Optical properties of the particles in the Crimea coastal waters (Black Sea)

A field experiment was carried out in summer 2002 on an oceanographic platform near the coast of Crimea, in the Black Sea. For the first time, the spectral volume scattering function (VSF) was measured for a wide range of scattering angles (i.e., from 0.6 to 177.3 degrees) using a recently developed device. Our analysis revealed that the mineral particles are the primary component influencing the scattering and backscattering coefficient in the study area. The good correlation obtained between the backscattering coefficient bbp and the nonalgal particles absorption coefficient showed that the absorption efficiency of the mineral particles is high in the second half of the experiment. The ratio Chla/cp (where Chla is the chlorophyll a concentration and cp is the beam attenuation coefficient) did not correlate with the backscattering ratio and thus could not be used in this experiment as an alternative proxy to estimate the bulk composition of the particles. The spectral variation of bp (the scattering coefficient) and bbp (the backscattering coefficient) was less steep than what can be found in the open ocean waters. That was explained by the influence of the absorption on the scattering process, especially in the blue, as a consequence of the anomalous dispersion. The average backscattering ratio bp varied spectrally within 4%. Nevertheless, a high spectral variability of bp (around 30%) was observed suggesting that the use of a flat spectral variation is not accurate in coastal zones.

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title: Environmental changes in the western Black Sea related to anthropogenic and natural conditions
abstract: The Black Sea northwestern shelf (NWS) ecosystem has been subjected to the strongest anthropogenic pressure of the entire Black Sea as about 80% of the freshwater runoff is discharged there. This paper presents a review of the global environmental changes related mainly to increased eutrophication in the western Black Sea basin. A case study (CoMSBlack 92a cruise) attempts to highlight the interaction of some natural and anthropogenic factors responsible for specific chemical and biological features in the western Black Sea environment. The significance of processes located near the Danube river mouth, which, due to their relatively small space scale, have not been recognized before, is clarified. Hydrological processess of specific importance to the distribution and transformation of antropogenic inputs are river plume dynamics, coastal upwelling and mixing and downwelling over the shelf break and slope in this part of the basin. A layer of low hydrological variability (“conservative layer”) appears to be a natural feature of the area, hypothesized to precondition specific biological and chemical processes there. The results suggest that the interplay between the Danube anthropogenic nutrient load with the natural hydrological fronts and gradients provides opportunities for enhanced biological activity thus contributing to the global environmental changes in the Black Sea NWS.

title: Environmental degradation of the Black Sea : challenges and remedies
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title: The Nutrient Stock of the Romanian Shelf of the Black Sea During the Last Three Decades
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title: Eutrophication: A Plausible Cause for Changes in Hydrochemical Structure of the Black Sea Anoxic Layer
abstract: Field observations have been used to analyze changes in chemical properties (distribution of nutrients and sulfide) of the Black Sea anoxic layer from 1960 to 1995. The results reveal notable changes (increase in the inventories of nutrient and sulfide) in the chemical structure of the anoxic zone, which has been considered for a long time as a conservative layer of the Black Sea. It is inferred that intensive eutrophication is the main reason of the change. The results of the analysis of long-term variations in chemical structure demonstrate not only intensive degradation of the ecosystem of the Black Sea in 70’s and 80’s, but also relative improvement of the situation in 90’s.

title: Boundary current instabilities, upwelling, shelf mixing and eutrophication processes in the Black Sea
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title: Patterns of Seasonal and Interannual Changes of Surface Chlorophyll Concentration in the Black Sea Revealed from the Remote Sensed Data
abstract: Several years of CZCS-measured surface pigment’s concentrations in the Black Sea are analyzed to appraise the seasonal and year-to-year fluctuations of phytoplankton biomass and understand the causes of these fluctuations in terms of the Black Sea’s general dynamics. The pattern of seasonal variations is typical for subtropical rather than temperate regions. The range of the absolute value of plant pigment surface concentration measured by remote sensing does not differ greatly from the values measured by direct methods. The pattern of year-to-year variations seems to correlate with cyclic oscillations of winter air temperature. In western shallow regions it is also correlated with the Danube discharge intensity. More intensive winter–spring blooms and a slightly lower level of pigment concentration during warm season are typical for years of with a mild winter. The causes of these regularities seem to be the peculiarities of hydrological and meteorological regimes of the Black Sea. The intensity of winter–spring bloom of phytoplankton appears to depend on hydrological mechanism (i.e., the intensity of water mixing during winter period due to thermic convection and wind mixing) rather than the illumination intensity.

