Inherent Optical Properties Of Seawater Research Articles

Variability of inherent optical properties of seawater in relation to the concentration and composition of suspended particulate matter in the coastal Arctic waters of western Spitsbergen

Inherent optical properties (IOPs) and characteristics of suspended particles in surface seawater samples were measured in summer months of 2021 and 2022 in Arctic fjords and coastal waters of western Spitsbergen in the Svalbard archipelago. The measured IOPs included the spectral backscattering and scattering coefficients of suspended particulate matter, as well as the spectral absorption coefficients of suspended particulate matter and its non-algal particulate and phytoplankton components. The particulate assemblages were characterized by measuring the mass concentrations of suspended particulate matter (SPM), particulate organic matter (POM), particulate inorganic matter (PIM), and phytoplankton pigments including chlorophyll-a (Chla). The investigated coastal waters exhibit high variability of particulate characteristics and associated IOPs. We observed more than two orders of magnitude variation in SPM and particulate IOPs, and Chla varied from below the detection limit to more than 3 mg m−3. The contribution of organic fraction to SPM (POM/SPM ratio) varied from 0.05 to 0.6, and the Chla/SPM ratio spanned more than three orders of magnitude with a maximum value of the order of 10−3. As a result, the mass-specific optical coefficients, especially the mass-specific backscattering and scattering coefficients of particles and mass-specific absorption coefficient of non-algal particulate matter, exhibit large variations. In addition, our study demonstrates the influences of changes in composition of suspended particulate matter parameterized in terms of POM/SPM and Chla/SPM ratios on IOPs. Various variants of spectral parametrizations of optical coefficients in terms of univariate or multivariable relationships with particulate characteristics are provided. These parameterizations are representative of the investigated coastal Arctic waters in Svalbard region and can be used for better interpretation of optical measurements, both in-situ and remote, in this Arctic environment.

Correlation between Turbidity and Inherent Optical Properties as an Initial Recognition for Backscattering Coefficient Estimation

Seawater turbidity is a common water quality indicator measured in situ and estimated from space on a regular basis. However, it is rarely correlated with the inherent optical properties of seawater, which convey information about seawater composition. In this study, we show a simple application of the turbidimeter’s weighting function in the estimation of the backscattering coefficient of a model inorganic suspension in seawater. First, we introduce a method to measure the instrument’s weighting function which describes the sensor’s angular response in terms of scattering angles. The determination of the sensor-specific weighting function led us to characterize its angular sensitivity to the presence of suspended particles. The highest sensitivity for the Seapoint turbidimeter is in the range of 114°–128° (containing 25% of the total signal). Next, we describe the correlations between turbidity and the scattering and backscattering coefficients on the example of the model of inorganic particle suspension using the calculations based on Mie theory. The correlations are analyzed for narrow size fractions of the particle size distribution of silica in the range of 0.59–190 µm. We established that there is a good linear correlation (characterized by the coefficient of determination r2 = 0.979) between the part of the scattering coefficient measured by the turbidimeter and the backscattering coefficient of all size fractions of the model inorganic suspension.

Multiple scattering effect of water clouds on spaceborne oceanic lidar signals

Multiple scattering effect in water clouds influences the shape and intensity of the return signals of spaceborne oceanic lidar. The contaminated signals will introduce errors into the retrieval of effective attenuation coefficient klidar of seawater and eventually affect the detection accuracy of seawater's inherent optical properties (IOPs). The multiple scattering effect of water clouds on spaceborne oceanic lidar signals is quantified in this paper. Two main parameters of the lidar signals from the seawater are analyzed, including the effective attenuation coefficient klidar and signal intensity ratio R with or without water clouds using a spaceborne oceanic lidar emulator. The results show that the lidar signals from the seawater below water clouds are both related to the cloud base height (CBH) and the cloud optical depth (COD). When the COD is less than 5, and CBH is relatively low (<2 km on average), the relative difference of the effective attenuation coefficient δ is within 30%. The δ is within 20% under the condition that the COD is less than 3 and the CBH is higher than 1.8 km. Moreover, this paper gives the relationship among R, COD and CBH. This work provides significant information for the development of a spaceborne oceanic profiling lidar.

