A simple band ratio technique to quantify the colored dissolved and detrital organic material from ocean color remotely sensed data
Résumé
Spectral reflectances of the ocean, R, as derived from ocean color remote sensing data at four wavelengths (412, 443. 490, and 555 nm), can be used to form two ratios of spectral reflectance, namely R(412)/R(443), and R(490)/R(555), thereafter denoted R-443(412) and R-555(490). The former is mainly sensitive to the colored dissolved organic material (CDOM), albeit influenced by the a [gal content as depicted by the chlorophyll concentration, ([Chl]); in contrast, the latter is essentially depending on [Chl], although it is also influenced by CDOM. Therefore the signatures of CDOM and [Chl] which are not truly separable. can nevertheless be identified by considering simultaneously the two ratios. The concomitant variations in these ratios can be established via a bio-optical model developed for Case I waters. This model implicitly includes a ``mean'' relationship between CDOM and [Chl], and thus produces a unique curve relating R-443(412) and R-555(490). Deviations with respect to this mean relationship can be introduced through a factor (phi), with (phi)>1 (excess) or <1 (deficit), applied to the CDOM[Chl] ratio. A family of R-443(412) and R-555(490) curves is thus generated, in correspondence with the discrete values given to (phi); this ``grid'' (or numerically, a 2-D lookup table) allows the phi-[Chl] couple to be unambiguously derived for any R-443(412) and R-555(490) couple. By applying this straightforward algorithm to actual reflectance ratios derived from ocean color imagery, the relative anomalies in CDOM with respect to its standard (Chl-related) values can be efficiently assessed. Within the global ocean (discarding the coastal zones), the phi factor is widely varying, between at least and 3, and is roughly log-normally distributed around similar to 1 (no anomaly). The spatial distributions of the phi factor in the whole ocean are Strongly featured according to latitude, season, and hydrographic regimes, and these features are regularly reproducible, from year to year (2002-2007). This simple method is also validated against available in situ data, and its results compare favorably, for instance, to those of the GSM (Garver-Siegel-Maritorena) inversion method, in terms of retrieved CDOM concentrations and distribution patterns. (C) 2009 Elsevier Inc, All rights reserved.