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Across Earth?s oceans, tiny marine plants called phytoplankton use chlorophyll to capture sunlight during photosynthesis and use the energy to produce sugars. Phytoplankton are the basis of the ocean food web, and they play a significant role in Earth's climate, since they draw down carbon dioxide, a greenhouse gas, at the same rate as land plants. About half of the oxygen we breathe arises from photosynthesis in the ocean.
Because of their role in the ocean's biological productivity and their impact on climate, scientists want to know how much phytoplankton the oceans contain, where they are located, how their distribution is changing with time, and how much photosynthesis they perform. They gather this information by using satellites to observe chlorophyll as an indicator of the number, or biomass, of phytoplankton cells.
Probably the most important and predominant pigment in the ocean is chlorophyll-α contained in microscopic marine plants known as phytoplankton. Chlorophyll-α absorbs blue and red light and reflects green light. If the ratio of blue to green is low for an area of the ocean surface, then there are more phytoplankton present. This relationship works over a very wide range of concentrations, from less than 0.01 to nearly 50 milligrams of chlorophyll per cubic meter of seawater.
These false-color maps show monthly, global observations of ocean chlorophyll collected by the Terra satellite. Values range from .01– 20 milligrams of chlorophyll per cubic meter of seawater and are displayed in shades of deep purple to red. Land surfaces are gray, and black shows where no data were collected (usually because of persistent clouds).
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Terra/MODIS