Polarized observations for advanced atmosphere-ocean algorithms using airborne multi-spectral hyper-angular polarimetric imager

Airborne measurements of the linear polarization state of light were carried out over coastal and open ocean conditions to study aerosol and water column properties and investigate the possibility of using a multi-spectral, hyper-angular imaging polarimeter for retrieving aerosol and hydrosol optical properties. The instrument, the Versatile Imager for the Coastal Ocean (VICO), is used to support the analysis of ocean color polarized observations and their implication for future space-borne polarimetry such as the polarimeters planned to be deployed with the NASA Plankton, Aerosol, Cloud, and ocean Ecosystem (PACE) mission. Several sets of images at different viewing angles from the visible to the near-infrared spectrum were collected and compared with simulations using a vector radiative transfer (VRT) code. The simulations were obtained based on measured seawater inherent optical properties from shipborne instruments and measured atmospheric parameters from the Aerosol Robotic Network (AERONET) and a shipborne sunphotometer at different locations. An uncertainty method has been derived by propagating uncertainties from the measured polarized radiances. The method demonstrates practicable uncertainty formulations that can be used to construct a measurement uncertainty budget for the polarized data products. Results from VICO and the VRT simulation are consistent for both radiance and polarization spectrum at all the measured viewing angles. The total and polarized water-leaving reflectances are retrieved at four bands and varied geometries. It is also shown that the polarized remote sensing reflectance measured at various angles could be used to distinguish between the aerosols’ and hydrosols’ optical signatures by exploiting the fact that the polarized reflectance is fairly insensitive to hydrosols for given acquisition geometries. This study thus provides an opportunity to investigate various relationships between the microphysical properties of the oceanic and atmospheric particulates such as refractive index and particle size properties. It also contributes to the development of polarization-based inverse ocean color algorithms. Finally, the provided analysis gives insights for the validation of the ocean color parameters that will be retrieved from the forthcoming polarimetric satellite missions.

The Nonlinear Effective Attenuation Coefficient of Spaceborne Oceanic Lidar Signal

Multiple scattering is an inevitable effect in spaceborne oceanic lidar because of the large footprint size and the high optical density of seawater. The effective attenuation coefficient klidar in oceanic lidar equation, which indicates the influence of the multiple scattering effect on the formation of lidar returns, is an important parameter in the retrieval of inherent optical properties (IOPs) of seawater. In this paper, the nonlinearity of klidar and the relationships between klidar and the IOPs of seawater are investigated by solving the radiative transfer equation with an improved semianalytic Monte Carlo model. klidar is found to decrease exponentially with the increase of depth in homogeneous waters. klidar is given as an exponential function of depth and IOPs of seawater. The results in this paper can help to have a better understanding of the multiple scattering effect of spaceborne lidar and improve the retrieval accuracy of the IOPs of seawater using spaceborne lidar.

A Hybrid Bio-Optical Transformation for Satellite Bathymetry Modeling Using Sentinel-2 Imagery

The article presents a new hybrid bio-optical transformation (HBT) method for the rapid modelling of bathymetry in coastal areas. The proposed approach exploits free-of-charge multispectral images and their processing by applying limited manpower and resources. The testbed area is a strait between two Greek Islands in the Aegean Sea with many small islets and complex seabed relief. The HBT methodology implements semi-analytical and empirical steps to model sea-water inherent optical properties (IOPs) and apparent optical properties (AOPs) observed by the Sentinel-2A multispectral satellite. The relationships of the calculated IOPs and AOPs are investigated and utilized to classify the study area into sub-regions with similar water optical characteristics, where no environmental observations have previously been collected. The bathymetry model is configured using very few field data (training depths) chosen from existing official nautical charts. The assessment of the HBT indicates the potential for obtaining satellite derived bathymetry with a satisfactory accuracy for depths down to 30 m.

