Phytoplankton grow by absorbing sunlight and nutrients in the water through their cell walls.
Phytoplankton contain chlorophyll, which is needed for photosynthesis. This is the process in which sunlight provides the energy needed to create carbohydrates and new plant cells out of the fusion of carbon dioxide with water molecules. The chlorophyll also gives the organisms their greenish tint.
In addition to creating carbohydrates, photosynthesis also produces oxygen, which is released into the atmosphere. Phytoplankton produce half of the earth's oxygen supply.
Phytoplankton depend on the upwelling of water from the ocean depths that provides nutrients, such as iron, that it needs to survive. This upwelling only occurs in cooler waters. During El Niño, the water temperature rises, the deep ocean currents find it more difficult to get to the surface, and the phytoplankton starve.
Since these one-celled organisms are the base of the ocean's food chain, if the phytoplankton starve, then the fish and small creatures that feed upon them also starve. In turn, the larger fish and marine mammals eventually suffer.
