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The ocean also harbors plant life-including microscopic plants called "phytoplankton," which serve as the foundation for the marine food chain. Like their land-based relatives, phytoplankton photosynthesize to produce carbohydrates. Then, small fish and whales eat the phytoplankton, bigger fish eat the smaller fish, and humans eat many of the bigger fish.

Periodic water temperature shifts, as in an El Nino or La Nina, can have dramatic effects on the health and distribution of phytoplankton, which in turn can have negative effects on those animals that depend upon phytoplankton for nutrition. Therefore, scientists plan to use EOS Terra data to closely monitor the abundance and distribution of phytoplankton all over the world. By precisely measuring even slight variations in ocean color, scientists can accurately estimate the abundance of the microscopic marine plant. Because phytoplankton selectively absorb red and blue light, and reflect green, scientists can measure the difference between incoming sunlight and that which is reflected back up into the atmosphere. This difference in sunlight represents the amount being used for photosynthesis, which allows scientists to derive the abundance of phytoplankton.

Covering more than 70 percent of the Earth's surface and containing 97 percent of its surface water, the ocean has been labeled "the heat engine of global climate" due to its influence on the timing and patterns of climate change. The ocean possesses a vast capacity to convert and store sunlight as heat. Because heat rises, it eventually escapes the ocean and rises up into the atmosphere where it directly influences temperature and precipitation patterns, and indirectly influences land vegetation through either precipitation or drought events. As previously mentioned, ocean temperature trends not only affect weather patterns, but they also affect the marine biosphere (see SST<>NDVI series). When the sea surface is cold-as during a La Nina-this allows the cold, deeper waters to flow upward, bringing critical life-sustaining nutrients with them. Yet, when sea surface temperatures increase-as during an El Nino-there is less vertical cycling of water, which means nutrients at the surface become more scarce and phytoplankton productivity drops. Larger fish and mammals that require phytoplankton to survive must either starve or move to where the food source is more abundant (see Picture of starving seals). South American fishermen who fish the Pacific named this phenomenon and know well that El Nino is a time when their livelihood is interrupted.

How will the frequency and severity of El Nino affect ocean productivity? How will concentrations of phytoplankton shift in response to changes in ocean circulation? How will changes in ocean productivity affect ocean transparency, and therefore impact the exchange of heat between the ocean and atmosphere? The EOS Terra satellite will measure these parameters with unprecedented accuracy to help scientists answer these questions.

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