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The three-stream radiation transfer model is used to investigate the fluctuation in the underwater diffuse attenuation coefficient of downwelling irradiance in the polar ocean with a high solar zenith angle and different direct radiation proportions. First, the applicability of the three-stream radiation model in the polar region is validated by using 18 in situ observation data from September to October 2009 in the Beaufort Sea. Statistics show that in the absence of sea ice, the average relative errors between the simulation and observation values for 490 nm downwelling irradiance (Ed(490)) and its diffuse attenuation coefficient (Kd(490)) are 7.04% and 9.88%, respectively. At the stations surrounded by sea ice, the radiation is relatively small due to ice blocking, and the average relative errors simulated by the model reach 15.89% and 15.55%, respectively. Second, simulations with different chlorophyll concentrations and different proportions of direct radiation reveal that a high solar zenith angle has a greater impact on Kd(490) in the surface water. Kd(490) is less affected by the light field (affected by the solar zenith angle and the proportion of direct radiation) at depths greater than 30 m, and meets the linear relationship with the inherent optical parameters (the sum of the absorption coefficient and backscattering coefficient). The surface Kd(490) is still consistent with that at a depth of more than 50 meters under a high solar zenith angle, implying that the surface Kd(490) can also be considered as an inherent optical parameter at a high solar zenith angle (greater than 60 degrees). The relative error of obtaining surface Kd(490) by using the linear relationship at the 50 m layer is found to be less than 8% in the seawater with chlorophyll concentration greater than 0.05 mg m−3. The effect of the solar zenith angle and proportion of direct radiation can be ignored when measuring the diffuse attenuation coefficient in the polar region. Finally, the model can correct the ice induced fluctuation in downward irradiance, allowing for optical research of seawater beneath the ice in the polar ocean. [ABSTRACT FROM AUTHOR]

Diffuse attenuation coefficient of photosynthetically available radiation (PAR), KPAR, is a key product of ocean color remote sensing. Current ocean color algorithms generally detect only the average KPAR within one optical depth, K P A R R S . Due to the marked vertical variations of KPAR, knowledge of K P A R R S is insufficient to accurately evaluate the submarine light field. By using field in situ observations, a two-step approach, based on the development of an ocean color algorithm for K P A R R S and the relationships between K P A R R S and the average KPAR from the surface down to depth Z ( K ¯ P A R Z ), was developed to remotely estimate the vertical variations in K ¯ P A R Z in the North Pacific from the MODerate-resolution Imaging Spectrometer (MODIS) imagery. The root mean square difference of log( K ¯ P A R Z ) in depths within the euphotic zone was around ±0.059 (in unit of m−1 for K ¯ P A R Z ), which corresponded to a deviation of ±15% for the estimated K ¯ P A R Z and the penetration depths of PAR. Our study may provide a promising approach to detect the vertical variations of K ¯ P A R Z and underwater PAR distributions in the North Pacific Ocean. [ABSTRACT FROM AUTHOR]

The diffuse attenuation coefficient of photosynthetically active radiation is an important inherent optical property of the subaquatic light field. This parameter, as a measure of the transparency of the medium, is a good indicator of water quality. Degradation of the optical properties of water due to anthropogenic disturbances is a common phenomenon in freshwater ecosystems. In this study, we used four algorithm-based Landsat 8 OLI and Sentinel-2A/B MSI images to estimate the diffuse attenuation coefficient of photosynthetically active radiation in Lake Villarrica located in south-central Chile. The algorithms' estimated data from the ACOLITE module were validated with in situ measurements from six sampling stations. Seasonal and intralake variations of the light attenuation coefficient were studied. The relationship between the diffuse attenuation coefficient of photosynthetically active radiation, meteorological parameters, and an optical classification was also explored. The best results were obtained with QAA v6 KdPAR Nechad (R2 = 0.931, MBE = 0.023 m−1, RMSE = 0.088 m−1, and MAPE = 35.9%) for spring and QAA v5 Kd490 algorithms (R2 = 0.919, MBE = −0.064 m−1, RMSE = −0.09 m−1, and MAPE = 30.3%) for summer. High KdPAR values are associated with the strong wind and precipitation events suggest they are caused by sediment resuspension. Finally, an optical classification of freshwater ecosystems was proposed for this lake. The promising results of this study suggest that the combination of in situ data and observation satellites can be useful for assessing the bio-optical state of water and water quality dynamics in Chilean aquatic systems. [ABSTRACT FROM AUTHOR]

Estimates of the diffuse attenuation coefficient ( K d ) at two different wavelengths and band-integrated (PAR) were obtained using different published algorithms developed for open ocean waters spanning in type from explicit-empirical, semi-analytical and implicit-empirical and applied to data from spectral radiometers on board six different satellites (MODIS-Aqua, MODIS-Terra, VIIRS–SNPP, VIIRS-JPSS, OLCI-Sentinel 3A and OLCI-Sentinel 3B). The resultant K d s were compared to those inferred from measurements of radiometry from sensors on board autonomous profiling floats (BGC-Argo). Advantages of BGC-Argo measurements compared to ship-based ones include: 1. uniform sampling in time throughout the year, 2. large spatial coverage, and 3. lack of shading by platform. Over 5000 quality-controlled matchups between K d s derived from float and from satellite sensors were found with values ranging from 0.01 to 0.67 m − 1 . Our results show that although all three algorithm types provided similarly ranging values of K d to those of the floats, for most sensors, a given algorithm produced statistically different K d distributions from the two others. Algorithm results diverged the most for low K d (clearest waters). Algorithm biases were traced to the limitations of the datasets the algorithms were developed and trained with, as well as the neglect of sun angle in some algorithms. This study highlights: 1. the importance of using comprehensive field-based datasets (such as BGC-Argo) for algorithm development, 2. the limitation of using radiative-transfer model simulations only for algorithm development, and 3. the potential for improvement if sun angle is taken into account explicitly to improve empirical K d algorithms. Recent augmentation of profiling floats with hyper-spectral radiometers should be encouraged as they will provide additional constraints to develop algorithms for upcoming missions such as NASA's PACE and SBG and ESA's CHIME, all of which will include a hyper-spectral radiometer. [ABSTRACT FROM AUTHOR]

Recent studies highlight the application of deriving the attenuation coefficient from spaceborne photon-counting lidar ATLAS/ICESat-2 over open oceans on global scales. However, its performance in the more optically complex and variable environments of marginal seas, which are more susceptible to human activity, has not been validated yet. In this study, we present an in-depth analysis of the consistency between diffuse attenuation coefficient (K d) detection from MODIS and ICESat-2 in China's Marginal Seas. Findings demonstrate that ICESat-2 possesses strong capabilities for the retrieval of the attenuation coefficient across differing aquatic environments. However, discrepancies exist between the lidar system attenuation coefficient obtained from ICESat-2 and the diffuse attenuation coefficient determined by MODIS, influenced by factors such as multiple scattering. Implementation of a novel multiple scattering correction model demonstrates a notable ability in significantly reducing the inconsistency. Validation with in-situ Biogeochemical Argo float measurements reveals an enhancement in the accuracy of lidar-derived diffuse attenuation coefficients upon correction, with the mean absolute percent difference between lidar-derived K d and Argo- K d decreasing from 26 % to 15.7 %. The multiple scattering model developed can bridge the gap between the passive and active remote sensing detection and improve the reliability of lidar-derived attenuation coefficients. Fusing these two missions will greatly improve ocean observation capabilities, providing unprecedented opportunities for precise and comprehensive assessment of marine light environments. This approach has broad implications for ocean science and the application of satellite remote sensing in environmental studies. • ICESat-2 lidar attenuation and MODIS diffuse attenuation are compared in marginal seas. • A new multiple scattering correction model adjusts lidar attenuation for better accuracy. • The model's performance is confirmed across different cumulative distances. • In-situ Biogeochemical Argo validation shows improved lidar attenuation after correction. [ABSTRACT FROM AUTHOR]

Increasing sediment load and deteriorating water clarity are the key challenges for many inland water bodies. The uncertainty associated with the bio-optical complexity of these water bodies limits remote-sensing approaches to monitor such fragile ecosystems. Therefore, we measured optical properties along with water quality parameters in the Chilika lagoon, the second-largest brackish water lagoon in the world. We evaluated exiting quasi-analytical algorithms (QAAs) to retrieve particle back-scattering coefficients (bbp) and diffuse attenuation coefficient (Kd) from surface remote-sensing reflectance (Rrs) data. However, existing QAAs underestimated the observed bbp and Kd values in the Chilika lagoon. Therefore, a modified inversion algorithm (MIA) using the combined features of the sixth version of QAA (QAA v6) and the IOPs inversion model of inland waters (IIMIW) is proposed and validated. Values of bbp at 665 nm and Kd at 490 nm were then used to estimate suspended particulate matter (SPM) and Secchi disk depth (ZSD), respectively. The bias for bbp and Kd was significantly reduced using MIA, which improved the accuracy of SPM and ZSD estimation. We further used MIA to retrieve bbp(665) and Kd(490) values from the Sentinel 2A/2B (S2A/S2B) multispectral instrument (MSI) Rrs data for each pixel over the Chilika lagoon to estimate SPM and ZSD respectively. Better root-mean-squared error (RMSE) value of 10.91 mg L−1 for SPM (ranging from 11.54 to 99.00 mg L−1) and 0.21 m for ZSD (ranging from 0.15 to 1.2 m) was observed when compared with in situ measurements (n = 77). The mean SPM trend from 90 different S2A/S2B MSI acquisition dates from 2017 to 2021 indicated that SPM concentration in the lagoon is closely related to wind velocity. Matching fish landing trends with SPM concentrations was also observed, which may open new opportunities to manage inland water bodies using remote-sensing observations. [ABSTRACT FROM AUTHOR]

