Besides acting as the first link in the food chain, phytoplankton are a critical part of ocean chemistry. The carbon dioxide in the atmosphere is in balance with Carbon dioxide in the ocean. During photosynthesis phytoplankton remove carbon dioxide from sea water, and release oxygen as a by-product. This allows the oceans to absorb additional carbon dioxide from the atmosphere. If fewer phytoplankton existed, atmospheric carbon dioxide would increase.

Click here for a diagram of the Carbon Cycle.

Phytoplankton also affect carbon dioxide levels when they die. Phytoplankton, like plants on land, are composed of substances that contain carbon. Dead phytoplankton can sink to the ocean floor. The carbon in the phytoplankton is soon covered by other material sinking to the ocean bottom. ln this way, the oceans act as a sink, a place to dispose of global carbon, which otherwise would accumulate in the atmosphere as carbon dioxide. Other global sinks include land vegetation and soil. However the carbon in these sinks frequently is returned to the atmosphere as carbon dioxide by burning or decomposition. Deforestation contributes to the accumulation of carbon dioxide in the atmosphere by reducing vegetation that takes up carbon dioxide. Carbon dioxide acts as a "greenhouse" gas in the atmosphere, and therefore may contribute to global warming. Sources of carbon dioxide in the Earth's atmosphere include decomposition of organic matter (such as trees), the carbon dioxide that animals and people exhale, volcanic activity, and human activities such as the burning of fossil fuels and wood.

No one yet knows how much carbon the oceans and land can absorb. Nor do we know how the Earth's environment will adjust to increasing amounts of carbon dioxide in the atmosphere. Studying the distribution and changes in global phytoplankton using ocean color and other tools will help scientists find answers to these questions.

SeaWiFS Teacher's Guide and Activities
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