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GCOS Essential Climate Variables (ECV) Definition: Phytoplankton (phyto = plant, planktos = to wander) are single celled plants which live in the surface waters of the oceans. Most of them simply drift around the ocean in the surface currents but some can move a tiny bit on their own. They use sunlight, carbon dioxide (CO2) and water, in a process called photosynthesis, to produce organic compounds which they use for food and to make their cells. One waste product is oxygen and this makes it possible for animals to live on earth. Phytoplankton remove almost as much carbon dioxide from the air as land plants and, therefore, help regulate our climate. Phytoplankton also need nutrients to grow. They need a wide variety of chemical elements but the two critical ones are nitrogen and phosphorous since they are needed in quite large amounts but are present in low concentrations in seawater. Nitrogen and phosphorous are like the fertilisers we add to land plants and are used to make proteins, nucleic acids and other cell parts the phytoplankton need to survive and reproduce. Phytoplankton need nutrients in well defined ratios. For every 106 atoms of carbon they make into organic matter, they need 16 atoms of nitrogen and 1 atom of phosphorous. Most can't use atmospheric nitrogen gas (N2) directly but need chemically reactive forms of nitrogen such as nitrate (NO3-) or ammonium (NH4+). There is always plenty of carbon dioxide so phytoplankton keep growing until they have used up all of the useable nitrogen or all of the phosphorous, which ever runs out first. In most of the ocean, nitrogen runs out first and growth is said to be nitrogen limited. The Eastern Mediterranean Sea is phosphorous limited, here growth stops when phytoplankton have used up all the phosphorous even though there is still nitrogen in the water. (from the Espere-ENC web site) Data, Product, Metadata and Information Access
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