Underwater ultraviolet radiation (UVR), which plays a significant role in photobiological and photochemical processes, is one of the key factors in marine ecosystems. A new algorithm KpcaUV, based on kernel principal component analysis (KPCA) and multiple linear regression (MLR), was proposed in this study for the retrieval of the UVR diffuse attenuation coefficient Kd(λ) from remote sensing reflectance Rrs(λ) in the global ocean. KPCA can be applied in all areas that principal components analysis (PCA) can be used. More importantly, KPCA can help mapping data into high dimensions and reducing the nonlinearity between inputs and outputs, which will improve the performance and robustness of algorithms when deriving large dynamic ranges parameters. Compared with SeaUVc, which is one of the most successful Kd(λ) retrieval algorithms in UVR, the results showed that KpcaUV (with R2 : 0.970 and RMSE: 14.0%) performed similar to SeaUVc (with R2 : 0.963 and RMSE: 15.6%) when implemented with high-quality data. Nevertheless, KpcaUV was more robust and consistent than SeaUVc when implemented on the satellite images with different levels of quality control. The RMSD of SeaUVc had a significant reduction from 26.8% (QA ≥ 0.6) to 12.7% (QA = 1.0), and the RMSD of KpcaUV varied less than SeaUVc from 14.6% (QA ≥ 0.6) to 10.1% (QA = 1). Hence, considering its good nonlinear-problem-solving ability and robustness when applied to multiple satellites, KpcaUV proposed by this study can be used to obtain Kd(380) for the continuous observation of the large area. [ABSTRACT FROM AUTHOR]

Accurate estimation of the underwater light field associated with photosynthetically available radiation (PAR) is critical to compute phytoplankton growth rate and net primary production (NPP), and to assess photo‐physiological response of phytoplankton, such as changes in cellular pigmentation. However, methods to estimate PAR used in many previous studies lack in accuracy, likely resulting in significant bias in light‐dependent products such as NPP derived from remote sensing, model simulations, or autonomous platforms. Here we propose and validate a new model for more accurate estimation of the subsurface PAR profile which uses chlorophyll concentration as its input. Validation is performed using 1,744 BGC‐Argo profiles of chlorophyll fluorescence that are calibrated with surface satellite‐derived chlorophyll concentration over their lifetime. The independent verification with the float's PAR sensors confirms the accuracy of satellite chlorophyll estimate worldwide and in the Southern Ocean in particular. Plain Language Summary: In this study, we propose a new model to compute the underwater light field available for phytoplankton growth. The model uses as its inputs the above water radiation (which can be obtained from public databases) and the subsurface distribution of chlorophyll‐a, a pigment shared by all phytoplankton that can be estimated from sensors deployed on robotic platforms and that is used by many ocean ecosystem models. We test the model accuracy using data observed by the light sensors on autonomous profiling floats and find it performs well. The comparison also highlights that estimates of surface chlorophyll concentrations from satellites are unbiased worldwide, in contrast to some published accounts. Key Points: The model takes into account the vertical change of chlorophyll and improves the estimate of the subsurface photosynthetically available radiation fieldThe model is applicable to autonomous platforms, ecosystem and biogeochemical models, and ocean color remote sensingModel comparison with independent in‐situ radiometry confirms that satellite chlorophyll‐a retrievals are unbiased worldwide [ABSTRACT FROM AUTHOR]

• A novel study was developed for diffuse attenuation retrieval in Amazon floodplains. • Semi-Analytical K d algorithm from Lee et al. (2013) performed well for turbid waters. • Uncertainties were lower than 22% for Sentinel-2 images in the visible bands. The modeling of underwater light field is essential for the understanding of biogeochemical processes, such as photosynthesis, carbon fluxes, and sediment transports in inland waters. Water-column light attenuation can be quantified by the diffuse attenuation coefficient of the downwelling irradiance (K d) using semi-analytical algorithms (SAA). However, the accuracy of these algorithms is currently limited in highly turbid environments, such as Amazon Floodplains, due to the SAA parametrization steps. In this study, we assessed an SAA approach for K d retrieval using a sizeable (n = 239) and diverse dataset (e.g., K d (490) ranging from almost 0 to up to 30 m−1 with mean values of 5.75 ± 3.94 m−1) in Amazon freshwater ecosystem. The main framework of this study consists of i) re-parametrization of a quasi-analytical algorithm using regional in-situ inherent optical properties (IOPs) and ii) application and validation of SAA for K d retrieval using in-situ and Sentinel-2/MSI (n = 49) derived from Remote Sensing Reflectance (R rs). Overall, the performance of the calibrated SAA was satisfactory for both in-situ and satellite R rs. The validation results with in-situ data achieved a Mean Absolute Percentage Error (MAPE) lower than 22%, Correlation Coefficient (R) > 0.80, Root Mean Square Error (RMSE) lower than 1.7 m−1, and bias between 0.73 and 1.34 for simulated visible bands of Sentinel-2/MSI (490, 560 and 660 nm) (VIS). The results using MSI imagery were similar to those of in-situ, with R > 0.9, MAPE < 20%, RMSE < 1.25 m−1, and bias between 0.98 and 1.10 for VIS bands, which illustrate the viability of this methodology for K d mapping in Amazon Floodplain Lakes. Therefore, this study demonstrates a successful application of satellite remote sensing data for the spatialization of the K d in the optically complex waters of Amazon Basin, which is essential for the ecological management of the Amazon Floodplain Lakes. [ABSTRACT FROM AUTHOR]

Impact of satellite-derived shortwave attenuation depth and its spatial variability on the upper ocean dynamics has been studied using a numerical ocean model over the Bay of Bengal. We conducted two simulations, differing in the spatial distribution of shortwave attenuation depth for the period 2014–2015. The control run use a constant attenuation depth of 23 m (the default case for Type-I water) while the experimental run (ER) use spatially varying attenuation depths derived from daily climatology of the diffuse attenuation coefficient ( K d 490). Simulated parameters like sea surface temperature (SST) and mixed-layer depth (MLD) are sensitive to K d 490 that limits the penetration of downwelling shortwave radiation into the ocean. It has been found that K d 490 alters the upper ocean thermodynamics significantly. Validation has been performed using satellite, moored-buoy and profile data, for the year 2015. During spring, the errors in SST in the ER are reduced up to 35% at buoy location. The impact of improving shortwave attenuation depth is found to be maximum in the upper ocean (50–150 m). Error in simulated temperature at 100 m depth is reduced by 15% in the ER. MLD, barrier layer thickness, and the depth of 26 °C isotherm also show significant improvements in the ER. [ABSTRACT FROM AUTHOR]

An optical algorithm is developed for the retrieval of the downwelling diffuse attenuation coefficient $K_{d}$ (490) in the Red Sea using a comprehensive hydrolight simulated data set ($N = 5000$). We found a robust relationship between the $K_{d}$ (490) and the ratio of remote sensing reflectance $R_{\text {rs}}$ (443)/ $R_{\text {rs}}$ (555), with an excellent determination coefficient ($R^{2} = 0.999$) and a low root-mean-square error (RMSE = 0.00033). The performance of the developed algorithm is evaluated with in situ data collected in the Red Sea by comparing obatined model output with existing empirical (NASA, Morel et al., Zhang and Fell, Tiwari and Shanmugam) and semianalytical (Lee et al.) algorithms. On the used in situ data from the Red Sea, the new algorithm shows good retrievals of $K_{d}$ (490) with a low bias, and a low RMSE compared to that of the existing algorithms. For satellite application, we applied our algorithm to selected MODIS–Aqua images acquired over the Red Sea, which captured spatial features of phytoplankton blooms and physical processes (e.g., cyclonic and anticyclonic circulations) in the Red Sea. The new algorithm has the potential to improve our understanding of water transparency and photosynthetic processes that rely on the availability of solar radiation. [ABSTRACT FROM AUTHOR]

