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

Spectral absorption coefficients of total particulate matter a(p) (lambda) were determined using the in vitro filter technique. The present analysis d eals with a set of 1166 spectra, determined in various oceanic (case 1) wat ers, with field chi a concentrations ([chl]) spanning 3 orders of magnitude (0.02-25 mg m(-3)). As previously shown [Bricaud et al.; 1995] for the abs orption coefficients of living phytoplankton a(phi)(lambda), the a(p)(lambd a) coefficients also increase nonlinearly with [chl]. The relationships (po wer laws) that link a(p)(lambda) and a(phi)(lambda) to [chl] show striking similarities. Despite large fluctuations, the relative contribution of nona lgal particles to total absorption oscillates around an average value of 25 -30% throughout the [chl] range. The spectral dependence of absorption by t hese nonalgal particles follows an exponential increase toward short wavele ngths, with a weakly variable slope (0.011 +/- 0.0025 nm(-1)). The empirica l relationships linking a(p)(lambda) to [chl] can be used in bio-optical mo dels. This parameterization based on in vitro measurements leads to a good agreement with a former modeling of the diffuse attenuation coefficient bas ed on in situ measurements. This agreement is worth noting as independent m ethods and data sets are compared. It is stressed that for a given [chl], t he a(p)(lambda) coefficients show large residual variability around the reg ression lines (for instance, by a factor of 3 at 440 nm). The consequences of such a variability, when predicting or interpreting the diffuse reflecta nce of the ocean, are examined, according to whether or not these variation s 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 affe cted by these variations.