Figure 1. Percentage of sunlight backscattered from upper ocean layers as a function of wavelength in nanometers (CZCS observing wavelengths in boldface), under three conditions: (A) clear open ocean water, low phytoplankton concentration; (B) moderate phytoplankton bloom, open ocean; (C) turbid coastal waters containing sediment as well as phytoplankton.
Simple, semi-empirical equations can be used to estimate the concentration of chlorophyll-a and its degradation products from satellite measurements of backscattered sunlight at three wavebands centered at 443, 520, and 550 nm, covering the blue and green regions of the spectrum (See figure, left).These radiances are not merely reflected from the sea surface, but are derived from sunlight that has entered the ocean, been selectively absorbed, scattered and reflected by phytoplankton and other suspended material in the upper layers, and then backscattered through the surface.This approach permits quantitative estimates of phytoplankton pigment concentrations within the upper tens of meters of the open ocean, and within somewhat lesser depths in coastal waters.
Sunlight backscattered by the atmosphere contributes 80-90% of the radiance measured by a satellite sensor at these key wavelengths.Such scattering arises from dust particles and other aerosols, and from molecular (Rayleigh) scattering.However, the atmosphereic contribution can be calculated and removed if additional measurements are made in the red and near-infrared spectral regions (e.g., 670 and 750 nm).Since blue ocean water reflects very little radiation at these longer wavelengths, the radiance measured is due almost entirely to scattering by the atmosphere.Long-wavelength measurements, combined with the predictions of models of atmospheric properties, can therefore be used to remove the contribution to the signal from aerosol and molecular scattering.
Embodying this approach, the CZCS measured reflected sunlight at 443, 520, 550, 670, and 750 nm with a spatial resolution of about 1 km across a swath 2200 km wide. An additional thermal-infrared spectral channel, at 11.5 microns, was included as well to permit concurrent measurements of sea-surface temperature. This pioneering instrument recorded over 60,000 two-minute ocean scenes over its extended 8-year mission lifetime, laying the foundation for systematic studies of ocean color from space.
Suspended sediment, detritus, and pigments other than chlorophyll also affect the spectrum of backscattered sunlight.When reflectance by these substances contributes a relatively high fraction of the total signal, as in the cases of some coastal waters, the semi-empirical equations used to estimate phytoplankton concentrations from CZCS measurements yield unreliable results.Research is underway to develop equations that can distinguish detrital, sediment, and phytoplankton signatures in observations of coastal waters.The application of these new equations requires measurements in more spectral channels than were provided by CZCS; future sensors are being designed to provide the required capabilities.