low absorption of light by the various optically active components of the lake water and the considerable scattering (over the entire spectral range investigated) due to the high SPM concentrations there. The remote sensing maximum at λ ≈ 690–720nm is higher still as a result of the natural fluorescence of chlorophyll a. Empirical relationships between the spectral reflectance band ratios at selected wavelengths and the various optically active components for these lake waters are also established: for example, the chlorophyll a concentration in surface water layer Ca=6.432 e4.556X, where X=[max Rrs(695 ≤ λ ≤ 720) – Rrs(λ=670)]/max Rrs(695 ≤ λ ≤ 720), and the coefficient of determination R2=0.95.