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 riseswith 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 havebeen 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.