Lidar Remote Sensing of Seawater Optical Properties: Experiment and Monte Carlo Simulation

Detecting the vertical profile of optical properties is an important task in the remote sensing of the upper ocean, especially for 3-D reconstruction. Ocean color remote sensing can only provide surface information, while the light detection and ranging (lidar) technique can provide depth-resolved data. Lidar can provide global-scale observations of the upper ocean for days and nights with minimal atmospheric correction errors. Unfortunately, due to the strong multiple scattering effects that occur when light propagates in seawater, the simple lidar equation may cause some deviations between the actual measurements and the simulation of the lidar signals. In this paper, we present a shipborne oceanic lidar, which was developed to detect the optical properties of seawater. For evaluating the performance of the lidar system, a Monte Carlo (MC) model was established to simulate lidar signals based on the simultaneous in situ inherent optical properties of seawater. The lidar measurements and the MC simulation can provide both the lidar signals and the retrieved lidar attenuation coefficient $\alpha $ . The results of the comparison indicate that the lidar-measured signals correspond well with the MC-simulated signals at different experiment stations in the Yellow Sea and at various receiving fields of view (FOVs). We also observed strong correlations between the lidar-measured $\alpha $ and MC-simulated $\alpha $ at different stations ( $r = 0.95$ ) and at various FOVs ( $r = 0.96$ ). The results indicate the reliability of the developed lidar system.

Characterization of the Light Field and Apparent Optical Properties in the Ocean Euphotic Layer Based on Hyperspectral Measurements of Irradiance Quartet

Although the light fields and apparent optical properties (AOPs) within the ocean euphotic layer have been studied for many decades through extensive measurements and theoretical modeling, there is virtually a lack of simultaneous high spectral resolution measurements of plane and scalar downwelling and upwelling irradiances (the so-called irradiance quartet). We describe a unique dataset of hyperspectral irradiance quartet, which was acquired under a broad range of environmental conditions within the water column from the near-surface depths to about 80 m in the Gulf of California. This dataset enabled the characterization of a comprehensive suite of AOPs for realistic non-uniform vertical distributions of seawater inherent optical properties (IOPs) and chlorophyll-a concentration (Chl) in the common presence of inelastic radiative processes within the water column, in particular Raman scattering by water molecules and chlorophyll-a fluorescence. In the blue and green spectral regions, the vertical patterns of AOPs are driven primarily by IOPs of seawater with weak or no discernible effects of inelastic processes. In the red, the light field and AOPs are strongly affected or totally dominated by inelastic processes of Raman scattering by water molecules, and additionally by chlorophyll-a fluorescence within the fluorescence emission band. The strongest effects occur in the chlorophyll-a fluorescence band within the chlorophyll-a maximum layer, where the average cosines of the light field approach the values of uniform light field, irradiance reflectance is exceptionally high approaching 1, and the diffuse attenuation coefficients for various irradiances are exceptionally low, including the negative values for the attenuation of upwelling plane and scalar irradiances. We established the empirical relationships describing the vertical patterns of some AOPs in the red spectral region as well as the relationships between some AOPs which can be useful in common experimental situations when only the downwelling plane irradiance measurements are available. We also demonstrated the applicability of irradiance quartet data in conjunction with Gershun’s equation for estimating the absorption coefficient of seawater in the blue-green spectral region, in which the effects of inelastic processes are weak or negligible.

Relationship between the effective attenuation coefficient of spaceborne lidar signal and the IOPs of seawater.