title: Water Constituents in the North-western Black Sea from Optical Remote Sensing and In situ Data
abstract: Satellite-based optical observations of surface waters have been used to assess the main environmental interactions in the north-western Black Sea, as a contribution to the EROS 21 project. Such observations allow evaluations of the presence and abundance of water constituents (primarily phytoplankton pigments), providing essential information on the processes which are taking place in the area and on their spatial and temporal scales. An analysis of ocean colour imagery was performed, using historical data collected by the CZCS (1978–1986), and by the MOS before, during and after the EROS 21 oceanographic cruise which took place in April/May 1997. The time series of CZCS-derived parameters (i.e. chlorophyll-like pigment concentration) originates from the archive generated by the OCEAN project. The MOS data were processed to apply sensor calibration, to correct for atmospheric contamination so as to assess water-leaving radiances for each visible channel, and to estimate geophysical parameters such as pigment concentration and in-water optical depth, which is correlated to suspended matter concentration. The atmospheric correction was performed with a novel algorithm developed especially for MOS application. The bio-optical algorithms used to derive in-water parameters were obtained by comparison with the concurrent in situ measurements of optically active parameters collected in the north-western Black Sea in the framework of the EROS 21 project. The multi-satellite data set highlights the differences between western and eastern sub-basins, inshore and offshore domains, northern and southern near-coastal areas. In the Danube delta area, the water constituents trace complex interactions of near-coastal and basin-wide features of the Black Sea.

title: A New Method for the Measurement of the Optical Volume Scattering Function in the Upper Ocean
abstract: A new method to measure the optical volume scattering function (VSF) of seawater is presented. The VSF is a fundamental property used in the calculation of radiative transfer for applications as diverse as upper-ocean heating by solar radiation to laser ranging of the sea bottom. The approach differs from traditional ones and involves use of a special periscope prism that allows the direct determination of the VSF over a wide range of angles (0.6°–177.3°) with an angular resolution of 0.3°. Measurements taken in the laboratory using Barnstead International, Inc., Nanopure water, cleaned seawater, and known additions of defined scatterers indicate close correspondence between experimental data and theoretical simulations based on Mie theory over the angle range from 12° to 170°. Field deployments in the Atlantic Ocean continental shelf water are shown to produce high quality measurements of the VSF in the angle range from 0.6° to 177.3°; such observations have not been available before. The new da...

title: Ocean Optics Protocols For Satellite Ocean Color Sensor Validation, Revision 2
abstract: This document stipulates protocols for measuring bio-optical and radiometric data for the SIMBIOS Project. It supersedes the earlier version, and is organized into four parts: Introductory Background, Instrument Characteristics, Field Measurements and Data Analysis, Data Reporting and Archival. Changes in this revision include the addition of three new chapters: (1) Fundamental Definitions, Relationships and Conventions; (2) MOBY, A Radiometric Buoy for Performance Monitoring and Vicarious Calibration of Satellite Ocean Color Sensors: Measurement and Data Analysis Protocols; and (3) Normalized Water-Leaving Radiance and Remote Sensing Reflectance: Bidirectional Reflectance and Other Factors. Although the present document represents another significant, incremental improvement in the ocean optics protocols, there are several protocols that have either been overtaken by recent technological progress, or have been otherwise identified as inadequate. Revision 4 is scheduled for completion sometime in 2003. This technical report is not meant as a substitute for scientific literature. Instead, it will provide a ready and responsive vehicle for the multitude of technical reports issued by an operational Project. The contributions are published as submitted, after only minor editing to correct obvious grammatical or clerical errors.

title: MEASUREMENT OF VISIBLE LIGHT ABSORPTION BY PARTICULATE MATTER IN THE OCEAN1
abstract: For the measurement of visible light absorption by particles in sea water, optical arrangements have been devised where scattered light is integrated and the photodetector “sees” mostly diffused light. High spectral resolution has been obtained by removing particulate matter from sea water using a membrane filter and scanning the filter against an unused filter as the reference blank in the Cary spectrophotometer. In this fashion spectral curves for particulate matter in the oceans have been obtained. These curves show pigment banding in the upper 100 m of water characteristic of absorption by chloroplastic pigments. Below this depth the absorption of particulate matter is characterized by a gradual increasing attenuation from long to short wavelengths. Optical density ratios of chloroplastic pigment bands seen in the particulate matter of the upper 100 m suggest the presence of absorbing components not generally seen in algal cultures. 
 
Differential filtration through graded filter sizes (5.0 µ-0.3 µ) followed by the spectral scanning of filters, shows that practically all of the chloroplastic pigments are retained by the 5.0 µ filter. Particles passing this filter show little selective attenuation of visible wavelengths. 
 
The nature of sea water particulate matter is discussed in terms of its absorbing characteristics as well as the significance to phytoplankton ecology.

title: Chlorophyll α specific absorption and fluorescence excitation spectra for light-limited phytoplankton
abstract: Methods are described for the measurement of spectral absorption coefficients, fluorescence excitation, and fluorescence yields for pigmented particles retained on filters. The corrections required for absorption coefficients include determining increased optical pathlength while corrections for fluorescence include determining system spectral variability, mean light level and reabsorption. The empirical technique is consistent with and validated by theoretical relationships for light transmission and fluorescence of absorbing particulate material embedded in a medium with intense scattering.These methods were applied to a study of photoadaptation in several phytoplankton species and revealed variations in the blue for chlorophyll α specific absorption [αph*(λ)] and fluorescence excitation [F*(λ)] of greater than 3− and 10-fold, respectively. Variations in the spectral shapes and the magnitude of αph*(λ) and F*(λ) with photoadaptation are determined largely by the effect of pigment absorption in discrete particles, sometimes referred to as the sieve or package effect. A model is presented expressing F*(λ) in terms of α*(λ) which predicts large variability in F*(λ) due to cell size and cellular pigmentation and which may help reconcile the previously reported, but unexplained variations in F*(λ). Spectral variations in the fluorescence yield appear to be caused by variations in the fraction of light absorbed by photosystem II which fluoresces as compared to photosystem I or photoprotective pigments which do not fluoresce. The techniques presented provide a rapid, reproducible, and simple approach for routine analysis, particularly for field applications where particle densities are too low for direct analysis of absorption spectra.

