This variation is due to varying populations of tiny aquatic plants called phytoplankton , or algae. Where phytoplankton production is small, such as the Sargasso Sea, the water is deep blue. By contrast, coastal waters rich in phytoplankton are green.
Phytoplankton are the base of the food chain in the sea.
Additional background information is available here . Exploration Let's make an analysis of the false color image of the world's oceans shown below. This image comes from the SeaWiFs Mission . All browse files in this data set are 360 pixels wide and 180 pixels high. These sections are not equal area, however.