Abstract To assess the seasonal and spatial variations and long-term trends in water optical properties in Lake Fuxian, investigations based on field work in four seasons and a long-term analysis of data from 1980 to 2014 were conducted. The results show that there was no significant variation in the euphotic depth (Z eu) across the four seasons, and no significant correlations between Z eu and potential influencing factors in seasons other than summer, suggesting that the water itself may be a major factor regulating the Z eu in general. Nevertheless, significant differences in Z eu between the north region (NR) and the south region (SR) were observed in all seasonal tests except spring. This finding relates to a higher abundance of chromophoric dissolved organic matter (CDOM) in the NR due to runoff, especially in the rainy seasons (summer and autumn). CDOM and its terrigenous component had an important impact on Z eu in summer, with the highest precipitation, and impacts from suspended solids and non-algal particles were also found in the NR in summer. The Secchi disk depth in the lake decreased clearly over the years, with significantly negative correlations with the increasing permanganate index and air temperature, implying that organic contaminants (CDOM and/or phytoplankton) are important regulators of water transparency. We estimate that the combined effects of climate warming and changes in land use and land cover are also indirect regulating factors. These findings should be considered in the protection of Lake Fuxian, owing to the importance of light penetration in aquatic ecosystems. Graphical abstract Unlabelled Image [ABSTRACT FROM AUTHOR]

The diffuse attenuation coefficient is an indicator of light availability in the surface layer, and is used in a broad range of applications, including numerical simulations, for the parameterization of the light transmission in the water column. In this study, a new dataset of the diffuse attenuation coefficient for the Eastern Mediterranean Sea test case is developed using an existing optical dataset of 2614 beam attenuation coefficient profiles. This method introduces a way of overcoming the difficulty of measuring the diffuse attenuation coefficient in-situ by utilizing the most routinely measured variable, the beam attenuation coefficient. The proposed approach uses existing semi-analytical relationships and a neural network. The neural network, a multi-layer perceptron regression model, is trained and validated with a dataset of 29398 concurrent bio-optical in-situ measurements from the PROSOPE cruise and remotely sensed surface variables. The model is applied to the Eastern Mediterranean dataset and the results are interpolated into a gridded gap-free field, with a grid resolution of 0.0416° x 0.0416°, which is assessed and compared with a satellite-derived product, investigating their significant differences. The resulting field's mean value is slightly reduced with respect to the satellite product, showing regions of higher turbidity, with the most prominent located in the northern Aegean Sea in regions of excess colored dissolved organic matter and around mesoscale features and in the Cretan and Levantine Sea in regions of higher mesoscale activity. • A diffuse attenuation coefficient dataset was developed using a neural network. • Beam attenuation measurements were used in the neural network. • The neural network was applied in the Eastern Mediterranean Sea. • The attenuation variability in the Eastern Mediterranean was assessed. • There was variability in the difference between our product and the satellite dataset. [ABSTRACT FROM AUTHOR]

The Ocean and Land Color Imager (OLCI) on the Sentinel-3A satellite, which was launched by the European Space Agency in 2016, is a new-generation water color sensor with a spatial resolution of 300 m and 21 bands in the range of 400-1020 nm. The OLCI is important to the expansion of remote sensing monitoring of inland waters using water color satellite data. In this study, we developed a dual band ratio algorithm for the downwelling diffuse attenuation coefficient at 490 nm (Kd(490)) for the waters of Lake Taihu, a large shallow lake in China, based on data measured during seven surveys conducted between 2008 and 2017 in combination with Sentinel-3A-OLCI data. The results show that: (1) Compared to the available Kd(490) estimation algorithms, the dual band ratio (681 nm/560 nm and 754 nm/560 nm) algorithm developed in this study had a higher estimation accuracy (N = 26, coefficient of determination (R2) = 0.81, root-mean-square error (RMSE) = 0.99m-1 and mean absolute percentage error (MAPE) = 19.55%) and validation accuracy (N = 14, R2 = 0.83, RMSE = 1.06 m-1 and MAPE = 27.30%), making it more suitable for turbid inland waters; (2) A comparison of the OLCI Kd(490) product and a similar Moderate Resolution Imaging Spectroradiometer (MODIS) product reveals a high consistency between the OLCI and MODIS products in terms of the spatial distribution of Kd(490). However, the OLCI product has a smoother spatial distribution and finer textural characteristics than the MODIS product and contains notably higher-quality data; (3) The Kd(490) values for Lake Taihu exhibit notable spatial and temporal variations. Kd(490) is higher in seasons with relatively high wind speeds and in open waters that are prone to wind- and wave-induced sediment resuspension. Finally, the Sentinel-3A-OLCI has a higher spatial resolution and is equipped with a relatively wide dynamic range of spectral bands suitable for inland waters. The Sentinel-3B satellite will be launched soon and, together with the Sentinel-3A satellite, will form a two-satellite network with the ability to make observations twice every three days. This satellite network will have a wider range of application and play an important role in the monitoring of inland waters with complex optical properties. [ABSTRACT FROM AUTHOR]

Estimates of light availability in the water column require accurate quantification at different depths of the diffuse attenuation coefficient ( K ). This study examines spatial and seasonal variability of K derived variables [euphotic zone depth ( Z ), attenuation depth ( Z )], their relationship with optically active constituents (OACs), and their impact on the underwater light field at Ibitinga Reservoir (São Paulo, Brazil). Radiometric data were acquired using profiling spectroradiometers operating from 320 to 950 nm in four campaigns (February-July 2014) concurrently with OAC water samplings. Minimum and maximum values of K PAR were, respectively, 0.99 and 3.45 m for average ranges of OACs varying from 14.4 to 16.2 mg/l for dissolved total carbon, 11.5-100.5 μg/l for chlorophyll- a, 1.8-14.5 mg/l for total suspended solid, and from 0.73 to 1.71 m for absorption coefficient of colored dissolved organic matter (CDOM) in 440 nm ( a ). CDOM removed blue wavelengths in the first meter of the water column throughout seasons, while green wavelengths were predominant with increasing depth. Furthermore, this study demonstrated that K variability is mainly influenced by the presence of phytoplankton and CDOM in the reservoir, and reveals that the seasonal variability of K was much larger than spatial variability. [ABSTRACT FROM AUTHOR]

The diffuse attenuation coefficient, K d (λ), is an important optical property. Environmental change and anthropogenic activities, however, have made it challenging to accurately assess K d (λ) patterns in the extremely turbid inland waters. We addressed this challenge by using new Landsat 8 Operational Land Imager (OLI) imagery. For the bio-optical complexity of water, we proposed an empirical band-ratio algorithm for estimating K d (490) using our in situ measurements. Based on the acceptable performance of an OLI image-based atmospheric correction and K d (490) validation, the algorithm was then applied to OLI images to estimate K d (490) patterns from April 2013 to April 2016, leading to several key findings: (1) Spatial-temporal patterns of K d (490) varied significantly in Dongting Lake. The temporal heterogeneity of K d (490) could be explained primarily by surface-runoff changes driven by regional precipitation. The spatial heterogeneity was due to sediment resuspension, resulting from sand dredging and shipping activities; (2) K d (490) values that were inversed at the intersection of Dongting Lake and Yangtze River were observed for the first time near the Chengliji site and resulted from the opposing temporal cycle of K d (490) variations between Dongting Lake and the Yangtze River; (3) There was a significant positive correlation between K d (490) and total suspended matter (TSM). This confirms that TSM often plays a principal role in the attenuation of light in extremely turbid water bodies; (4) The empirical band-ratio algorithm worked well, not only for the broader Landsat archives, but also for the narrower Sentinel-2/3 for K d (490) estimation, which demonstrates that the algorithm could be used to quantitatively monitor multi-decade records of Landsat observations and future applications of inland water quality in turbid inland waters, such as Dongting Lake and Poyang Lake. [ABSTRACT FROM AUTHOR]

The 2SeaColor model (Salama & Verhoef, 2015) was proposed to analytically retrieve the diffuse attenuation coefficient ( K d ) from remote sensing reflectance ( R rs ), but its parameterization was based on approximations and subjected to large uncertainties. In this study, we present an improvement on the parameterization equations in the inverse scheme of the 2Seacolor model. The improved model is then validated with three in-situ datasets and compared with the Zhang model (Zhang & Fell, 2007) and the Lee model (Lee, Darecki et al., 2005). Validation with radiometric data shows that the 2SeaColor model provides the best estimates of K d for the full range of observations, with the largest determination coefficient ( R 2 = 0.935) and the smallest root mean squared error (RMSE = 0.078 m − 1 ). For clear waters, where K d (490) < 0.2, the Zhang model provides the most accurate K d estimations, but results from the Lee model and the 2SeaColor are rather comparable. For turbid waters, where K d (490) > 0.2 m − 1 , the 2SeaColor model is found to be more accurate, with an RMSE of 0.186 m − 1 , compared to RMSEs of 0.279 m − 1 and 0.388 m − 1 for the Zhang model and the Lee model, respectively. The 2SeaColor model is finally applied to the GOCI (Geostationary Ocean Color Imager) level 2 product (L2P) to produce K d maps over the Yangtze Estuary, resulting in a reasonable distribution and expected range of K d , as for example K d (490) was varying from 0.04 to 9.82 m − 1 for the image acquired at 02:16 UTC, on March 8th 2013. The analytical 2SeaColor model is able to provide consistently stable and fairly accurate K d estimates in both clear and turbid waters without the need of tuning empirical coefficients from field measurements, and thus has great potential for estimating K d over optically complex waters. [ABSTRACT FROM AUTHOR]