title: Particulate backscattering ratio at LEO 15 and its use to study particle composition and distribution
abstract: Particulate scattering and backscattering are two quantities that have traditionally been used to quantify in situ particulate concentration. The ratio of the backscattering by particles to total scattering by particles (the particulate backscattering ratio) is weakly dependent on concentration and therefore provides us with information on the characteristics of the particulate material, such as the index of refraction. The index of refraction is an indicator of the bulk particulate composition, as inorganic minerals have high indices of refraction relative to oceanic organic particles such as phytoplankton and detrital material that typically have a high water content. We use measurements collected near the Rutgers University Long-term Ecosystem Observatory in 15 m of water in the Mid-Atlantic Bight to examine application of the backscattering ratio. Using four different instruments, the HOBILabs Hydroscat-6, the WETLabs ac-9 and EcoVSF, and a prototype VSF meter, three estimates of the ratio of the particulate backscattering ratio were obtained and found to compare well. This is remarkable because these are new instruments with large differences in design and calibration. The backscattering ratio is used to map different types of particles in the nearshore region, suggesting that it may act as a tracer of water movement. We find a significant relationship between the backscattering ratio and the ratio of chlorophyll to beam attenuation. This implies that these more traditional measurements may be used to identify when phytoplankton or inorganic particles dominate. In addition, it provides an independent confirmation of the link between the backscattering ratio and the bulk composition of particles.

title: Phase function effects on oceanic light fields.
abstract: Numerical simulations show that underwater radiances, irradiances, and reflectances are sensitive to the shape of the scattering phase function at intermediate and large scattering angles, although the exact shape of the phase function in the backscatter directions (for a given backscatter fraction) is not critical if errors of the order of 10% are acceptable. We present an algorithm for generating depth- and wavelength-dependent Fournier-Forand phase functions having any desired backscatter fraction. Modeling of a comprehensive data set of measured inherent optical properties and radiometric variables shows that use of phase functions with the correct backscatter fraction and overall shape is crucial to achieve model-data closure.

title: Relationship of light scattering at an angle in the backward direction to the backscattering coefficient.
abstract: We revisit the problem of computing the backscattering coefficient based on the measurement of scattering at one angle in the back direction. Our approach uses theory and new observations of the volume scattering function (VSF) to evaluate the choice of angle used to estimate b(b). We add to previous studies by explicitly treating the molecular backscattering of water (b(bw)) and its contribution to the VSF shape and to b(b). We find that there are two reasons for the tight correlation between observed scattering near 120 degrees and the backscattering coefficient reported by Oishi [ Opt. 29, 4658, (1990)], namely, that (1) the shape of the VSF of particles (normalized to the backscattering) does not vary much near that angle for particle assemblages of differing optical properties and size, and (2) the ratio of the VSF to the backscattering is not sensitive to the contribution by water near this angle. We provide a method to correct for the water contribution to backscattering when single-angle measurements are used in the back direction (for angles spanning from near 90 degrees to 160 degrees ) that should provide improved estimates of the backscattering coefficient.

title: Variations of light absorption by suspended particles with chlorophyll a concentration in oceanic (case 1) waters: Analysis and implications for bio-optical models
abstract: Spectral absorption coefficients of total particulate matter ap(λ) were determined using the in vitro filter technique. The present analysis deals with a set of 1166 spectra, determined in various oceanic (case 1) waters, with field chl a concentrations (〈chl〉) spanning 3 orders of magnitude (0.02–25 mg m−3). As previously shown [Bricaud et al., 1995] for the absorption coefficients of living phytoplankton ao(λ), the ap(λ) coefficients also increase nonlinearly with 〈chl〉. The relationships (power laws) that link ap(λ) and ao(λ) to 〈chl〉 show striking similarities. Despite large fluctuations, the relative contribution of nonalgal particles to total absorption oscillates around an average value of 25–30% throughout the 〈chl〉 range. The spectral dependence of absorption by these nonalgal particles follows an exponential increase toward short wavelengths, with a weakly variable slope (0.011±0.0025 nm−1). The empirical relationships linking ap(λ) to (〈chl〉) can be used in bio-optical models. This parameterization based on in vitro measurements leads to a good agreement with a former modeling of the diffuse attenuation coefficient based on in situ measurements. This agreement is worth noting as independent methods and data sets are compared. It is stressed that for a given (〈chl〉), the ap(λ) coefficients show large residual variability around the regression lines (for instance, by a factor of 3 at 440 nm). The consequences of such a variability, when predicting or interpreting the diffuse reflectance of the ocean, are examined, according to whether or not these variations in ap are associated with concomitant variations in particle scattering. In most situations the deviations in ap actually are not compensated by those in particle scattering, so that the amplitude of reflectance is affected by these variations.

