Spectral measurements of downwelling and upwelling daylight were made in waters different with respect to turbidity and pigment content and from these data the spectral values of the reflectance ratio just below the sea surface, R(λ), were calculated. The experimental results are interpreted by comparison with the theoretical R(λ) values computed from the absorption and back‐scattering coefficients. The importance of molecular scattering in the light back‐scattering process is emphasized. The R(λ) values observed for blue waters are in full agreement with computed values in which new and realistic values of the absorption coefficient for pure water are used and presented. For the various green waters, the chlorophyll concentrations and the scattering coefficients, as measured, are used in computations which account for the observed R(λ) values. The inverse process, i.e. to infer the content of the water from R(λ) measurements at selected wavelengths, is discussed in view of remote sensing applications.

Spectral values of absorption of light by dissolved organic matter were measured in samples originating from diverse parts of the ocean, quite different with respect to pigment and particle content. The use of llO-cm cells and then of lo-cm cells, with a highly sensitive spectrophotometer, allowed measurement throughout the UV-visible range (200-700 nm) even for the low concentrations of yellow substance encountered in the open sea. The concentration appears influenced predominantly by natural and industrial land discharges. In oceanic waters, it remains low and seems to be related to the biological activity averaged over a long period rather than to the local and temporary phytoplankton content. However, even at such low concentrations, yellow substance in the open sea may have an effect on absorption and hence on ocean color similar to that of low or moderate algal biomass. The spectral dependence law of absorption appears to vary within a restricted range, and an average law can be considered representative of rapid measurements at one selected wavelength.

The variations of the spectral absorption coefficient of seawater can be attributed to three principal factors-phytoplankton, nonchlorophyllous particles, and yellow substanceswhose concentrations are represented in this study in terms of chlorophyll a and pheophytin a content (C), the total scattering coefficient (b), and the absorption by filtered seawater at a given wavelength (Y). By assuming an exponential absorption by yellow substances, we developed a new iterative method for identifying the in situ apparent specific spectral absorption curves of the other two factors. The weighting coefficients for each of the three components are calculated for over 80 spectral absorption measurements from different representative marinc regions. The reconstruction of spectral absorption curves from the three coefficients and the three specific absorption curves fits the original data with an overall average error of 3%. Relationships between these coefficients and C, b, and Y are examined. Results show that, though the spectral form of absorption by pigments can be considered as more or less invariant, the absorption "efficiency" per unit C depends on the nature of the water, and hence on the type of phytoplankton population considered. An optical classification based on the three optical coefficients related to three absorbing agents is proposed.

Abstract. Light absorption by colored dissolved organic matter (CDOM) [a CDOM (λ)] plays an important role in the heat budget of the Arctic Ocean, contributing to the recent decline in sea ice, as well as in biogeochemical processes. We investigated a CDOM (λ) in the Southern Beaufort Sea where a significant amount of CDOM is delivered by the Mackenzie River. In the surface layer, a CDOM (440) showed a strong and negative correlation with salinity, indicating strong river influence and conservative transport in the river plume. Below the mixed layer, a weak but positive correlation between a CDOM (440) and salinity was observed above the upper halocline, resulting from the effect of removal of CDOM due to brine rejection and lateral intrusion of Pacific summer waters into these layers. In contrast, the relationship was negative in the upper and the lower haloclines, suggesting these waters originated from Arctic coastal waters. DOC concentrations in the surface layer were strongly correlated with a CDOM (440) (r 2 = 0.97), suggesting that this value can be estimated in this area, using a CDOM (440) that is retrieved using satellite ocean color data. Implications for estimation of DOC concentrations in surface waters using ocean color remote sensing are discussed.