Saturation of water reflectance in extremely turbid media based on field measurements, satellite data and bio-optical modelling.

Evidence of water reflectance saturation in extremely turbid media is highlighted based on both field measurements and satellite data corrected for atmospheric effects. This saturation is obvious in visible spectral bands, i.e., in the blue, green and even red spectral regions when the concentration of suspended particulate matter (SPM) reaches then exceeds 100 to 1000 g.m-3. The validity of several bio-optical semi-analytical models is assessed in the case of highly turbid waters, based on comparisons with outputs of the Hydrolight radiative transfer model. The most suitable models allow to reproduce the observed saturation and, by inversion, to retrieve information on the SPM mass-specific inherent optical properties.

Quantitative filter technique measurements of spectral light absorption by aquatic particles using a portable integrating cavity absorption meter (QFT-ICAM).

The accurate determination of light absorption coefficients of particles in water, especially in very oligotrophic oceanic areas, is still a challenging task. Concentrating aquatic particles on a glass fiber filter and using the Quantitative Filter Technique (QFT) is a common practice. Its routine application is limited by the necessary use of high performance spectrophotometers, distinct problems induced by the strong scattering of the filters and artifacts induced by freezing and storing samples. Measurements of the sample inside a large integrating sphere reduce scattering effects and direct field measurements avoid artifacts due to sample preservation. A small, portable, Integrating Cavity Absorption Meter setup (QFT-ICAM) is presented, that allows rapid measurements of a sample filter. The measurement technique takes into account artifacts due to chlorophyll-a fluorescence. The QFT-ICAM is shown to be highly comparable to similar measurements in laboratory spectrophotometers, in terms of accuracy, precision, and path length amplification effects. No spectral artifacts were observed when compared to measurement of samples in suspension, whereas freezing and storing of sample filters induced small losses of water-soluble pigments (probably phycoerythrins). Remaining problems in determining the particulate absorption coefficient with the QFT-ICAM are strong sample-to-sample variations of the path length amplification, as well as fluorescence by pigments that is emitted in a different spectral region than that of chlorophyll-a.

Improved correction methods for field measurements of particulate light backscattering in turbid waters.

Monte Carlo simulations are used to compute the uncertainty associated to light backscattering measurements in turbid waters using the ECO-BB (WET Labs) and Hydroscat (HOBI Labs) scattering sensors. ECO-BB measurements provide an accurate estimate of the particulate volume scattering coefficient after correction for absorption along the short instrument pathlength. For Hydroscat measurements, because of a longer photon pathlength, both absorption and scattering effects must be corrected for. As the standard (sigma) correction potentially leads to large errors, an improved correction method is developed then validated using field inherent and apparent optical measurements carried out in turbid estuarine waters. Conclusions are also drawn to guide development of future short pathlength backscattering sensors for turbid waters.

On the modeling of hyperspectral remote-sensing reflectance of high-sediment-load waters in the visible to shortwave-infrared domain.

We evaluated three key components in modeling hyperspectral remote-sensing reflectance in the visible to shortwave-infrared (Vis-SWIR) domain of high-sediment-load (HSL) waters, which are the relationship between remote-sensing reflectance (R(rs)) and inherent optical properties (IOPs), the absorption coefficient spectrum of pure water (a(w)) in the IR-SWIR region, and the spectral variation of sediment absorption coefficient (a(sed)). Results from this study indicate that it is necessary to use a more generalized R(rs)-IOP model to describe the spectral variation of R(rs) of HSL waters from Vis to SWIR; otherwise it may result in a spectrally distorted R(rs) spectrum if a constant model parameter is used. For hyperspectral a(w) in the IR-SWIR domain, the values reported in Kou et al. (1993) provided a much better match with the spectral variation of R(rs) in this spectral range compared to that of Segelstein (1981). For a(sed) spectrum, an empirical a(sed) spectral shape derived from sample measurements is found working much better than the traditional exponential-decay function of wavelength in modeling the spectral variation of R(rs) in the visible domain. These results would improve our understanding of the spectral signatures of R(rs) of HSL waters in the Vis-SWIR domain and subsequently improve the retrieval of IOPs from ocean color remote sensing, which could further help the estimation of sediment loading of such waters. Limitations in estimating chlorophyll concentration in such waters are also discussed.

Variations of suspended particulate matter concentrations of the Mackenzie River plume (Beaufort Sea, Arctic Ocean) over the last two decades.

This work addresses the last 20 years' evolution of the suspended particulate matter (SPM) concentrations in the Beaufort Sea (Canadian Arctic Ocean) directly influenced by the Mackenzie River discharge. The SPM variations in the coastal zone are highlighted and related to the freshwater and solid discharges of the river measured in situ at the Arctic Red River station (150 km upstream of the river delta). The correlation between the variations of the river discharge and SPM concentration within the surface layer of the coastal waters is obvious. Rather unexpectedly, both have been slightly but significantly decreasing from 2003 to 2018-2019 and started to increase very recently (2019-2022). This change of regime could be explained by changing winter precipitation and groundwater distribution, progressively accumulating sediments within the thawing permafrost layer and its recent release into the groundwater together with thermokarst lakes' rapid drainage.

Uncertainties associated to measurements of inherent optical properties in natural waters.

