History and Other Background Info
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.
Further readings:
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Atmos Composition
