Phytoplankton, which convert energy from the sun, are the primary producers of energy in the aquatic food web. Many marine animals directly eat phytoplankton, including larger zooplankton, krill and even massive baleen whales, which filter plankton out of the water.
Phytoplankton use chlorophyll to convert sunlight into energy and food through photosynthesis. Phytoplankton occur almost everywhere with water and sunlight, and there are thousands of different types of phytoplankton. Phytoplankton categories include dinoflagellates, diatoms and cyanobacteria, often known as blue-green algae. Not all algae are phytoplankton, but all phytoplankton are algae.
Phytoplankton consume carbon dioxide and produce oxygen during photosynthesis, similarly to forests and plants on land. Two-thirds of the world's photosynthesis occurs in the oceans. Most carbon dioxide captured by phytoplankton moves through the carbon cycle and back into the environment after the phytoplankton decomposes or after other animals eat the phytoplankton. However, some carbon also drifts down into the deep ocean.
Some phytoplankton produce toxins, such as those present in red tides and harmful types of algal blooms. Huge phytoplankton blooms also cause other problems when the dead phytoplankton sink to ocean and lake floors. Bacteria eat the dead phytoplankton and deplete the water's oxygen levels, suffocating other animal life. Phytoplankton algae blooms occur when an overabundance of nutrients is present.