Multiple scattering is an inevitable effect in spaceborne oceanic lidar because of the large footprint size and the high optical density of seawater. The effective attenuation coefficient klidar in the oceanic lidar equation, which indicates the influence of the multiple scattering effect on the formation of lidar returns, is an important parameter in the retrieval of inherent optical properties (IOPs) of seawater. In this paper, the relationships between klidar of the spaceborne lidar signal and the IOPs of seawater are investigated by solving the radiative transfer equation with an improved semianalytic Monte Carlo model. Apart from the geometric loss factors, klidar is found to decrease exponentially with the increase of depth in homogeneous waters. klidar is given as an exponential function of depth and IOPs of seawater. The mean percentage errors between klidar calculated by the exponential function and the simulated ones in three typical stratified waters are within 0.5%, proving the effectiveness and applicability of this klidar-IOPs function. The results in this paper can help researchers have a better understanding of the multiple scattering effect of spaceborne lidar and improve the retrieval accuracy of the IOPs and the chlorophyll concentration of case 1 water from spaceborne lidar measurements.

Correction for the non-nadir viewing geometry of AERONET-OC above water radiometry data: an estimate of uncertainties.

The effects of non-nadir viewing geometry in above-water radiometry data were investigated using field measurements and two different correction approaches: one centered on chlorophyll-a concentration (Chla) developed for Case-1 waters, and the other relying on seawater inherent optical properties (IOP) proposed for any water type. With specific reference to data from the Ocean Color component of the AErosol RObotic NETwork (AERONET-OC), the study focused on the assessment of the uncertainties affecting corrections for non-nadir view of data collected with 40° in-air viewing angle and with 90° relative azimuth between viewing direction and sun. The study analyzed AERONET-OC water-leaving radiance data from different European seas to determine differences between corrections performed with the Chla- and the IOP-based approaches. Additionally, data collected in waters characterized by different optical complexity and comprising water-leaving radiances measured at nadir and with 28.6° in-water viewing angle (corresponding to 40° in-air) and 90° relative azimuth, were used to investigate the uncertainties of the two correction approaches. Results from the analysis of data from AERONET-OC sites characterized by a variety of optically complex waters, indicate corrections with uncertainties between 20% and 35% from 412 nm to 667 nm for the IOP-based approach. Conversely, uncertainties for the Chla-based one largely vary with wavelength and water type, with values of approximately 55% at 412 nm, 20-40% between 490 nm and 551 nm, and exceeding 60% at 667 nm.

Monte Carlo simulation of spectral reflectance and BRDF of the bubble layer in the upper ocean.

The presence of bubbles can significantly change the radiative properties of seawater and these changes will affect remote sensing and underwater target detection. In this work, the spectral reflectance and bidirectional reflectance characteristics of the bubble layer in the upper ocean are investigated using the Monte Carlo method. The Hall-Novarini (HN) bubble population model, which considers the effect of wind speed and depth on the bubble size distribution, is used. The scattering coefficients and the scattering phase functions of bubbles in seawater are calculated using Mie theory, and the inherent optical properties of seawater for wavelengths between 300 nm and 800 nm are related to chlorophyll concentration (Chl). The effects of bubble coating, Chl, and bubble number density on the spectral reflectance of the bubble layer are studied. The bidirectional reflectance distribution function (BRDF) of the bubble layer for both normal and oblique incidence is also investigated. The results show that bubble populations in clear waters under high wind speed conditions significantly influence the reflection characteristics of the bubble layer. Furthermore, the contribution of bubble populations to the reflection characteristics is mainly due to the strong backscattering of bubbles that are coated with an organic film.