Monte Carlo simulations are used to explain and quantify the errors in inherent optical properties (IOPs) (absorption and attenuation coefficients) measured using the WET Labs AC-9 submarine spectrophotometer, and to assess correction algorithms. Simulated samples with a wide range of IOPs encountered in natural waters are examined. The relative errors on the measured absorption coefficient are in general lower than 25%, but reach up to 100% in highly scattering waters. Relative errors on attenuation and scattering coefficients are more stable, with an underestimation mainly driven by the volume scattering function. The errors in attenuation and scattering spectral shapes are small.

Role of measurement uncertainties in observed variability in the spectral backscattering ratio: a case study in mineral-rich coastal waters.

The particulate backscattering ratio (b(bp)/b(p)) is a useful indicator of the angular scattering characteristics of natural waters. Recent studies have shown evidence both for and against significant spectral variability in b(bp)/b(p) in the visible domain, but most show significant variability in its magnitude. We present results from a case study in which both backscattering and scattering coefficients were measured at nine wavelengths in a region of UK coastal waters where optical scattering is strongly influenced by inorganic particles and where a wide range of turbidities is found in a small geographic area. Using a new approach based on regression analysis of in situ signals, it is shown that, for this study site, most of the apparent variability in the magnitude of the backscattering ratio can be attributed to measurement uncertainties. Regression analysis suggests that b(bp)/b(p) is wavelength dependent for these mineral-rich waters. This conclusion can only be avoided by positing the existence of undocumented, systematic, wavelength-dependent errors in backscattering measurements made by two independently calibrated sensors. These results are important for radiative transfer simulations in mineral-dominated waters where the backscattering ratio has often been assumed to be spectrally flat. Furthermore, spectral dependence also has profound implications for our understanding of the relationship between b(bp)/b(p) and particle size distributions in coastal waters since the commonly assumed power-law distribution is associated with a spectrally flat particulate backscattering ratio for nonabsorbing particles.

Near-infrared light scattering by particles in coastal waters.

We report the first measurements of the scattering coefficient of natural marine particles, which extend over the near-infrared spectral region to up to 870 nm. The measurements were conducted in three different European estuaries (Gironde, Tamar and Elbe) using an in situ absorption and attenuation-meter. The observed particulate scattering coefficients varied from 1 to nearly 100 m(-1). The spectral shape in the near-infrared very closely matched a lambda(-gamma) spectral dependence, which is expected when the particle size followed a power-law distribution. The spectral slope of the scattering spectrum, gamma, spanned from 0.1 to 1.2 and showed significant regional and temporal variations. These variations were certainly related to the particle size distribution, which will have to be studied in future works. Using our near-infrared data as a reference, we assessed the use of the attenuation coefficient spectrum in the visible range to estimate the near-infrared particulate scattering slope and found values different by 10% on average.

Apparent and inherent optical properties of turbid estuarine waters: measurements, empirical quantification relationships, and modeling.

Spectral measurements of remote-sensing reflectance (Rrs) and absorption coefficients carried out in three European estuaries (Gironde and Loire in France, Tamar in the UK) are presented and analyzed. Typical Rrs and absorption spectra are compared with typical values measured in coastal waters. The respective contributions of the water constituents, i.e., suspended sediments, colored dissolved organic matter, and phytoplankton (characterized by chlorophyll-a), are determined. The Rrs spectra are then reproduced with an optical model from the measured absorption coefficients and fitted backscattering coefficients. From Rrs ratios, empirical quantification relationships are established, reproduced, and explained from theoretical calculations. These quantification relationships were established from numerous field measurements and a reflectance model integrating the mean values of the water constituents' inherent optical properties. The model's sensitivity to the biogeochemical constituents and to their nature and composition is assessed.

Remote-sensing reflectance of turbid sediment-dominated waters. Reduction of sediment type variations and changing illumination conditions effects by use of reflectance ratios.

Variations of sediment type (grain size and refractive index) and changing illumination conditions affect the reflectance signal of coastal waters and limit the accuracy of sediment-concentration estimations from remote-sensing measurements. These effects are analyzed from numerous in situ remote-sensing measurements carried out in the Gironde and Loire Estuaries and then reduced and partly eliminated when reflectance ratios between the near infrared and the visible are considered. These ratios showed high correlation with the sediment concentration. On the basis of the obtained relationships, performing correspondence functions were established that allow an accurate estimation of suspended sediments in the estuaries from Système Probatoire d'Observation de la Terre, Landsat, and Sea-Viewing Wide Field-of-View Sensor data, independently of the date of acquisition.

Use of a Spectralon panel to measure the downwelling irradiance signal: case studies and recommendations.

Field determinations of the remote sensing reflectance signal are necessary to validate ocean color satellite sensors. The measurement of the above-water downwelling irradiance signal Ed(0+) is commonly made with a reference plaque of a known reflectance. The radiance reflected by the plaque (L(dspec)) can be used to determine Ed(0+) if the plaque is assumed to be near Lambertian. To test this assumption, basic experiments were conducted on a boat under changing sky conditions (clear, cloudy, covered) and with different configurations for simultaneous measurements of L(dspec) and Ed(0+). For all measurement configurations, results were satisfactory under a clear sky. Under cloudy or covered skies, shadow effects on the plaque induced errors up to 100% in the determination of Ed(0+). An appropriate measurement configuration was defined, which enabled Ed(0+) to be determined with an accuracy of better than +/- 15% regardless of the sky conditions.