Phytoplankton Dynamics
Time-dependent changes in phytoplankton biomass (phytoplankton dynamics) are
induced by a complex interplay of physical, chemical, and biological processes.
Of particular importance are the availabilities of plant nutrients and sunlight
for photosynthesis. In the tropics, where adequate sunlight is available
throughout the year, phytoplankton dynamics are controlled by the rate at which
plant nutrients are supplied to upper ocean layers. Sea-surface winds play a
key role in this process. Under favorable wind conditions over coastal areas
and along the Equator, cool nutrient-rich subsurface waters well up to the
surface, stimulating the rapid increases in phytoplankton biomass known as "blooms." Upwelling systems off Peru, northwest Africa, and the US West Coast
are among the most productive regions of the world's oceans and support
important fisheries.
In contrast to tropical waters, temperate and polar seas generally have adequate
nutrient concentrations, but winter sunlight intensity is too low for
phytoplankton growth. Phytoplankton blooms therefore tend to be seasonal.
During the spring, phytoplankton productivity rises with increasing sunlight
intensity, eventually causing a rapid buildup of phytoplankton biomass. The
"spring phytoplankton bloom" is a major event in the North Atlantic Ocean, in
the polar seas, and in temperate coastal waters around the world.
To date, the principal use of CZCS imagery has been to study phytoplankton
blooms in coastal waters and to use the imagery to locate fronts, eddies,
coastal currents, and other circulation features. The imagery provides a
real-time map of near-surface chlorophyll concentration that is impossible to
obtain by other methods. Image sequences have been used to study the timing
and duration of phytoplankton blooms and to track the movement and persistence
of ocean features. These studies have made important contributions to our
knowledge of ocean dynamics.
Future research will emphasize use of CZCS imagery to study phytoplankton
dynamics on ocean-basin and global scales. Through combination of the results
from many orbits to produce a composite image, CZCS chlorophyll maps of entire
basins and of the global ocean can be constructed. Weekly and monthly
composites yield information on seasonal phytoplankton blooms. Annual
composites can yield insights into the study of interannual variablility. The
composite images contained in this document are
among the most recently produced and illustrate the exciting potential of this
technique.
On the basis of these and similar images, biological oceanographers have become
keenly interested in the possibility of using composite ocean-color imagery to
study large-scale phytoplankton dynamics. However, some key issues must be
resolved before routine analysis of composite images is possible. For example,
the number of observations represented by each pixel (picture element) within a
composite image is not constant. In some parts of the image, a pixel may
represent the mean of many observations because cloud-free conditions were
common during the compositing interval. In another part of the same image, a
pixel may represent only a single observation. Clearly, the statistical
implications of image composites need to be carefully considered before their
application to the study of large-scale processes.
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Atmos Composition