MODELLING OF SEAWATER POLLUTED BY LIGHT AND HEAVY CRUDE OIL DROPLETS

Significant amounts of crude oil transported from offshore fields to the refineries using tankers or pipelines, demand increased control of seawater pollution. Tanker accidents resulting in oil spills drive much attention, as they influence local marine life and coastal industry. However, the most significant annual amount of crude oil enters the sea in the form of oilin-water emulsion as a result of standard tanker operations, offshore oil extraction and daily work of refineries. Many branches of science are challenged to provide new methods for oil detection, less expensive, more sensitive and more accurate. Remote satellite or airborne detection of large oil spills is possible using joint techniques as microwave radars, ultraviolet laser fluorosensors and infrared radars. Some methods are capable to deal with oil streaks detection and estimation of oil thickness. Although there is currently, no method to detect small concentration of oil droplets dispersed in seawater. Oil droplets become additional absorbents and attenuators in water body. They significantly change seawater inherent optical properties, which imply the change of apparent optical properties, detectable using remote sensing techniques. To enable remote optical detection of oil-in-water emulsion, a study of optical properties of two types of crude oil was conducted. Radiative transfer theory was applied to quantify the contribution of oil emulsion to remote sensing reflectance (R rs ). Spectra of R rs from in situ measurements in Baltic Sea were compared to R rs spectra of seawater polluted by 1 ppm of crude oil emulsion, collected using radiative transfer simulation. The light crude oil caused a 9-10% increase of Rrs while the heavy one reduced R rs up to 30% (model accuracy stayed within 5% for considered spectral range). Results are discussed concerning their application to shipboard and offshore oil content detection.

Evaluation of the Quasi-Analytical Algorithm for estimating the inherent optical properties of seawater from ocean color: Comparison of Arctic and lower-latitude waters

We evaluated the performance of the Quasi-Analytical Algorithm (QAA, v5 with modifications) for deriving the spectral total absorption, a(λ), and backscattering, bb(λ), coefficients of seawater and partitioning of a(λ) into phytoplankton and non-phytoplankton components from input spectrum of remote-sensing reflectance, Rrs(λ), with field data collected in the Arctic and lower-latitude open waters from the Atlantic and Pacific Oceans. The systematic error based on median ratio between QAA-derived and measured a(λ) varied from about 1% to ±10% depending on light wavelength and the oceanic region. The QAA typically overestimated bb(λ) from 3% to 14% compared with field measurements. These results were obtained with a correction for Raman-scattering contribution to Rrs and separate parameterization of molecular and particulate backscattering in the Rrs vs. bb/a relationship. Without these features the earlier versions of the QAA can overestimate bb(λ) by as much as 35% in clear waters. The use of pure seawater backscattering coefficients accounting for water temperature and salinity improved the accuracy of QAA-derived a(λ) in Arctic waters. The absorption-partitioning component of the QAA significantly underestimated phytoplankton absorption and overestimated non-phytoplankton absorption in both Arctic and lower-latitude waters.

Simple statistical formulas for estimating biogeochemical properties of suspended particulate matter in the southern Baltic Sea potentially useful for optical remote sensing applications

Simple statistical formulas for estimating various biogeochemical properties of suspended particulate matter in the southern Baltic Sea are presented in this paper. These include formulas for estimating mass concentrations of suspended particulate matter (SPM), particulate organic matter (POM), particulate organic carbon (POC) and total chlorophyll a (Chl a). Two different approaches have been adopted. The first approach was to use the available empirical material (the results of field measurements and laboratory analyses of discrete water samples) and find statistical formulas for estimating the biogeochemical properties of suspended particulate matter from those of inherent optical properties (IOPs), which are potentially retrievable from remote sensing measurements. The second approach was to find formulas that would enable biogeochemical properties of suspended particulate matter to be estimated directly from spectral values of the remote-sensing reflectance Rrs. The latter was based on statistical analyses of a synthetic data set of Rrs obtained from numerical simulations of radiative transfer for which the available empirical material on seawater IOPs and biogeochemistry served as input data. Among the empirical formulas based on seawater IOPs that could be used as a step in two-stage remote sensing algorithms (the other step is estimating certain IOPs from reflectance), the best error statistics are found for estimates of SPM and POM from the particulate backscattering coefficient bbp in the blue region of light wavelengths (443nm), and for estimates of POC and Chl a from the coefficient of light absorption by the sum of all non-water (i.e. suspended and dissolved) constituents of seawater an, in the blue (443nm) and green (555nm) parts of the spectrum respectively. For the semi-empirical formulas under consideration, which could serve as starting points in the development of local onestage (direct) remote sensing algorithms, the best error statistics are found when SPM, POM and POC are estimated from the same blue-to-red band reflectance ratio (Rrs(490)/Rrs(645)) (with estimated SPM reaching a better precision than estimated POM and POC), and when Chl a is estimated from the green-to-red band ratio (Rrs(555)/Rrs(645)).

