The past ten years have witnessed a revolution in the way oceanographers view the biological, chemical and physical interactions in the world's oceans. Satellite measurements of ocean color have played a key role, permitting a quantum leap in our understanding of oceanographic processes from regional to global scales.As a result of this new capability, determinations of ocean productivity, visualization of surface currents, and the rate at which the oceans sequester atmospheric carbon dioxide are all now within our reach.Such observations are also critcal to major new initiatives aimed at establishing the role of the oceans in the biogeochemical cycles of elements which influence both climate and the distribution of life on Earth.
The measurement of ocean color from space has revealed, for the first time, the global-scale variability in the distribution and concentration of phytoplankton -- microscopic, single-celled aquatic plants that provide the ultimate source of food for marine life.Like green terrestrial plants, phytoplankton contain chlorophyll-a and other pigments that absorb sunlight; this process provides the energy needed for photosynthesis of organic carbon.The rate at which photosynthesis proceeds in known as primary productivity.
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.
Since phytoplankton pigments absorb energy primarily in the red and blue regions of the spectrum and reflect green light, there is a relationship between the spectrum of sunlight backscattered by upper ocean layers and the distribution of phytoplankton pigments in these layers.Satellite measurements of ocean radiance at selected wavelengths can thus be used to estimate near-surface phytoplankton concentrations and the extent of primary productivity (Figure 1).
In addition to sustaining the marine food chain, phytoplankton strongly influence ocean chemistry.During photosynthesis, they remove carbon dioxide dissolved in seawater to produce sugars and other simple organic molecules, and release oxygen as a by-product.Phytoplankton also require inorganic nutrients (e.g., nitrogen, phosphorus, silicon) as well as trace elements (e.g., iron) to synthesize complex molecules, such as proteins.Ocean productivity thus plays a key role in the global biogeochemical cycles of carbon, oxygen, and other elements critical to both marine and terrestrial life.The rising atmospheric concentration of carbon dioxide, which may produce a global warming (the "greenhouse effect"), underscores the additional importance of the carbon cycle to the Earth's climate.The magnitude and variability of primary production are poorly known on a global scale, largely because of the high spatial and temporal variability of marine phytoplankton concentrations.Oceanographic vessels move too slowly to map dynamic, large-scale variations in productivity; global coverage by shipborne instruments is impossible.Only satellite observations can provide the rapid, global coverage required for studies of ocean productivity worldwide.
The first observations of ocean color from space (and the only long-term satellite observations to date) were carried out by the Coastal Zone Color Scanner (CZCS), a radiometer visible and infrared spectral channels that operated on NASA's Nimbus-7 research satellite from 1978 to 1986.Despite being designed as a proof-of-concept mission with a nominal 1-year life-time, CZCS gathered a rich harvest of new data which have permitted an unprecidented view of the world's oceans.The CZCS global data sets lay the scientific foundation for a new generation of satellite ocean color measurements in the 1990s.