title: Regional models for phytoplankton absorption as a function of chlorophyll a concentration
abstract: Empirical relationships for predicting phytoplanktonic absorption at 676 and 436 nm from water column chlorophyll a concentration are presented for distinct geographic regions defined by latitude. The forms of the predictive equations are controlled by underlying biological mechanisms and lend insight into these mechanisms. These region-specific models allow prediction of phytoplanktonic absorption from more easily measured parameters such as chlorophyll a concentration or in situ fluorescence and increase accuracy in modeling optical properties or primary production rates from specific absorption coefficients. Phytoplanktonic absorption can be predicted from chlorophyll a concentrations estimated from satellite-based ocean color measurements. Temperate and tropical regions exhibited statistically indistinguishable relationships at low chlorophyll so these regions were combined and treated as one. Nonlinear relationships between phytoplanktonic absorption at both 436 and 676 nm and chlorophyll a concentration for the combined temperate/tropical region suggested that pigment packaging effects were important and variable. Higher slopes between absorption and chlorophyll a at low chlorophyll supported the concept of low pigment packaging effects (thus higher specific absorption) in oligotrophic, low chlorophyll a waters. Subpolar waters displayed a distinct pattern and were defined as a separate region. Near-linear and linear relationships between phytoplanktonic absorption at 436 and 676 nm and chlorophyll a concentration indicated that influences of pigment packaging on phytoplankton specific absorption coefficients were relatively constant and uncoupled from water column chlorophyll a concentration in the subpolar region. Optical depth correlated inversely with specific absorption at 436 nm in the subpolar region, illustrating the role of photoadaptation in determining specific absorption and predictive relationships. Differences between predictive quadratic equations for particulate and phytoplanktonic absorption indicated that detritus made only a small contribution to the nonlinear nature of the relationships. Absorption by detritus at 436 nm ranged from 25 to 90% of the total particulate absorption. The proportion of absorption by detritus did not exhibit any strong patterns as a function of chlorophyll a concentration, arguing against the previously invoked explanation that proportional increases in absorption by detritus as environments become more oligotrophic cause the observed nonlinearity between absorption and chlorophyll a concentration.

title: Volume Scattering Functions for Selected Ocean Waters
abstract: Volume scattering functions for three general types of natural ocean waters have been obtained and are presented here. The three types of water are (1) deep clear oceanic water, (2) nearshore ocean water, and (3) very turbid harbor water. Also included are the results of laboratory experiments using sea water, filtered fresh water, and artificial scattering and absorbing agents. The beam transmission was obtained for all waters investigated. A brief description of the instruments used is given.

title: Effect of the particle-size distribution on the backscattering ratio in seawater.
abstract: Mie theory is used to model the backscattering ratio (the ratio of the backscattering coefficient to the total scattering coefficient) of marine particles with the assumption that they follow a Junge-type size distribution. Results show that the backscattering ratio is very sensitive to the presence of submicrometer particles and depends strongly on the shape of the size distribution. However, it is not affected significantly by absorption and does not vary with wavelength over the visible range. The implications for modeling of backscattering and ocean color in terms of phytoplankton pigment concentration are discussed.

title: A model for estimating bulk refractive index from the optical backscattering ratio and the implications for understanding particle composition in case I and case II waters
abstract: A model based on Mie theory is described that estimates bulk participate refractive index n¯p from in situ optical measurements alone. Bulk refractive index is described in terms of the backscattering ratio and the hyperbolic slope of the particle size distribution (PSD). The PSD slope ξ is estimated from the hyperbolic slope of the particulate attenuation spectrum γ according to the relationship γ ≈ ξ − 3, verified with Mie theory. Thus the required in situ measurements are the particulate backscattering coefficient, the total particulate scattering coefficient, and the particulate attenuation coefficient. These parameters can be measured with commercially available instrumentation with rapid sampling rates and real-time data return. Application of the model to data from the Gulf of California yielded results that agreed with expectations, e.g., predicted n¯p was 1.04–1.05 in the chlorophyll maximum and 1.14–1.18 near sediments. Below the chlorophyll maximum in case I type waters, predicted n¯p values were between 1.10 and 1.12, suggesting the presence of a significant inorganic mineral component in the background or detrital organic particles with low water content.

title: Theoretical results concerning light absorption in a discrete medium, and application to specific absorption of phytoplankton
abstract: The radiation absorption within a medium supposedly non-absorbing in itself but containing an absorbing substance as particles is examined theoretically. Absorption by such particles can be treated independently of (Mie) scattering under an assumption concerning the complex refractive index that is reasonable in case of algal cells (index close to that of water and weakly absorbing substance). The effect of discreteness on absorption properties is ruled by a function of the dimensionless parameter that combines through their product the size (d) and the absorption coefficient of the cell material (acm). The limiting value of this function (d → 0) describes the case of a true solution of the same material, whereas its variations with d and acm imply that the specific absorption coefficient, for a given substance, is variable in magnitude and in spectral behaviour. Consequently, Beer's law, which rests on the existence of a constant specific coefficient, generally cannot apply when canopy changes intervene in an algal population. Various cases of such changes are studied along with their consequences on absorptive properties. 
 