Evaluation of a compact sensor for backscattering and absorption

Seawater inherent optical properties (IOPs) are key parameters in a wide range of applications in environmental studies and oceanographic research. In particular, the absorption coefficient (a) is the typical IOP used to obtain the concentration of chlorophyll-a in the water-a critical parameter in biological oceanography studies and the backscattering coefficient (b(b)) is used as a measure of turbidity. In this study, we test a novel instrument concept designed to obtain both the absorption and backscattering coefficients. The instrument would emit a collimated monochromatic light beam into the water retrieving the backscattered light intensity as a function of distance from the center of illumination. We use Monte Carlo modeling of light propagation to create an inversion algorithm that translates the signal from such an instrument into values of a and b(b). Our results, based on simulations spanning the bulk of natural values of seawater IOP combinations, indicate that a 6.2 cm diameter instrument with a radial resolution of 1 cm would be capable of predicting b(b) within less than 13.4% relative difference and a within less than 57% relative difference (for 90% of the inverted a values, the relative errors fall below 29.7%). Additionally, these errors could be further reduced by constraining the inversion algorithm with information from concurrent measurements of other IOPs. Such a compact and relatively simple device could have multiple applications for in situ optical measurements, including a and b(b) retrievals from instrumentation mounted on autonomous underwater vehicles. Furthermore, the same methodology could possibly be used for an out-of-water sensor.

Spectral variations in the near-infrared ocean reflectance

The optical properties of natural waters beyond the visible range, in the near-infrared (NIR, 700–900 nm), have received little attention because they are often assumed to be mostly determined by the large absorption coefficient of pure water, and because of methodological difficulties. It is now growingly admitted that the NIR represents a potential optical source of unambiguous information about suspended sediments in turbid waters, thence the need for better understanding the NIR optical behaviour of such waters. It has recently been proposed (Ruddick et al., Limnology and Oceanography. 51, 1167–1179, 2006) that the variability in the shape of the surface ocean reflectance spectrum in the NIR is negligible in turbid waters. In the present study, we show, based on both in situ and remote sensing data, that the shape of the ocean reflectance spectrum in the NIR does vary in turbid to extremely turbid waters. Space-borne ocean reflectance data were collected using 3 different sensors (SeaWiFS, MODIS/Aqua and MERIS) over the Amazon, Mackenzie and Rio de la Plata turbid river plumes during extremely clear atmospheric conditions so that reliable removal of gas and aerosol effects on reflectance could be achieved. In situ NIR reflectance data were collected in different European estuaries where extremely turbid waters were found. In both data sets, a flattening of the NIR reflectance spectrum with increasing turbidity was observed. The ratio of reflectances at 765 nm and 865 nm, for instance, varied from ca. 2 down to 1 in our in situ data set, while a constant value of 1.61 had been proposed based on theory in a previous study. Radiative transfer calculations were performed using a range of realistic values for the seawater inherent optical properties, to determine the possible causes of variations in the shape of the NIR reflectance spectrum. Based on these simulations, we found that the most significant one was the gradual increase in the contribution of suspended sediments to the color of surface waters, which often leads to the flattening of the reflectance spectrum. Changes in the scattering and absorption properties of particles also contributed to variations in the shape of the NIR surface ocean reflectance spectrum. The impact of such variations on the interpretation of ocean color data is discussed.

