Independence and interdependencies among global ocean color properties: Reassessing the bio-optical assumption

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Abstract

[1] The bio-optical assumption states that ocean optical properties should covary with the chlorophyll concentration. However, chlorophyll is not the only constituent capable of absorbing and scattering light as phytoplankton, detrital particles, colored dissolved organic material (CDOM), and suspended solids all contribute independently to ocean color. Here we employ a semi-analytical ocean color algorithm, the Garver-Siegel-Maritorena model (GSM01), to decompose ocean color imagery from the Sea-viewing Wide Field of view Sensor (SeaWiFS) into optically relevant components for phytoplankton absorption (related to the chlorophyll concentration, Chl), colored non-algal absorption and particulate backscatter. Semi-analytical Chl retrievals are consistent with available open ocean observations but show important differences compared with the standard SeaWiFS algorithm (OC4v4). These differences may be accounted for by the absorption due to colored detrital materials (CDM; the sum of colored dissolved organic material and detrital particulates). Values of CDM are a large and variable fraction of the total non-water absorption at 440 nm varying from 30 to 40% in the subtropical gyres to more than 60% at high latitudes. Retrievals of particulate backscatter (BBP) vary comparatively little. In the tropical and subtropical oceans, changes in BBP are largely unrelated to Chl, while at higher latitudes, BBP increases mirror changes in Chl. For much of the ocean, alterations in the relationship between BBP and Chl are consistent with a physiological response by phytoplankton to changes in mixed layer light, nutrients, and temperature. Regional scale correspondences among the three ocean color properties illustrate both independent and interdependent variations which can be explained by assessing the regional scale forcing mechanisms driving changes in ocean color properties.

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