A critical parameter to assess the environmental status of water bodies is the transparency of the water, as it is strongly affected by different water quality related components (such as the presence of phytoplankton, organic matter and sediment concentrations). One parameter to assess the water transparency is the diffuse attenuation coefficient. However, the number of subsurface irradiance measurements obtained with conventional instrumentation is relatively low, due to instrument costs and the logistic requirements to provide regular and autonomous observations. In recent years, the citizen science concept has increased the number of environmental observations, both in time and space. The recent technological advances in embedded systems and sensors also enable volunteers (citizens) to create their own devices (known as Do-It-Yourself or DIY technologies). In this paper, a DIY instrument to measure irradiance at different depths and automatically calculate the diffuse attenuation Kd coefficient is presented. The instrument, named KdUINO, is based on an encapsulated low-cost photonic sensor and Arduino (an open-hardware platform for the data acquisition). The whole instrument has been successfully operated and the data validated comparing the KdUINO measurements with the commercial instruments. Workshops have been organized with high school students to validate its feasibility. [ABSTRACT FROM AUTHOR]

• A simple method is proposed for calculating the asymptotic value of the diffuse attenuation coefficient of sea water. • The scaling law deduced from the radiative transfer equation is verified for realistic ocean scattering models. • Over a wide range of ocean water IOPs, the asymptotic attenuation coefficient is described by a universal analytical formula. A simple method is proposed for calculating the asymptotic attenuation coefficient K ∞ of sea water. The method is based on the scaling analysis of the radiative transfer equation within the small-angle approximation. From the scaling, it follows that K ∞ is analytically expressed in terms of the absorption coefficient, the transport scattering coefficient, and two dimensionless numeric constants (scaling exponents) depending on a specific scattering phase function. For a given phase function, the scaling exponents can be determined by numerical calculations of the downwelling irradiance. We test the method on a number of oceanlike scattering models. A direct numerical integration of the radiative transfer equation is carried out with the DISORT code for the Petzold, Morel et al., Fournier–Forand and Kopelevich phase functions. Our numerical results agree perfectly with the small-angle scaling and allow us to establish an explicit expression for K ∞ for each phase function. We find that the scaling exponents depend rather weakly on the specific angular profile of the phase function and can be taken equal to their values for the Henyey–Greenstein function (this approximation leads to a relative error in the value K ∞ less than 5 %). So that, within a few percent accuracy, the coefficient K ∞ is governed only by the absorption and transport scattering coefficients and is described by a universal formula. [ABSTRACT FROM AUTHOR]

Secchi disk depth ( Z SD ) is a measure of water transparency, whose interpretation has wide applications from diver visibility to studies of climate change. This transparency has been explained in the past 60 + years with the underwater visibility theory, the branch of the general visibility theory for visual ranging in water. However, through a thorough review of the physical processes involved in visual ranging in water, we show that this theory may not exactly represent the sighting of a Secchi disk by a human eye. Further, we update the Law of Contrast Reduction, a key concept in visibility theory, and develop a new theoretical model to interpret Z SD . Unlike the classical model that relies strongly on the beam attenuation coefficient, the new model relies only on the diffuse attenuation coefficient at a wavelength corresponding to the maximum transparency for such interpretations. This model is subsequently validated using a large (N = 338) dataset of independent measurements covering oceanic, coastal, and lake waters, with results showing excellent agreement (~ 18% average absolute difference, R 2 = 0.96) between measured and theoretically predicted Z SD ranging from < 1 m to > 30 m without regional tuning of any model parameters. This study provides a more generalized view of visual ranging, and the mechanistic model is expected to significantly improve the current capacity in monitoring water transparency of the global aquatic environments via satellite remote sensing. [ABSTRACT FROM AUTHOR]

Addressing critical earth science questions often requires time scales beyond the life of any single satellite sensor. Overlap between satellite-based datasets allows for the quantification of continuity (and discrepancies) between sensors. Toward that end, collocated matchups between Sea-viewing Wide Field-of-View Sensor (SeaWiFS), Moderate Resolution Imaging Spectroradiometer (MODIS), and Visible Infrared Imager Radiometer Suite (VIIRS) water clarity data from the Gulf of Mexico were analyzed at simultaneous, daily, and monthly time scales. Simultaneous data indicated strong agreement between sensors, with unbiased percent difference (UPD) generally less than 10% for both SeaWiFS/MODIS and VIIRS/MODIS matchups, with no apparent temporal trends. Spatially, UPD was highest near frontal boundaries and at high sensor zenith angles, while bias showed nearshore/offshore trends. UPD and bias statistics did not diminish for daily matchups; however, large degradation was seen for comparisons of monthly means between sensors, particularly SeaWiFS/MODIS matchups. Data coverage represented an important factor contributing to uncertainties in monthly mean data, as higher UPD was observed when fewer valid satellite measurements were recorded. Requiring a minimum of 15 samples per pixel per month minimizes the uncertainties in monthly mean products, with UPD between satellites roughly equivalent to that for simultaneous matchups. Overall, these findings demonstrate high consistency between three satellite instruments for most locations, while several “hot spots” of inconsistency are also revealed, which should be avoided in time-series studies. The findings also highlight the need to quantify uncertainties in often-used satellite products (particularly monthly mean composites) as well as the need to have a sufficient number of observations to assure the fidelity of monthly means. [ABSTRACT FROM PUBLISHER]

The objectives of this study are to evaluate the applicability of three existing retrieval models of total suspended matter (TSM) in the Bohai, Yellow, and East China Seas (BYES), and to propose a multilayer back propagation neural network (MBPNN) model. Based on the comparison of TSM predicted by these models with in situ measurements taken in the BYES, it was found that the MBPNN model produces a superior performance to the three existing models selected. Near-infrared band-based and shortwave infrared band-based combined model was used to remove the atmospheric effects from the moderate resolution imaging spectroradiometer (MODIS) data, and the TSM concentration was quantified from the MODIS data after atmospheric correction using the MBPNN model. It was found that the MBPNN model produces 41.4% uncertainty in deriving TSM concentration from the MODIS data. The MBPNN model-based climatological seasonal mean TSM concentration indicated that the highest TSM concentrations were found around the river estuaries, while the lowest values were found in the far offshore regions of the BYES. The MBPNN model-based monthly mean TSM concentration revealed that the highest TSM concentrations occurred in February, and the lowest in July. The diffuse attenuation coefficient at 490 nm ( K d (490)) depended heavily on TSM concentration in the BYES. The significant relationship ( R 2 = 0.72, p < 0.05) between K d (490) and TSM concentration indicated that the BYES waters are TSM dominated types. [ABSTRACT FROM AUTHOR]

Satellite-based observations of the diffuse attenuation coefficient for the downwelling spectral irradiance at the wavelength of 490 nm, K d (490) and the diffuse attenuation coefficient for the downwelling photosynthetically available radiation (PAR), K d (PAR) in the ocean can play important roles for ocean–atmospheric circulation, biogeochemical, and ecosystem models. Since existing K d (PAR) models for the satellite ocean color data have wide regional variations, we need to improve the K d (PAR) algorithm for global ocean applications. In this study, we propose a new blended K d (PAR) model for both open oceans and turbid coastal waters. The new method has been assessed using in situ optical measurements from the NASA Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Bio-Optical Archive and Storage System (SeaBASS) database. Next, the new method is applied to the Moderate Resolution Imaging Spectroradiometer (MODIS) and the Visible Infrared Imaging Radiometer Suite (VIIRS) to derive K d (PAR) products, and is compared with in situ measurements. Results show that there are significant improvements in model-derived K d (PAR) values using the new approach compared to those from some existing K d (PAR) algorithms. In addition, matchup comparisons between MODIS-derived and in situ-measured K d (PAR) data for the global ocean show a good agreement with mean and median ratios of 1.109 and 1.035, respectively. Synoptic maps of MODIS- and VIIRS-derived K d (PAR) data generated using the new method provide very similar and consistent spatial patterns in the U.S. East Coast region, although there are some slight differences between two satellite-derived K d (PAR) images (~ 1–5% higher in VIIRS K d (PAR) compared with those from MODIS-Aqua in the shallow water region), which are possibly due to differences in spectral bands and sensor performance (e.g., calibrations). Monthly maps of VIIRS-derived K d (PAR) data for the global ocean are also generated using the new K d (PAR) model, and provide spatial and temporal K d (PAR) distributions that show consistent results with those from previous studies. Thus, results show that satellite-derived K d (PAR) data using the new K d (PAR) model, e.g., from MODIS and VIIRS, can provide more accurate K d (PAR) data to science communities, in particular, as an important input for ocean–atmospheric circulation, biogeochemical, and ecosystem models. [ABSTRACT FROM AUTHOR]

