Variations of light absorption by suspended particles with chlorophyll <i>a</i> concentration in oceanic (case 1) waters: Analysis and implications for bio‐optical models

Abstract

Spectral absorption coefficients of total particulate matter a p (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 a ø (λ), the a p (λ) coefficients also increase nonlinearly with 〈chl〉. The relationships (power laws) that link a p (λ) and a ø (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 a p (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 a p (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 a p are associated with concomitant variations in particle scattering. In most situations the deviations in a p actually are not compensated by those in particle scattering, so that the amplitude of reflectance is affected by these variations.