The theoretical conclusions are exemplified by some experimental results concerning algal cultures. The absorption properties of the cell material for each species can be extrapolated from the actual absorption spectra of intact cells. 
 
Problems that originate from the non-constancy of the specific absorption coefficient (spectral values or mean value) are examined in view of two kinds of applications: the photosynthetic (quantum) yield evaluation and the algal biomass assessment by remote sensing, for which the constancy is generally postulated.

title: Light backscattering efficiency and related properties of some phytoplankters
abstract: By using a set-up that combines an integrating sphere with a spectroradiometer LI-1800 UW, the backscattering properties of nine different phytoplankters grown in culture have been determined experimentally for the wavelengths domain ν = 400 up to 850 nm. Simultaneously, the absorption and attenuation properties, as well as the size distribution function, have been measured. This set of measurements allowed the spectral values of refractive index, and subsequently the volume scattering functions (VSF) of the cells, to be derived, by operating a scattering model previously developed for spherical and homogeneous cells. The backscattering properties, measured within a restricted angular domain (approximately between 132 and 174°), have been compared to theoretical predictions. Although there appear some discrepancies between experimental and predicted values (probably due to experimental errors as well as deviations of actual cells from computational hypotheses), the overall agreement is good; in particular the observed interspecific variations of backscattering values, as well as the backscattering spectral variation typical of each species, are well accounted for by theory. Using the computed VSF, the measured backscattering properties can be converted (assuming spherical and homogeneous cells) into efficiency factors for backscattering (Qbb). Thhe spectral behavior of Qbb appears to be radically different from that for total scattering Qb. For small cells, Q (λ) is practically constant over the spectrum, whereas Qb(λ) varies approximately according to a power law (λ−2). As the cell size increases, Qbb conversely, becomes increasingly featured, whilst Qb becomes spectrally flat. The chlorophyll-specific backscattering coefficients (bb∗ appear highly variable and span nearly two orders of magnitude. The chlorophyll-specific absorption and scattering coefficients, a∗ and b∗, are mainly ruled by the interspecific variations in cellssize (D) and intracellular pigment concentration (Ci) (actually by the variations of the product DCi). Though bb∗ is involved in the modelling of the diffuse reflectance of waters, the impact of its actual variation is greatly limited because typical bb∗ values, even at their maximum (10−3 m2 mg−1), are very low. This result confirms that living algae have a negligible influence on the backscattering process by oceanic waters; other particles (bacteria, detritus, etc.) associated with algae are mainly responsible for this process.

title: Scattering and Polarization Properties of Polydispersed Suspensions with Partial Absorption
abstract: The angular scattering and polarization properties of polydispersed suspensions of nonabsorbing and partially absorbing spheres have been computed using the complete Mie series. The results with three types of size distributions are presented and compared with observations. These show a strong dependence of angular intensity and polarization patterns on the size distribution, the size range, and the dielectric and absorbing properties of the individual particles. A peculiarity of scattering at angles near 45 degrees, observed experimentally and independently by two authors, is corroborated by the numerical results. Prominent observational features characteristic of natural fog, such as an extremely bright aureole, rainbows and counter coronas are reproduced in a model corresponding to a cloud of spherical water droplets, with a wide distribution in droplet radius and a maximum concentration at a 4-micron radius.

title: Light scattering and suspended particles in the eastern equatorial Pacific Ocean
abstract: The total light scattering coefficient, the volume scattering function at 45°, and the suspended particle size distribution have been determined at the surface, at the depth of maximum turbidity, and at 100 meters at several stations in the eastern equatorial Pacific. The total scattering coefficient and volume scattering function were also measured at several other depths. Total light scattering was measured in situ with a b meter, the volume scattering function was measured in the ship's laboratory with a Brice-Phoenix light scattering photometer, and particle size distributions were determined in the ship's laboratory with a Coulter Counter. The total scattering coefficient is directly proportional to the volume scattering function at 45°. The total number of observed particles, total particulate cross section, and total particulate volume per cubic centimeter of sea water were computed from the measured particle size distributions. Both the total scattering coefficient and the 45° volume scattering function are roughly proportional to these properties of the suspended particles. A significant fraction of the light scattering is believed to be due to particles smaller than 1 μ, which were not measurable by the Coulter Counter.

title: Significant relationship between the backward scattering coefficient of sea water and the scatterance at 120 degrees.
abstract: Computations of Mie scattering by marine particles indicate that the ratios between the backward scattering coefficient and the scatterance at 120 degrees are ~6.9 +/- 0.4, 7.2 +/- 0.6, and 7.1 +/- 0.5 for Junge, normal and log-normal particle size distributions, respectively. In situ observations of the scattering confirm this result.

