Phytoplankton and Atmospheric Carbon Dioxide
Posted February 1, 1998
If you took an ocean voyage, you might not see very much in the way of living things. Most of the living things in the ocean are not whales, sharks, or large fish,
but plankton. Plankton are microscopic organisms, such as diatoms, protists, and other species, that float or swim in the upper layers of ocean water. Phytoplankton, the producers in
the ocean ecosystem, use sunlight to convert carbon dioxide and water into energy. Phytoplankton are, in turn, consumed by other organisms. As phytoplankton grow and reproduce, they
take carbon dioxide out of the atmosphere and fix it in the bodies of organisms.
But sunlight only penetrates into the surface layers of the open ocean. The phytoplankton in this layer provide food for zooplankton, small fish, and even for plankton-eating
baleen whales. Once the carbon dioxide is fixed by photosynthesis, it is not released again until the ocean organisms die, fall to the ocean bottom, and decompose. When carbon dioxide
in the surface layer of the open ocean is used up by the phytoplankton, this reduces the partial pressure of carbon dioxide in those waters. The reduction of pressure pumps carbon dioxide
into the ocean water from the atmosphere. Scientists hypothesize that this movement of carbon dioxide from the atmosphere, into ocean surface waters, then into organisms, and finally
into ocean bottom sediments, works like a "biological pump" to help govern the levels of carbon dioxide in the atmosphere.
Oceanographers think that the chemistry of the oceans over thousands of years is driven somewhat by the action of this biological pump. To test whether or not
atmospheric carbon dioxide has been increasing over time, a finding that would support the idea of global warming, some scientists are measuring the amount of carbon found in ocean bottom
sediments. They hope to relate the amount of carbon found in organic matter in sediments with normal changes found seasonally and annually in levels of atmospheric gases above open ocean
waters. Such information would help in estimating the carbon budget of the upper layers of the oceans.
References
Doney, Scott C. "The Ocean's Productive Deserts." Nature, October 30, 1997, Vol. 389, No. 6654, pp. 905-906.
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