Monte Carlo code for high spatial resolution ocean color simulations

A Monte Carlo code for ocean color simulations has been developed to model in-water radiometric fields of downward and upward irradiance (E(d) and E(u)), and upwelling radiance (L(u)) in a two-dimensional domain with a high spatial resolution. The efficiency of the code has been optimized by applying state-of-the-art computing solutions, while the accuracy of simulation results has been quantified through benchmark with the widely used Hydrolight code for various values of seawater inherent optical properties and different illumination conditions. Considering a seawater single scattering albedo of 0.9, as well as surface waves of 5 m width and 0.5 m height, the study has shown that the number of photons required to quantify uncertainties induced by wave focusing effects on E(d), E(u), and L(u) data products is of the order of 10(6), 10(9), and 10(10), respectively. On this basis, the effects of sea-surface geometries on radiometric quantities have been investigated for different surface gravity waves. Data products from simulated radiometric profiles have finally been analyzed as a function of the deployment speed and sampling frequency of current free-fall systems in view of providing recommendations to improve measurement protocols.

Relationship between the irradiance reflectance and inherent optical properties of seawater

A new expression for relationship between the surface irradiance reflectance and Inherent Optical Properties (IOPs) of homogeneous seawater is presented. Physical parameters largely responsible for the relationship are identified. These are the average cosines of the backward volume scattering function and of the downward radiance. Lorenz‐Mie computations show that the average cosine of the backward volume scattering function is quasi‐constant (0.493 ± 0.135) for most marine waters, even if the backward volume scattering function itself is considerably variable. As a result, a base magnitude of the proportionality between the irradiance reflectance and IOPs are largely set by the average cosine of the backward volume scattering function and eventually determined by the average cosine of the downward radiance that is primarily driven by the sun angle. Independent numerical simulations of radiative transfer support results of the new expression to show that it gives significant improvement in both physical understanding and numerical prediction of the relationship.

Multiparameter retrieval of water optical properties from above-water remote-sensing reflectance using the simulated annealing algorithm

Water-leaving radiance, measured just above the ocean surface, contains important information about near-surface or subsurface processes that occur on or below the deep ocean and coastal water. As such, retrieving seawater inherent optical properties (IOPs) is an important step to determining water type, subsurface light field, turbidity, pigment concentration, and sediment loading. However, the retrieval (or inversion) of seawater IOPs from just above water radiance measurements is a multiparameter nonlinear problem that is difficult to solve by conventional optimization methods. The applicability of the simulated annealing algorithm (SA) is explored as a nonlinear global optimizer to solve this multiparameter retrieval problem. The SA algorithm is combined with widely known semianalytical relations for seawater's IOPs to parameter invert these properties from simulated and measured water-leaving reflectance spectra. Furthermore, given the versatility of the SA algorithm, the scheme is extended to retrieve water depth from input reflectance data. Extensive tests and comparisons with in situ and simulated data sets compiled by the International Ocean-Color Coordinating Group are presented. Field data include reflectance spectra acquired with a handheld GER 1500 spectroradiometer and absorption measurements, performed with the AC-9 instrument on waters around Singapore's nearby islands.

Deep seawater inherent optical properties in the Southern Ionian Sea

The NEMO (NEutrino Mediterranean Observatory) Collaboration has been carrying out since 1998 an evaluation programme of deep sea sites suitable for the construction of the future Mediterranean km 3 Čerenkov neutrino telescope. We investigated the seawater optical and oceanographic properties of several deep sea marine areas close to the Italian Coast. Inherent optical properties (light absorption and attenuation coefficients) have been measured as a function of depth using an experimental apparatus equipped with standard oceanographic probes and the commercial transmissometer AC9 manufactured by WETLabs. This paper reports on the visible light absorption and attenuation coefficients measured in deep seawater of a marine region located in the Southern Ionian Sea, 60–100 km SE of Capo Passero (Sicily). Data show that blue light absorption coefficient is about 0.015 m −1 (corresponding to an absorption length of 67 m) close to the one of optically pure water and it does not show seasonal variation.