title: Instruments and methods for measuring the backward-scattering coefficient of ocean waters.
abstract: The backward-scattering coefficient bb is an important optical property that plays a central role in studies of ocean-color remote sensing, suspended particle distributions, water clarity, and underwater visibility. We investigate the fixed-angle backscattering sensor approach for the application of measuring bb . Analysis shows that the sensor response to volume scattering can be expressed as the integral of the volume scattering function (VSF) over the backward angles (90 -180 degrees ) weighted by the sensor-response function. We present a procedure for determining the sensor-response function that contains all the information necessary to calibrate the sensor fully to measure the VSF at a nominal backscattering angle. It is shown that, for fixed-angle backscattering sensors,bb is most accurately estimated when the sensor-response function covers the middle range of backscattering angles, roughly 110 -160 degrees , where the shape of the VSF has the least variability. Backscattering at and near the end angles, namely, 90 degrees and 180 degrees , are least correlated with bb . We describe a variety of spectral backscattering sensors that we have developed, and we present their sensor-response functions.

title: Neural network for emulation of an inverse model: operational derivation of Case II water properties from MERIS data
abstract: An algorithm was developed to derive the concentrations of phytoplankton pigment, suspended matter and gelbstoff, and the aerosol path radiance from 'Rayleigh corrected' top-of-atmosphere reflectances over turbid coastal waters. The procedure is designed for MERIS, the Medium Resolution Imaging Spectrometer, which will be flown onboard the Earth observation satellite Envisat of the European Space Agency (ESA). The algorithm is a neural network (NN) which is used to parameterize the inverse of a radiative transfer model. It is used in this study as a multiple nonlinear regression technique. The NN is a feedforward backpropagation model with two hidden layers. The NN was trained with computed reflectances covering the range of 0.5-50mugl-1 phytoplankton pigment, 1-100mgl-1 suspended matter, gelbstoff absorption at 420nm of 0.02-2m-1 and a horizontal visibility of 2-50km. Inputs to the NN are the reflectances of the 16 spectral channels which were under discussion for MERIS. The outputs are the three water c...

title: Artificial neural networks for modeling the transfer function between marine reflectance and phytoplankton pigment concentration
abstract: A neural network methodology is developed to estimate the near-surface phytoplankton pigment concentration of case I waters from spectral marine reflectance measurements (ocean color) at the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) visible wavelengths. The advantages of neural network approximation, i.e., association of nonlinear complexity, smoothness, and reduced sensitivity to noise, are demonstrated. When applied to in situ California Cooperative Oceanic Fisheries Investigations data, the neural network algorithm performs better than the reflectance ratio algorithms. Relative rms errors on pigment concentration are reduced from 61 and 62 to 38%, and absolute rms errors are reduced from 4.43 and 3.52 to 0.83 mg m 23 . When applied to SeaWiFS- derived imagery, there is statistical evidence that the neural network algorithm filters residual atmospheric correction errors more efficiently than the standard SeaWiFS bio- optical algorithm.

title: Inversion of oceanic constituents in case I and II waters with genetic programming algorithms.
abstract: A stochastic inverse technique based on agenetic programming (GP) algorithm was developed toinvert oceanic constituents from simulated data for case I and case II water applications. The simulations were carried out with the Ordre Successifs Ocean Atmosphere (OSOA) radiative transfer model. They include the effects of oceanic substances such as algal-related chlorophyll, nonchlorophyllous suspended matter, and dissolved organic matter. The synthetic data set also takes into account the directional effects of particles through a variation of their phase function that makes the simulated data realistic. It is shown that GP can be successfully applied to the inverse problem with acceptable stability in the presence of realistic noise in the data. GP is compared with neural network methodology for case I waters; GP exhibits similar retrieval accuracy, which is greater than for traditional techniques such as band ratio algorithms. The application of GP to real satellite data [a Sea-viewing Wide Field-of-view Sensor (SeaWiFS)] was also carried out for case I waters as a validation. Good agreement was obtained when GP results were compared with the SeaWiFS empirical algorithm. For case II waters the accuracy of GP is less than 33%, which remains satisfactory, at the present time, for remote-sensing purposes.

title: Optical modeling of the upper ocean in relation to its biogenous matter content (case I waters)
abstract: The aim of the present study is to review and tentatively to interpret the optical behavior of oceanic case I waters, those waters for which phytoplankton and their derivative play a predominant role in determining their optical properties. Chlorophyll-like pigment concentration is used as the index to quantify the algal material (living and detrital), and statistical relationships between this index and the depth of the euphotic layer, the spectral values of the attenuation coefficient for downwelling irradiance, or the scattering coefficient are investigated. On the basis of these statistical relationships a pigment-dependent optical model is developed. It allows the propagation of the visible radiant energy within the ocean or the backscattered radiation from the upper layer to be predicted as a function of the local phytoplanktonic content. Other geophysical or geochemical applications are derived which concern the heating rate due to penetrating visible radiations or the rate of energy storage due to photosynthesis. The nonlinear trends observed in the algal biomass-attenuation relationships are analyzed by (1) considering the rather regular change of the living-to-detrital organic carbon ratio which seems to occur in oceanic waters ranging from oligotrophic to eutrophic, and (2) accounting for the respective contributions of absorption (by pigmented cells) and of scattering (by all kind of particulates) in the attenuation process of radiant energy.