• Euphotic zone depth (Z eu) estimated from absorption coefficient data for the Chilika lagoon. • Sentinel S2A/S2B MSI data for large-scale characterization of underwater light environment shown. • Spatio-temporal dynamics of Z eu in the Chilika lagoon linked to macrophyte growth. Euphotic zone depth (Z eu) is a key indicator of underwater light environment that regulates different biogeochemical and physical processes in aquatic ecosystem. Remote sensing approaches offer a unique opportunity to frequently monitor Z eu , although its estimation from diffuse attenuation coefficient (K d) requires depth profiles of the down-welling irradiance (E d). Measuring E d values at different water depths (i.e., E d profiling) in shallow waterbodies is a challenging task with the available profiling equipment. In this study, K d values were estimated from measured absorption coefficient (a) data at multiple locations in the Chilika lagoon, which is a shallow waterbody located in the eastern coast of India and is a prestigious Ramsar site. Specifically, remote sensing reflectance (R rs), a values (wavelength: 400–715 nm at 4 nm interval), Secchi disc depth (Z SD), suspended particulate matter (SPM) and chlorophyll- a (Chl- a) contents were measured during Oct. 07–08, 2018 and Feb. 02–08, 2019 in the Chilika lagoon. Atmospherically corrected Sentinel 2A/2B (S2A/S2B) MSI- R rs data (level 2A) were then used to develop algorithms for mapping Z eu for the entire lagoon. Modeling results showed that R rs values at band 4 (665 nm) or band 5 (704 nm) extracted from the S2A/S2B MSI data may be used to estimate K d of photosynthetically active radiation (PAR) (observed range: 1.29 m−1 to 10.74 m−1) with the root-mean-squared error (RMSE) as low as 0.81 m−1. However, uncertainties associated with atmospheric correction of S2A/S2B MSI-single band R rs values are high when compared with in situ R rs. Thus, a multivariate modeling approach was adopted with R rs - band ratio and band difference variables selected by stepwise, forward, and backward variable selection method with p < 0.05. Modeling results showed that the band differences between band 2 and band 4 and band 3 and band 5 may be used to estimate K d (PAR) with improved mean absolute percentage difference (MAPD) while ensuring low uncertainty associated with atmospheric correction. Spatio-temporal dynamics from S2A/S2B MSI for 85 different dates during Jan. 2017 to June 2021 provided seasonal patterns for Z eu in the Chilika lagoon. [ABSTRACT FROM AUTHOR]

• Euphotic zone depth (Z eu) estimated from absorption coefficient data for the Chilika lagoon. • Sentinel S2A/S2B MSI data for large-scale characterization of underwater light environment shown. • Spatio-temporal dynamics of Z eu in the Chilika lagoon linked to macrophyte growth. Euphotic zone depth (Z eu) is a key indicator of underwater light environment that regulates different biogeochemical and physical processes in aquatic ecosystem. Remote sensing approaches offer a unique opportunity to frequently monitor Z eu , although its estimation from diffuse attenuation coefficient (K d) requires depth profiles of the down-welling irradiance (E d). Measuring E d values at different water depths (i.e., E d profiling) in shallow waterbodies is a challenging task with the available profiling equipment. In this study, K d values were estimated from measured absorption coefficient (a) data at multiple locations in the Chilika lagoon, which is a shallow waterbody located in the eastern coast of India and is a prestigious Ramsar site. Specifically, remote sensing reflectance (R rs), a values (wavelength: 400–715 nm at 4 nm interval), Secchi disc depth (Z SD), suspended particulate matter (SPM) and chlorophyll- a (Chl- a) contents were measured during Oct. 07–08, 2018 and Feb. 02–08, 2019 in the Chilika lagoon. Atmospherically corrected Sentinel 2A/2B (S2A/S2B) MSI- R rs data (level 2A) were then used to develop algorithms for mapping Z eu for the entire lagoon. Modeling results showed that R rs values at band 4 (665 nm) or band 5 (704 nm) extracted from the S2A/S2B MSI data may be used to estimate K d of photosynthetically active radiation (PAR) (observed range: 1.29 m−1 to 10.74 m−1) with the root-mean-squared error (RMSE) as low as 0.81 m−1. However, uncertainties associated with atmospheric correction of S2A/S2B MSI-single band R rs values are high when compared with in situ R rs. Thus, a multivariate modeling approach was adopted with R rs - band ratio and band difference variables selected by stepwise, forward, and backward variable selection method with p < 0.05. Modeling results showed that the band differences between band 2 and band 4 and band 3 and band 5 may be used to estimate K d (PAR) with improved mean absolute percentage difference (MAPD) while ensuring low uncertainty associated with atmospheric correction. Spatio-temporal dynamics from S2A/S2B MSI for 85 different dates during Jan. 2017 to June 2021 provided seasonal patterns for Z eu in the Chilika lagoon. [ABSTRACT FROM AUTHOR]

The paper “A neural network model for remote sensing of diffuse attenuation coefficient in global oceanic and coastal waters: Exemplifying the applicability of the model to the coastal regions in Eastern China Seas” uses the ratio of water-leaving radiance to downward irradiance multiple by π as the remote sensing reflectance for NOMAD dataset and the implement routine of diffuse attenuation coefficient retrieval models. Our results indicate that the quasi-analytical algorithm-based semi-analytical model (QSM) and spectrum-based neural network model (SNNM) produce a superior performance to these mentioned in the original paper, but these results do not affect the main conclusions in the original paper. [ABSTRACT FROM AUTHOR]

El Hierro Island, located at the Canary Islands Archipelago in the Atlantic coast of North Africa, has been rocked by thousands of tremors and earthquakes since July 2011. Finally, an underwater volcanic eruption started 300 m below sea level on October 10, 2011. Since then, regular multidisciplinary monitoring has been carried out in order to quantify the environmental impacts caused by the submarine eruption. Thanks to this natural tracer release, multisensorial satellite imagery obtained from MODIS and MERIS sensors have been processed to monitor the volcano activity and to provide information on the concentration of biological, chemical and physical marine parameters. Specifically, low resolution satellite estimations of optimal diffuse attenuation coefficient ( K d ) and chlorophyll- a ( Chl - a ) concentration under these abnormal conditions have been assessed. These remote sensing data have played a fundamental role during field campaigns guiding the oceanographic vessel to the appropriate sampling areas. In addition, to analyze El Hierro submarine volcano area, WorldView-2 high resolution satellite spectral bands were atmospherically and deglinted processed prior to obtain a high-resolution optimal diffuse attenuation coefficient model. This novel algorithm was developed using a matchup data set with MERIS and MODIS data, in situ transmittances measurements and a seawater radiative transfer model. Multisensor and multitemporal imagery processed from satellite remote sensing sensors have demonstrated to be a powerful tool for monitoring the submarine volcanic activities, such as discolored seawater, floating material and volcanic plume, having shown the capabilities to improve the understanding of submarine volcanic processes. [ABSTRACT FROM AUTHOR]

Abstract: Freshwater resources are deteriorating rapidly due to human activities and climate change. Remote sensing techniques have shown potential for monitoring water quality in shallow inland lakes, especially in data-scarce areas. The purpose of this study was to determine the spectral diffuse attenuation coefficient (Kd(λ)) of the water column, in order to map the euphotic depth (Zeu) of Lake Naivasha, Kenya using the Medium Resolution Imaging Spectrometer (MERIS). Intensive in situ radiometric and limnological data collection was undertaken at Lake Naivasha. Atmospheric correction was done on the MERIS images using MERIS Neural Network algorithms, Case 2 Waters (C2R) and Eutrophic Lakes processors and the bright pixel atmospheric correction algorithm (BPAC). The Eutrophic Lakes processor gave the most accurate atmospherically corrected remote sensing reflectances at 490nm compared to the other processors, with mean absolute percentage error (MAPE) of 47% and a root mean square error (RMSE) 43%, with BPAC giving negative reflectances in the blue spectral range. In situ Kd and Zeu models were calibrated and validated using above- and under-water radiometric measurements, and tested on the Neural Network atmospheric correction processed MERIS images. The Eutrophic Lakes estimates were the most accurate, with an RMSE of 0.26m−1 and MAPE of 18% for Kd(490) and RMSE of 0.17m and MAPE of 14% for Zeu. The Zeu maps produced from MERIS images clearly show the variation of euphotic depth both in space and time. These results indicate the suitability of MERIS to monitor euphotic depth and other water quality parameters of shallow inland water bodies. [Copyright &y& Elsevier]

Abstract: For global oceanic and coastal waters, a multilayer back propagation neural network (MBPNN) is developed to retrieve the diffuse attenuation coefficient for the downwelling spectral irradiance at the wavelength 490nm (K d(490)). The applicability of Lee's quasi-analytical algorithm-based semi-analytical model, Wang's switching model, Chen's semi-analytical model, Jamet's neural network model, and the MBPNN model is evaluated using the NASA bio-optical marine algorithm dataset (NOMAD) and the Eastern China Seas dataset. Based on the comparison of K d(490) predicted by these five models, with field measurements taken in global oceanic and coastal waters, it is found that the MBPNN model provides a stronger performance than the Lee, Wang, Chen, and Jamet's models. The atmospheric effects on the MODIS data are eliminated using near-infrared band-based and shortwave infrared band-based combined models, and the K d(490) is quantified from the MODIS data after atmospheric correction using the MBPNN model. The study results indicate that the MBPNN model produces ~28% uncertainty in estimating K d(490) from the MODIS data. Finally, an exemplification of the applicability of the model to the coastal regions in the Eastern China Seas is carried out. Our results suggest that the K d(490) shows a large variation in the Eastern China Seas, ranging from 0.02 to 4.0m−1, with an average value of ~0.17m−1. [Copyright &y& Elsevier]

