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 aø(λ), the ap(λ) coefficients also increase nonlinearly with 〈chl〉. The relationships (power laws) that link ap(λ) and aø(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.