title: Inherent optical property inversion of ocean color spectra and its biogeochemical interpretation: 1. Time series from the Sargasso Sea
abstract: A nonlinear statistical method for the inversion of ocean color spectra is used to determine three inherent optical properties (IOPs), the absorption coefficients for phytoplankton and dissolved and detrital materials, and the backscattering coefficient due to particulates. The inherent optical property inversion model assumes that (1) the relationship between remote-sensing reflectance and backscattering and absorption is well known, (2) the optical coefficients for pure water are known, and (3) the spectral shapes of the specific absorption coefficients for phytoplankton and dissolved and detrital materials and the specific backscattering coefficient for particulates are known. This leaves the magnitudes for the three unknown coefficients to be determined. A sensitivity analysis is conducted to determine the best IOP model configuration for the Sargasso Sea using existing bio-optical models. The optical and biogeochemical measurements used were collected as part of the Bermuda Bio-Optics Project and the U.S. Joint Global Ocean Flux Study Bermuda Atlantic Time Series (BATS). The results demonstrate that the IOP model is most sensitive to changes in the exponential decay constant used to model absorption by dissolved and detrital materials. The retrieved chlorophyll a estimates show excellent correspondence to chlorophyll a determinations (r2 = 81%), similar to estimates from standard band ratio pigment algorithms, while providing two additional retrievals simultaneously. The temporal signal of retrieved estimates of absorption by colored dissolved and detrital materials is mirrored in ratios of K d (410) to K d (488), a qualitative indicator for nonalgal light attenuation coefficients. The backscatter coefficient for particles is nearly constant in time and shows no correspondence with the temporal signal observed for chlorophyll a concentrations. Last, the TOP model is evaluated using only those wavelengths which closely match the Sea Viewing Wide Field of View Sensor wave bands. This results in only a to 6% decrease in hindcast skill with the BATS biogeochemical data set. This is encouraging for the long-range goal of applying the IOP model to data from upcoming ocean color satellite missions.

title: Light attenuation and scattering by phytoplanktonic cells: a theoretical modeling.
abstract: A theoretical modeling is proposed to predict the efficiency factors for attenuation, total scattering, and backscattering for spherical and homogeneous phytoplanktonic cells in suspension. The input parameters of this modeling are the actual size distribution, the spectral values of absorption by the living cells, and an adjustable value of the real part of the refractive index. The variations in these parameters lead to very diverse spectral behavior of the efficiency factors. Theoretical predictions are compared to experimental results for some species to evaluate the reliability of the model for algal cells of various indices and morphologies.

title: An alternative approach to absorption measurements of aquatic particles retained on filters
abstract: We made modifications to the procedure used for absorption measurements of aquatic particles retained on glass-fiber filters that extend the procedure’s application range to “case 2” waters with high suspended sediment content and allow the determination of phytoplankton pigment absorption in situations where the standard solvent extraction method is not effective. The first result was achieved by combining light-transmission and light-reflection measurements (the latter made possible by the use of a commercially available integrating-sphere attachment for the dual-beam spectrophotometer) so as to remove the spurious contribution to the measured absorption caused by sample backscattering. The second result was obtained by bleaching the sample with a NaClO solution-a method that proved satisfactory even with water-soluble pigments (e.g. the important class of the phycobilins) and solvent-resistant Chlorophyceae. This process also allows for depigmentation of the particle suspension, and thus it was used to evaluate the empirical expression for converting the filter-retained sample absorbance to the equivalent particle suspension value. Both features of the modified procedure have been positively tested through measurements carried out on solvent-resistant phytoplankton species and on samples of inorganic suspended sediment. Light-transmission measurements on particles retained in glass-fiber filters (Yentsch 1962) are convenient for determining the light absorption spectrum of particle suspensions consisting mainly of natural phytoplankton and organic and inorganic detritus (Gordon and Morel 1983). The procedure has two basic advantages: particles can be concentrated so that instrumental accuracy requirements can be met regardless of the high dilution occurring in situ, and information is provided on the absorption of the cells in vivo. The main problem in data analysis arises from the large modification of light transmission due to multiple scattering by the filter, which results in an overestimate of particle absorption relative to suspension state (Butler 1962). Several empirical expressions have been derived for converting the absorption of filter-retained particles (for convenience also referred to as “sample” in the following) to the equivalent absorption of particle suspension. A review on this subject is given by Cleveland and Weidemann ( 199 3). Kishino et al. (1984, 1985) proposed a procedure for discriminating absorption by phytoplankton pigments from absorption by detritus based on measurements performed before and after pigment extraction by methanol. Although some doubts remain about the effectiveness of the procedure (Bricaud and Stramski 1990) and its validity range shows some limitations (little or no effect on some algal species), solvent extraction is the method generally used to identify pigment absorption in natural phytoplankton populations. It is current practice to minimize the loss of forward

title: Absorption spectra of inorganic particles in the Irish Sea and their relevance to remote sensing of chlorophyll
abstract: Mineral suspended solids contribute significantly to the total suspended sediment load in shelf seas and estuaries. Even in moderate concentrations, mineral suspended solids affect algorithms for retrieving chlorophyll concentrations from remotely-sensed colour data. Brown and Simpson (1990) showed empirically that the slope of a log-log graph of chlorophyll against the blue-green reflectance ratio increased as the concentration of mineral suspended solids increased. In an attempt to explain this, we have measured the absorption spectrum of over 100 samples of mineral suspended solids collected from the Menai Strait in the Irish Sea. The absorption spectra of the mineral suspended solids particles on filters consistently show an exponential decrease with increasing wavelength across the visible spectrum. The absorption coefficient at a single wavelength is proportional to the concentration of mineral suspended solids. This spectrum has been incorporated into a simple model of the blue-green ratio...