Abstract: Photochemical processes driven by high-energy ultraviolet radiation (UVR) in inshore, estuarine, and coastal waters play an important role in global biogeochemical cycles and biological systems. A key to modeling photochemical processes in these optically complex waters is an accurate description of the vertical distribution of UVR in the water column which can be obtained using the diffuse attenuation coefficients of downwelling irradiance (K d(λ)). The SeaUV/SeaUVc algorithms (Fichot et al., 2008) can accurately retrieve K d (λ=320, 340, 380, 412, 443 and 490nm) in oceanic and coastal waters using multispectral remote sensing reflectances (Rrs (λ), SeaWiFS bands). However, SeaUV/SeaUVc algorithms are currently not optimized for use in optically complex, inshore waters, where they tend to severely underestimate K d(λ). Here, a new training data set of optical properties collected in optically complex, inshore waters was used to re-parameterize the original SeaUV/SeaUVc algorithms, resulting in improved K d(λ) retrievals for turbid, estuarine waters. Although the updated SeaUV/SeaUVc algorithms perform best in optically complex waters, the original SeaUV/SeaUVc models still perform well in most coastal and oceanic waters. Therefore, we propose a composite set of SeaUV/SeaUVc algorithms, optimized for K d(λ) retrieval in almost all marine systems, ranging from oceanic to inshore waters. The composite algorithm set can retrieve K d from ocean color with good accuracy across this wide range of water types (e.g., within a mean relative error of 13% for K d(340)). A validation step using three independent, in situ data sets indicates that the composite SeaUV/SeaUVc can generate accurate K d(λ) values at λ=320–490nm from ocean color on a global scale. Taking advantage of the inherent benefits of our statistical methods, we pooled the validation data with the training set, obtaining an optimized composite model for estimating K d(λ) in UV wavelengths for almost all marine waters. This “optimized composite” set of SeaUV/SeaUVc algorithms will provide the optical community with improved ability to quantify the role of solar UV radiation in photochemical and photobiological processes in the ocean. [Copyright &y& Elsevier]

Abstract: Especially for a shallow and extremely turbid lake, light availability in the water column can determine the euphotic zone, and the horizontal and vertical distribution of algae species. Light attenuation is traditionally quantified as the diffuse attenuation coefficient of the photosynthetically available radiation (K d(PAR)). Global coverage of K d(PAR) at high spatial and temporal resolution can be provided by satellite measurements, and these data can be used to improve our understanding of the physical, chemical and biological processes in Lake Taihu, a large shallow lake in China, of considerable importance as a source of drinking water for several large cities. The primary dominated contributor to K d(PAR) was determined firstly using linear regression. There was a significant positive correlation between K d(PAR) and concentrations of total suspended matter (TSM). The determination coefficient between K d(PAR) and TSM (R 2 =0.91) was significantly higher than that between K d(PAR) and chlorophyll-a concentrations (Chl-a) (R 2 =0.11), and between K d(PAR) and absorption of chromophoric dissolved organic matter (CDOM) (R 2 =0.01). Our results therefore could demonstrate that TSM usually plays a dominant role in the attenuation of light in Lake Taihu. A retrieval model of K d(PAR) values for Lake Taihu was subsequently developed using top-of-atmosphere (TOA) radiance of Medium Resolution Imaging Spectrometer (MERIS) image data at band 10, which was strongly correlated with in situ K d(PAR) (R 2 =0.74, p <0.005, N=48). In contrast, the atmospheric corrected reflectance of MERIS image data was not strongly correlated with in situ K d(PAR). Thus, atmospheric correction was not a prerequisite for estimations of K d(PAR) in this highly turbid and hyper-eutrophic lake, and a simple empirical model of the K d(PAR) estimation for Lake Taihu was effective. With the simple model, the seasonal and spatial distributions of K d(PAR) in Lake Taihu were studied using the MERIS measurements from 2003 to 2010. Our results show distinct seasonal, spatial, and wind driven, K d(PAR) values in Lake Taihu. The highest and the lowest K d(PAR) values were found in summer and in winter, respectively. The increase of K d(PAR) in summer can be attributed to the phytoplankton blooms, and wind-driven sediment resuspension. Spatially, the K d(PAR) values were high in the southern part and the lake center, and low K d(PAR) in East Lake Taihu. Based on the MERIS derived K d(PAR) values, a significant correlation was found between K d(PAR) and wind speeds, suggesting a critical role of wind speeds in the K d(PAR) variations in Lake Taihu. [Copyright &y& Elsevier]

Abstract: Each year, spatial patterns of ocean color in the Baltic Sea differ in temporal evolution and magnitude. We have investigated the interannual variability of the spatially averaged vertical diffuse attenuation coefficient at 490nm (Kd(490)) in response to atmospheric forcing and river discharge. Our results indicate that atmospheric forcing does not have a significant influence on interannual anomalies of Kd(490) in the Baltic Sea. This is dissimilar to the North Atlantic at the same latitudes, where interannual variability of phytoplankton blooms (ocean color chlorophyll concentration, Chl, and Kd) is to a large degree controlled by local weather. In contrast, in the Baltic Sea the interannual variability of Kd(490) is significantly influenced by river runoff. Higher values of Kd(490) are observed in years with larger inflow of water from rivers. Without access to more detailed information about the concentrations of various optically significant water components we can only speculate about the possible reasons for this relationship. Most likely a combination of several factors causes the observed positive correlation between Kd(490) and river discharge. These factors include development of more intense phytoplankton blooms in response to a larger supply of nutrients delivered by rivers. This conclusion is supported by satellite Chl data. However, an advection of optically important material with river water most likely also contributes to the bserved variability of Kd(490). The diffuse attenuation coefficient plays a critical role in many oceanographic processes. For example, Kd is essential for quantification of radiative heating of the ocean, in models of primary production and other photoprocesses, and in studies discussing water turbidity and water quality. Better understanding of the variability of Kd in the Baltic Sea can improve our knowledge of this marine environment. [Copyright &y& Elsevier]

Relationships between water clarity, light attenuation, and concentration of suspended particulates are important in water optics and remote sensing, but are not well described yet, especially for optically complex turbid inland waters. In this study, based on 3-year data from Chinese lakes (Lake Taihu, Lake Chaohu, and Three Gorges reservoir), we propose a new approach to describe the inter-relationships of these bio-optical variables. This approach includes a pre-classification step of the waters into three types based on a semi-analytical parameter C before establishing the relationships. Our results showed that the pre-classification of waters increased model accuracies both for Z (Secchi depth) versus K (diffuse attenuation coefficient) and K versus TSM (total suspended matter concentration). The quasi-theoretical model described better the relationship between Z and K than the empirical model. For the K versus TSM relationship, linear models proved suitable for the Type 2 and Type 3 waters, whereas the power function model gave a better fit for the Type 1 water. Testing of the proposed relations with an independent dataset showed mean absolute percentage errors (MAPE) mostly below 30%. The findings of this study clarify the relationships between Z, K, and TSM, and improve our bio-optical understanding of complex turbid inland waters. [ABSTRACT FROM AUTHOR]

The main objective of our study was to reconstruct the multi-decadal changes of the underwater light field in two large, shallow and polymictic Estonian lakes Võrtsjärv and Peipsi in order (i) to assess the potential role that light limitation may have had on phytoplankton growth in the past and (ii) to get an insight into the factors driving underwater light climate in shallow turbid lakes in the long term. We reconstructed the long-term variations of the diffuse attenuation coefficient of water (Kd,PAR) in the photosynthetically active region (PAR, 400-700 nm) partly on the basis of measured beam attenuation spectra and partly using regression analysis. From Kd,PAR we calculated the depth of the euphotic zone (z1%) and the mean light availability in the mixed layer (Emix). The reconstructed time series of these bio-optical parameters gave a plausible picture of the long-term development of light conditions in the two lakes studied, which was in accordance with their eutrophication history and changes in their water levels. Better light availability in both lakes generally coincided with years of a low water level, and the coincidence was more distinct in the shallower Võrtsjärv. Values of Emix revealed a probable light limitation in Peipsi in autumn and in Võrtsjärv throughout the year. [ABSTRACT FROM AUTHOR]

