Free-floating photosynthetic organisms (phytoplankton), dissolved organic matter and sediments affect the spectral signature of the light reflected by the upper ocean by increasing absorption and scattering. "Ocean color" takes into account the spectral variations of light scattered by the sea surface upper-layer, and thus varies from deep blue in pure waters to dark green in phytoplankton-rich waters and "milky" green in sediment-rich waters.

In the open ocean, where phytoplankton and associated biogenous materials are the prevailing components, the chlorophyll-a pigment concentration (Chl) is the index usually adopted to specify the bio-optical state of the water body. This pigment, which is contained in all photosynthesis-organisms, presents relatively strong and weak absorption in the blue and green part of the spectrum, respectively. This spectral property allows the retrieval of Chl from observation of ocean color.

By observing ocean color from space (IOCCG website), the chlorophyll pigment concentration may be estimated and its spatial and temporal variability may be studied on both regional and global scales. This information is crucial to the understanding of marine ecosystems and the evaluation of energy fluxes affecting the food chains.

Phytoplankton also plays a geochemical role as a "biological CO2 pump" due to its ability to take up atmospheric carbon dioxide through organic matter photosynthesis. International programs such as IGBP and JGOFS have emphasized the key role of oceans in the global carbon cycle. In order to quantify and model this process, satellite remote sensing pigment maps have to be translated into "primary production" maps (carbon uptake by surface or volume and time unit) via models.

Moreover, phytoplankton affects the heating rate of the upper ocean layers and consequently the oxygen and CO2 fluxes at the ocean/atmosphere interface. In addition, because phytoplankton behaves like a passive tracer in certain cases, remote sensing of ocean color can be used to depict dynamic ocean features like eddies or the meanders of currents such as the Gulf Stream.

Besides Chl, the absorption, a, and the backscattering, bb, coefficients are essential for satellite ocean color data applications, as bb determines the amplitude of the remote sensed reflectance, and a modifies its spectral shape. These coefficients provide new additional biogeochemical information (a may be used to discriminate different phytoplankton species, and bb is a proxy of the suspended particulate matter). Moreover, while the chlorophyll data as detected from space are now currently used to constraint models of oceanic carbon cycle, new generation of biological models now integrate explicitly two or more species of plankton, as well as dissolved and particulate organic carbon. The evaluation of such models by new (compared to Chl) biogeophysical derived-satellite data is crucial as pointed out by members of IGBP & SCOR.

Pseudo-true color image (marine reflectance at 443, 490, and 565 nm), chlorophyll concentration, and the backscattering to absorption ratio, bb/a, as seen by POLDER the 10 th of April 1997 (orbit 6508). bb and a represent the backscattering by particles at 565 nm, and the absorption by particles and dissolved matter at 443 nm, respectively. This ratio, that presents different patterns to the Chl ones, allows to distinguish different families of particles.

POLDER has the conventional capabilities of an imaging sensor for monitoring ocean color: a wide field of view, enabling global, repetitive observations, a resolution suited to the open ocean, and channels with well suited spectral and radiometric properties.

Furthermore, POLDER's original multidirectional and polarization capabilities will allow to :

systematically avoid perturbations due to glitter,
improve corrections for atmospheric effects (scattering by atmospheric molecules and aerosols) and other perturbations (such as reflection by foam). As the ocean contribution is generally less than 10% of the signal measured by the sensor, the accuracy of these corrections largely determines the accuracy of marine reflectances retrieved. The multidirectional and polarized measurements will lead to a better "pixel by pixel" understanding of the nature of atmospheric aerosols, which are highly variable over space and time.
potentially distinguish between mineral particles and phytoplankton.