title: Light absorption by aquatic particles in the near-infrared spectral region
abstract: In this study, we used a special measurement geometry with samples placed inside the integrating sphere to address whether significant absorption by aquatic particles exists in the near-infrared (near-IR) spectral region from about 700 to 850 nm. Our tests with inorganic dyes and MgCO 3 particles showed that placing a small sample (1 cm cuvette) inside a relatively large integrating sphere (15 cm diameter) reduced the scattering error to a negligible level with no adverse effect on the absorption measurement. Our measurements of absorption by various particle suspensions suggest that absorption is generally negligible in the near-IR regardless of the type of particles. We examined four species of phytoplankton, phytodetritus derived from phytoplankton cultures, three samples of natural assemblages of mineral particles that show distinct reddish or brownish color, and three samples of aquatic particles from coastal and inland waters that have varying proportions of organic and inorganic particles.

title: Bio-optical properties of oceanic waters: A reappraisal
abstract: The apparent optical properties (AOPs) of oceanic case 1 waters were previously analyzed (Morel, 1988) and statistically related to the chlorophyll concentration ((Chl)) used as a global index describing the trophic conditions of water bodies. From these empirical relationships a bio-optical model of the upper layer was developed. With objectives and structure similar to those of the previous study the present reappraisal utilizes AOPs determined during recent Joint Global Ocean Flux Study cruises, namely, spectral attenuation for downward irradiance Kd(l) and irradiance reflectance R(l). This revision also benefits from improved knowledge of inherent optical properties (IOPs), namely, pure water absorption coefficients and particle scattering and absorption coefficients, and from better pigment quantification (via a systematic use of high- performance liquid chromatography). Nonlinear trends, already observed between optical properties and algal biomass, are fully confirmed, yet with numerical differences. The previous Kd(l) model, and subsequently the R(l) model, is modified to account for these new relationships. The R(l) values predicted as a function of (Chl) and the predicted ratios of reflectances at two wavelengths, which are commonly used in ocean color algorithms, compare well with field values (not used when developing the reflectance model). This good agreement means that semianalytical ocean color algorithms can be successfully applied to satellite data. Going further into purely analytical approaches, ideally based on radiative transfer computations combined with a suite of relationships between the IOPs and (Chl), remains presently problematic, especially because of the insufficient knowledge of the phase function and backscattering efficiency of oceanic particles.

title: Diffuse reflectance of oceanic waters: its dependence on Sun angle as influenced by the molecular scattering contribution.
abstract: A spectral model of the inherent optical properties (absorption and scattering coefficients a and b, respectively) of oceanic case 1 waters with varying chlorophyll concentrations C is operated. It provides the initial conditions for Monte Carlo simulations aimed at examining the diffuse reflectance directly beneath the surface R and its variations with the solar zenith angle zeta. In most oceanic waters, molecular scattering is not negligible, and molecular backscattering may largely exceed backscattering. The variable contributions (depending on C and wavelength) of water molecules and particles in the scattering process result in considerable variations in the shape of the volume-scattering function. R(zeta) is sensitive to this shape. From the simulations, R (which increases as zeta increases) appears to be linearly related to cos zeta, with a slope that is strongly dependent on eta(b), the ratio of molecular backscattering to particle backscattering. The value of the single-scattering albedo (?= b/a + b) has a negligible influence on the R(zeta) function provided that ? < 0.8, a condition that is always fulfilled when dealing with oceanic case 1 waters. Practical formulas for R(zeta) are proposed. They include the influence of the diffuse sky radiation. The history of each photon and the number of collisions it experiences before exiting have been recorded. These histories and also a probabilistic approach allow the variations of R with cos zeta, eta(b), and ? to be understood.

title: Spectral beam attenuation coefficient retrieved from ocean color inversion
abstract: A reflectance inversion model in presented in which particle backscattering is parameterized by the particle backscattering ratio, attenuation and absorption coefficients rather than a hyperbolic function. This parameterization removes backscattering spectral constraints and yields independent estimates of the magnitude and spectral slope of the particle beam attenuation, which have been found to correlate with the particle concentration and size distribution, respectively. The model yields estimates of component absorption and backscattering consistent with observations and Mie models, particularly for environments dominated by phytoplankton and other strongly absorbing particles.

title: Light and Water: Radiative Transfer in Natural Waters
abstract: Radiometry. Photometry. Optical Properties of Water.  Monte Carlo Methods.  Solving the RTE.  Underwater Light Fields. Epilogue. Chapter Problems.