Abstract: Optical data collected in coastal waters off South Florida and in the Caribbean Sea between January 2009 and December 2010 were used to evaluate products derived with three bio-optical inversion algorithms applied to MODIS/Aqua, MODIS/Terra, and SeaWiFS satellite observations. The products included the diffuse attenuation coefficient at 490nm (Kd_490) and for the visible range (Kd_PAR), and euphotic depth (Zeu, corresponding to 1% of the surface incident photosynthetically available radiation or PAR). Above-water hyperspectral reflectance data collected over optically shallow waters of the Florida Keys between June 1997 and August 2011 were used to help understand algorithm performance over optically shallow waters. The in situ data covered a variety of water types in South Florida and the Caribbean Sea, ranging from deep clear waters, turbid coastal waters, and optically shallow waters (Kd_490 range of ~0.03–1.29m−1). An algorithm based on Inherent Optical Properties (IOPs) showed the best performance (RMSD<13% and R2 ~1.0 for MODIS/Aqua and SeaWiFS). Two algorithms based on empirical regressions performed well for offshore clear waters, but underestimated Kd_490 and Kd_PAR in coastal waters due to high turbidity or shallow bottom contamination. Similar results were obtained when only in situ data were used to evaluate algorithm performance. The excellent agreement between satellite-derived remote sensing reflectance (Rrs) and in situ Rrs suggested that the different product uncertainties resulted primarily from algorithm inversion as opposed to atmospheric correction. A simple empirical model was developed to derive Zeu from Kd_490 for satellite measurements of nearshore waters. MODIS/Aqua gave the best results in general relative to in situ observations. Our findings lay the basis for synoptic time-series studies of water quality in coastal ecosystems, yet more work is required to minimize the bottom interference in the Florida Keys optically shallow waters. [Copyright &y& Elsevier]

The diffuse attenuation coefficient of photosynthetically active radiation (PAR) (400-700 nm) ( K(PAR)) is one of the most important optical properties of water. Our purpose was to create K(PAR) prediction models from the Secchi disk depth (SDD) and beam attenuation coefficient of particulate and dissolved organic matter ( C(PAR), excluding pure water) in the PAR range. We compare their performance and prediction precision by using the determination coefficient ( r), relative root mean square error (RRMSE), and mean relative error (MRE). Our dataset comprised 1,067 measurements, including K(PAR), SDD, and C(PAR) taken in shallow, eutrophic, Lake Taihu, China, from 2005 to 2010. The prediction models of K(PAR) were based on the linear model with an intercept of zero, using the inverse SDD, and the nonlinear model using SDD. The linear model generated a slope of 1.369, which was not significantly different from 1.7, the index used worldwide, but significantly lower than the value of 2.26. The nonlinear model gave a slightly more reliable prediction of K(PAR) with a r of 0.804. Compared to the SDD, C(PAR) was more significantly correlated to K(PAR) based on the linear model, with a significantly higher r and lower RMSE and RE. Considering the measurement simplicity of C(PAR) and data acquisition feasibility from high-frequency autonomous buoys and satellites, our results demonstrated that this prediction model reliably estimates K(PAR), and could be used to significantly expand optical observations in an environment where the conditions for underwater PAR measurement are limited. [ABSTRACT FROM AUTHOR]

This article investigates the performance of MERIS reduced resolution data to monitor water quality parameters in the Berau estuary waters, Indonesia. Total suspended matter (TSM), Chlorophyll-a (Chl-a) concentration and diffuse attenuation coefficient (Kd) were derived from MERIS data using three different algorithms for coastal waters: standard global processor (MERIS L2), C2R and FUB. The outcomes were compared to in situ measurements collected in 2007. MERIS data processed with C2R gave the best retrieval of Chl-a, while MERIS L2 performed the best for TSM retrieval, but large deviations from in situ data were observed, pointing at inversion problems over these tropical waters for all standard processors. Nevertheless, MERIS can be of use for monitoring equatorial coastal waters like the Berau estuary and reef system. Applying a Kd(490) local algorithm to the MERIS RR data over the study area showed a sufficient good correlation to the in situ measurements (R2 = 0.77). [ABSTRACT FROM AUTHOR]

Abstract: This paper presents a model to calculate depth-resolved marine photochemical fluxes from remotely sensed ocean color and modeled solar irradiance. The basic approach uses three components: 1) below-sea-surface spectral downward scalar irradiance calculated from a radiative transfer model (STAR) and corrected for clouds using TOMS UV reflectivities; 2) surface-ocean spectral diffuse attenuation coefficients and absorption coefficients for chromophoric dissolved organic matter retrieved from SeaWiFS ocean color using the SeaUV/SeaUVc algorithms; and (3) spectral apparent quantum yield for the photochemical reaction considered. The output of the model is a photochemical rate profile, Ψ PR (z), where z represents depth. We implemented the model for carbon monoxide (CO) photochemistry using an average apparent quantum yield spectrum and generated a monthly climatology of depth-resolved CO photoproduction rates in the global ocean. The climatology was used to compute global budgets and investigate the spatial and seasonal variabilities of CO photoproduction in the ocean. The model predicts a global CO photoproduction rate of about 41TgCyr−1, in good agreement with other recent published estimates ranging from 30 to 84TgCyr−1. The fate of photochemically derived CO and its role in global biogeochemical cycles remains uncertain however, with biological consumption and sea–air exchange competing for its removal in the surface ocean. Knowledge of the vertical distribution of CO photoproduction is critical in the quantification of the relative magnitudes of these sink mechanisms. The depth-resolution capabilities of this model, together with US Naval Research Laboratory climatology for mixed layer depths allowed further estimation that >95% of the total water-column CO photoproduction occurs within the mixed layer on a global, yearly basis. Despite this compelling figure, the model also suggests significant spatio-temporal variability in the vertical distribution of CO photoproduction in the subtropical gyres, where up to 40% of water-column CO can be produced below the mixed layer during summertime. While the approach can be applied to other photochemical fluxes (e.g. DIC formation or DMS removal), accurate quantification of such processes with remote sensing will be limited until the mechanisms regulating observed oceanic variability in the apparent quantum yields are better understood. Minor modification to this model can also make it applicable for the determination of the effects of UV and visible solar radiation on sensitive biological systems. [Copyright &y& Elsevier]

Transport theory methods can be applied to optical oceanography to solve forward and inverse problems. The combination of delta function representing forward and backward scattering with isotropic scattering is used to obtain scalar and plane irradiances for Henyey-Greenstein phase function. Once the irradiances are obtained, the apparent optical properties can be found analytically and numerically. [ABSTRACT FROM AUTHOR]

Accurate quantification of photo-induced marine processes requires a realistic description of UV radiation (UVR) penetration in the surface ocean. Remote sensing of ocean color could supply global views of UVR attenuation if reliable algorithms were made available. Although a few models for the retrieval of UV diffuse attenuation coefficients, K d(λ), have been proposed in the past, robust and properly validated algorithms are still needed. We have developed and validated two algorithms for the retrieval of K d(λ) at λ =320, 340, 380, 412, 443, 490 nm from multispectral remote-sensing reflectances, Rrs (λ) (SeaWiFS wavebands). These algorithms, SeaUV and SeaUV C , were developed from a large “training dataset” comprising simultaneous in situ measurements of Rrs (λ) and K d(λ) made in a variety of water types ranging from blue oligotrophic to turbid estuarine systems. Four orthogonal dimensions of ocean color defined from the original Rrs (λ) spectra are used as predictors in multiple linear regressions of K d(λ) to develop the algorithms. In the case of SeaUV C , an optical classification permits the development of water-type specific parameterizations. Validation against an independent synthetic Hydrolight® data set and an in situ data set showed that the algorithms can be used with confidence over the range of water types for which they have been developed. SeaUV and SeaUV C will find applicability in the fields of marine photochemistry and photobiology where their performance and inherent simplicity make them suitable for batch implementation on large data sets and at global scales. [Copyright &y& Elsevier]

The vertical attenuation coefficient of diffuse downwelling irradiance at 490 nm (K d 490) is a parameter that we routinely derive from SeaWiFS images of the Baltic Sea. Here, through model simulations, we examine the relationship between K d(490), and the vertical attenuation coefficient of PAR (K d PAR), as this later coefficient determines the light available for aquatic photosynthesis. A simple semi-analytical model is used to predict K d(490) and K d(PAR), as a function of the concentrations of chlorophyll, colored dissolved organic material (CDOM), suspended inorganic, and suspended organic particulate material. A series of model simulations based on variations in these optically significant constituents over a range realistic for the Baltic Sea, are used to define the relationship between the two attenuation coefficients. This relationship was verified, using data collected independently from the data set used to derive model coefficients, and appears robust when applied to the Baltic Sea. Comparison to other studies and model sensitivity analyses suggest that the relationship will be dependent on relatively large regional variations in CDOM absorption. A relationship between K d(490) and Secchi disk depth was also developed and verified. This relationship while useful was more uncertain. The uncertainty was related to a greater influence of scattering on Secchi disk depth estimates and the corresponding parameterization of scattering in our model. [Copyright &y& Elsevier]

