Meeting the Challenge of Monitoring Chlorophyll in the Ocean from Outer Space

Abstract

Detecting and measuring, from space, the chlorophyll (Chl) content within the upper layer of the ocean, where the concentration is so low and the Chl-bearing phytoplanktonic cells are so small, can appear to be an impossible task. Further, a satellite-borne sensor directed towards the ocean captures the dominant atmospheric signal, due to light scattered by air molecules and aerosols, so that the marine signal must be extracted from this invasive background. Ocean color sensors, however, have been developed and now provide, on a daily basis, maps of the Chl distribution over the oceans of the world. This chapter explains how this challenge was faced and fi nally solved. These solutions involved the so-called 'bio-optical' properties of open ocean waters; namely, the existence in these waters of empirical relationships between the whole biogenic material, which governs the bulk optical properties, and a single component, the Chl content which can be detected and estimated from space by ocean color sensors. With newly developed sensors, it is now also possible to detect natural sun-stimulated Chl a fl uorescence of phytoplankton which also provides a new promising approach to derive information about the physiological state of algal populations. The method of retrieving the marine signal and making the 'atmospheric correction' is briefl y described. Paradoxically, this correction itself provides highly valuable information on the atmospheric aerosols, a crucial component of the radiative budget of our planet.