To human eyes, the ocean appears as shades of blue, sometimes blue-green. From outer space, satellite sensors can distinguish even slight variations in color to which our eyes are not sensitive. Different shades of ocean color reveal the presence of differing concentrations of sediments, organic materials, or even phytoplankton, all of which can be measured by satellites.
Due to their pigment (chlorophyll), phytoplankton preferentially absorb the red and blue portions of the light spectrum (for photosynthesis) and reflect green light. Therefore, the ocean over regions with high concentrations of phytoplankton will appear as certain shades, from blue-green to green, depending upon the type and density of the phytoplankton population there (Figure 5).
When considering Earth's sources of oxygen, we usually think of vast forests such as the Amazon, but about half of the oxygen we breathe comes from elsewhere; it is produced by phytoplankton. Phytoplankton are tiny, single-celled plants that live in the ocean, and they serve as the
Chlorophyll concentration (mg m 3)
Figure 5 Nine years of ocean chlorophyll. The image shows chlorophyll measured by SeaWiFS from 18 September 1997 to 31 July 2006. Chlorophyll is shown in milligrams (a milligram is one-thousandth of a gram) per cubic meter of seawater. The greatest concentrations appear in yellow, and the sparsest appear in deep blue. Because this image shows values averaged over nearly 9 years, greater amounts of chlorophyll are observed in areas with recurring blooms. Some of the greatest concentrations appear along coastlines. Consistently high concentrations appear at the high latitudes, and medium-level concentrations appear over much of the ocean, particularly along the El Niño/La Niña route in the Pacific. Marine biologists often refer to the darkest blue areas as 'deserts', because the concentration of key nutrients in the water is usually so low that phytoplankton cannot grow. Credit: NASA image created by Jesse Allen, Earth Observatory, using data provided courtesy of the SeaWiFS Project (http://oceancolor.gsfc.nasa.gov/SeaWiFS/), NASA/Goddard Space Flight Center, and ORBIMAGE.
base of the oceanic food chain. Yet as important as phytoplankton are to life on Earth, their interaction with our planet has only recently been studied on a global scale. The satellite sensor that has pioneered the study of phytoplankton globally is the sea-viewing wide field-of-view sensor (SeaWiFS) based on legacy instruments such as the CZCS on NIMBUS-7. (http://earthobservatory.nasa.gov/ Newsroom/NewImages/images.php3?img_id=17405).
Like their land-based relatives, phytoplankton require sunlight, water, and nutrients for growth. Because sunlight is most abundant at and near the sea surface, phytoplankton remain at or near the surface. Also like terrestrial plants, phytoplankton contain the pigment chlorophyll, which gives them their greenish color. Chlorophyll is used by plants for photosynthesis, in which sunlight is used as an energy source to fuse water molecules and carbon dioxide into carbohydrates - plant food. Phytoplankton (and land plants) use carbohydrates as 'building blocks' to grow; fish and humans consume plants to get these same carbohydrates.
The atmosphere is a rich source of carbon dioxide, and millions of tons of this gas settle into the ocean every year. However, phytoplankton still require other nutrients, such as iron, to survive. When surface waters are cold, ocean water from deeper depths upwells, bringing these essential nutrients toward the surface where the phytoplankton may use them. However, when surface waters are warm (as during an El Niño), they do not allow the colder, deeper currents to upwell and effectively block the flow of life-sustaining nutrients. (The El-Nino phenomenon is described in: http://earthobservatory.nasa.gov/Library/ElNino/.) As phytoplankton starve, so too do the fish and mammals that depend upon them for food. Even in ideal conditions an individual phytoplankton only lives for about a day or two. When it dies, it sinks to the bottom. Consequently, over geological time, the ocean has become the primary storage sink for atmospheric carbon dioxide. About 90% of the world's total carbon content has settled to the bottom of the ocean, primarily in the form of dead biomass.
Prior to the launch of SeaWiFS, scientists could only study phytoplankton on a relatively small scale. By measuring chlorophyll on a global scale over time, this sensor has been able to track how phytoplankton thrive and diminish as light and nutrient levels change. Massive phytoplankton blooms spread across the North Atlantic in the Northern Hemisphere each spring, and intense blooms also occur in the South Atlantic off the Patagonian Shelf of South America during spring in the Southern Hemisphere. Blooms fostered by changes in nutrient-rich water, though less regular, are also dramatic, especially when El Nino gives way to La Nina, and cold, nutrient-rich waters well up across the Pacific.
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