OCEAN COLOR REMOTE-SENSING AND THE SUBSURFACE VERTICAL STRUCTURE OF PHYTOPLANKTON PIGMENTS

In order to use to the best advantage the phytoplankton pigment concentration estimates obtained by observing the ocean from space, it is important to evaluate what information they actually provide concerning the vertical distribution of the pigments within the surface layer. By using an optical model, the satellite estimate is predicted for oceans with various vertical pigment profiles. Simulations made for selected modeled profiles show that the satellite estimate differs from the concentration at the very surface only when abrupt vertical variations occur within the first few meters of the ocean upper layer. Simulations made for a large number of actual profiles measured in situ show that such situations are exceptional in the ocean and that the satellite, as a general rule, provides an estimate of a truly "near-surface" concentration. The currently used "optically weighted concentration" is shown to provide an inadequate representation of the satellite estimate where stratification effects are significant. In common oceanic situations, in contrast, this concept is quite acceptable but can be safely replaced by a more simple (arithmetic and non-optical) depth average of the upper pigment profile.