Abstract: The paper describes the feasibility of ballast water detection with SeaWiFS-derived diffuse attenuation coefficients and the corresponding number of in situ plankton cell density along the ship’s routes. Onboard sampling of the ballast water have been carried out by the LNG carrier of 110,000 gross tons during the six cruises between Japan and Qatar from May 2002 to July 2003. SeaWiFS images of the ballast water exchange areas along the ship’s route were acquired and processed by using SeaDAS. Based on the plankton analysis of the sampled ballast water, phytoplankton and zooplankton species were identified and their numbers of cell were counted for each ballast water exchange. It is found out that the observed cell density is highly correlated with the corresponding SeaWiFS-derived diffuse attenuation coefficient at 490nm (K(490)). By using this correlation the criterion of 10 cells/ml imposed by IMO regulations corresponds to 0.24m−1 of K(490). It is expected that this value of K(490) will be the criterion of future satellite-based ballast water quality standard. [Copyright &y& Elsevier]

Abstract: To characterize the water column, the diffuse attenuation coefficient of downwelling irradiance, K d(z, λ) (m−1) is one of the most important optical properties of seawater. The purpose of this research was to determine the downwelling diffuse attenuation coefficient of water around Roatan Island, Honduras. In situ K d analysis showed low attenuation coefficient values in green and blue and increased exponentially after 570 nm. The blue, green and red portion of the spectrum showed a K d value of 0.138, 0.158, and 0.503 m−1, respectively. Error analysis revealed a significantly high uncertainty in the red region (600–700 nm) and, as expected, low estimation uncertainty in blue and green. When compared with IKONOS derived K d (490 nm), it was observed that the differences were negligible, being 0.0084 and 0.0054 m−1 for station #1 and #2, respectively. Based on the fact that 90% of the diffused reflected light from a water body comes from a surface layer of water of depth 1/ K d, the results showed that a typical satellite sensor (such as IKONOS) can penetrate up to 8 m in the blue band, 6 m in green, and 2 m in the red region. [Copyright &y& Elsevier]

Abstract: Spatial patterns of bio-optical properties were studied in coastal, estuarine and Lakshadweep waters during the pre-monsoon period. This was carried out by analysing the profiles of downward irradiance, upward radiance and water samples collected from 26 stations. The apparent optical properties such as subsurface reflectance (R) and diffuse attenuation coefficient (k) derived from these profiles were processed to generate inherent optical properties, a (total absorption coefficient) and b b (total backscattering coefficient). Highly variable surface-coloured dissolved organic matter (CDOM) absorption (1.8–21.6 m−1 at 440 nm) was observed in the coastal waters. The distribution of optical properties a and b b reflected the distribution of optically active constituents with CDOM contributing to most of the variation in coastal waters, sediment in estuarine waters, and chlorophyll a and water molecules in Lakshadweep waters. The spectral remote sensing reflectance (R rs(λ)) is a linear function of a and b b. k was lowest and R was at maximum in the green part of the electromagnetic spectrum (EMR) in estuarine and coastal waters while this is true in the blue region of EMR in Lakshadweep waters. A linear relationship was seen between downwelling diffuse attenuation coefficient (k d) at 490 nm and attenuation of photosynthetically available radiation (k PAR). Examination of the light field revealed maximum chlorophyll at greater depth in Lakshadweep waters and at shallow depth in coastal waters, with no well-defined maxima in the estuarine waters. An exponential inverse relation is evident between k d (490) and depth of chlorophyll maxima. Through a radiative transfer computation, a ratio between water-leaving radiance at 400 and 670 nm is found to be an index of CDOM and hence could be used to retrieve it through an optical sensor. [Copyright &y& Elsevier]

The deep-water macroalgal assemblage was described at 14 sites off the central California coast during 1999 and 2000 from SCUBA and remotely operated vehicle sampling. The stipitate kelp Pleurophycus gardneri Setchell & Gardner, previously thought to be rare in the region, was abundant from 30 to 45 m, forming kelp beds below the well-known giant kelp forests. Macroalgae typically formed three broadly overlapping zones usually characterized by one or a few visually dominant taxa: 1) the upper “Pleurophycus zone” (30–45 m) of stipitate kelps and Desmarestia spp. with a high percent cover of corallines, low cover of uncalcified red algae, and rare green algae; 2) a middle “Maripelta zone” (40–55 m) with other uncalcified red algae and infrequent corallines and green algae; and 3) a zone (55–75 m) of infrequent patches of nongeniculate coralline algae. The green alga Palmophyllum umbracola Nelson & Ryan, not previously reported from the Northeast Pacific, was found over the entire geographical range sampled from 35 to 54 m. Year-round profiles of water column irradiance revealed unexpectedly clear water with an average K 0 of 0.106·m- 1 . The low percent surface irradiance found at the average lower macroalgal depth limits in this study (0.56% for brown algae, 0.12% for uncalcified red algae, and 0.01% for nongeniculate coralline algae) and lack of large grazers suggest that light controls the lower distributional limits. The ubiquitous distribution, perennial nature, and similar lower depth limits of deep-water macroalgal assemblages at all sites suggest that these assemblages are a common persistent part of the benthic biota in this region. [ABSTRACT FROM AUTHOR]

The composition of phytoplankton and the concentration of pigments in their cells make their absorption and specific absorption coefficients key parameters for bio-optical modeling. This study investigated whether the multispectral vertical diffuse attenuation coefficient of downward irradiance (Kd) gradients could be a good framework for accessing phytoplankton genera. In situ measurements of remote sensing reflectance (Rrs), obtained in an Amazon Floodplain Lake (Lago Grande do Curuai), were used to invert Kd, focusing on Sentinel-3/Ocean and Land Color Instrument (OLCI) sensor bands. After that, an analysis based on the organization of three-way tables (STATICO) was applied to evaluate the relationships between phytoplankton genera and Kd at different OLCI bands. Our results indicate that phytoplankton genera are organized according to their ability to use light intensity and different spectral ranges of visible light (400 to 700 nm). As the light availability changes seasonally, the structure of phytoplankton changes as well. Some genera, such as Microcystis, are adapted to low light intensity at 550–650 nm, therefore high values of Kd in this range would indicate the dominance of Microcysts. Other genera, such as Aulacoseira, are highly adapted to harvesting blue-green light with higher intensity and probably grow in lakes with lower concentrations of colored dissolved organic matter that highly absorbs blue light (405–498). These findings are an important step to describing phytoplankton communities using orbital data in tropical freshwater floodplains. Furthermore, this approach can be used with biodiversity indexes to access phytoplankton diversity in these environments. [ABSTRACT FROM AUTHOR]

Measuring the diffuse attenuation coefficient ( K d ) allows for monitoring the water body's environmental status. This parameter is of particular interest in water quality monitoring programs because it quantifies the presence of light and the euphotic zone's depth. Citizen scientists can meaningfully contribute by monitoring water quality, complementing traditional methods by reducing monitoring costs and significantly improving data coverage, empowering and supporting decision-making. However, the quality of the acquisition of in situ underwater irradiance measurements has some limitations, especially in areas where stratification phenomena occur in the first meters of depth. This vertical layering introduces a gradient of properties in the vertical direction, affecting the associated K d . To detect and characterize these variations of K d in the water column, it needs a system of optical sensors, ideally placed in a range of a few cm, improving the low vertical accuracy. Despite that, the problem of self-shading on the instrumentation becomes critical. Here, we introduce a new concept that aims to improve the vertical accuracy of the irradiance measurements: the underwater annular irradiance ( E a ). This new concept consists of measuring the irradiance in an annular-shaped distribution. We first compute the optimal annular angle that avoids self-shading and maximizes the light captured by the sensors. Second, we use different scenarios of water types, solar zenith angle, and cloud coverage to assess the robustness of the corresponding diffuse attenuation coefficient, K a . Finally, we derive empirical functions for computing K d from K a . This new concept opens the possibility to a new generation of optical sensors in an annular-shaped distribution which is expected to (a) increase the vertical resolution of the irradiance measurements and (b) be easy to deploy and maintain and thus to be more suitable for citizen scientists. [ABSTRACT FROM AUTHOR]

The diffuse attenuation coefficient of downwelling irradiance ( K d ) is an essential parameter for inland waters research by remotely sensing the water transparency. Lately, K d semi-analytical algorithms substituted the empirical algorithms widely employed. The purpose of this research was to reparametrize a semi-analytical algorithm to estimate K d and then apply it to a Sentinel-2 MSI time-series (2017–2019) for the Três Marias reservoir, Brazil. The results for the K d semi-analytical reparametrization achieved good accuracies, reaching mean absolute percentage errors (MAPE) for bands B2, B3 and B4 (492, 560 and 665 nm), lower than 21% when derived from in-situ remote sensing reflectance ( R r s ), while for MSI Data, a derived MAPE of 12% and 38% for B2 and B3, respectively. After the application of the algorithm to Sentinel-2 images time-series, seasonal patterns were observed in the results, showing high K d values at 492 nm during the rainy periods, mainly in the tributary mouths, possibly due to an increase in the surface runoff and inflows and outflow rates in the reservoir watershed. [ABSTRACT FROM AUTHOR]