The term “plankton,” from Greek planktos for “wandering,” is applied to any organism that floats or drifts with the movement of the ocean water.

Most plankton are microscopic and are usually single-celled, a chain of cells, or a loose group of cells. Algal and cyanobacterial plankton are referred to as phytoplankton. The heterotrophic crustaceans and larvae of animals are referred to as zooplankton.

The group of organisms known as phytoplankton (literally, “plant” plankton) do not constitute a taxonomic group but rather refer to a collection of diverse, largely algal and cyanobacterial, microorganisms that live in water and are at the base of the food chain.

The phytoplankton, including diatoms, unicellular cyanobacteria and coccolithophorids in nutrient-poor waters, and cryptomonads, manufacture organic material from carbon dioxide, usually through photosynthesis. Phytoplankton are responsible for one-half of the world’s primary photosynthesis and produce one-half of the oxygen in the atmosphere.

Eighty to ninety percent of the weight of phytoplankton is water, with the rest made up of protein, fat, salt, carbohydrates, and minerals. Some species have compounds of calcium or silica that make up their shells or skeletons. Phytoplankton include many of the algal phyla: Chrysophyta (chrysophytes), Phaeophyta (golden-brown algae), coccolithophores, silico flagellates, and diatoms.

The most common type of phytoplankton is the diatom (phylum Bacillariophyta), a single-celled organism that can form complex chains. Dinoflagellates (phylumDinophyta) are the most complex of the phytoplankton. They are unicellular and mobile.

Green algae (phylum Chlorophyta) are usually found in estuaries or lagoons in the late summer and fall. Some species can cause toxic algal blooms associated with coastal pollution and eutrophication. Cyanobacteria (often called blue-green algae but not true algae) are prominent near shore waters with limited circulation and brackish waters.


Phytoplankton are primary producers, responsible for a half the world’s primary photosynthesis: the conversion of light energy and inorganic matter into bioenergy and organicmatter. Each year, 28 billion tons of carbon and 250 billion to 300 billion tons of photosynthetically produced materials are generated in the oceans by phytoplankton.

All animal organisms eliminate carbon dioxide into the atmosphere, and plants remove carbon dioxide from the air through photosynthesis. In the oceans’ carbon cycle, carbon dioxide from the atmosphere dissolves in the ocean.

Photosynthesis by marine plants, mainly phytoplankton, converts the carbon dioxide into organic matter. Carbon dioxide is later released by plants and animals during respiration, while carbon is also excreted as waste or in the dead bodies of organisms.

Bacteria decompose organic matter and release the carbon dioxide back into the water. Carbon may be deposited as calcium carbonate in biogenous sediments and coral reefs (made of skeletons and shells of marine organisms).

Food Chain

Because they are primary producers of organic matter through photosynthesis, phytoplankton play a key role in the world’s food chain: They are its very beginning. Sunlight usually penetrates only 200 to 300 feet deep into ocean waters, a region called the photic zone. Most marine plant and animal life and feeding take place in this zone.

Phytoplankton, the first level in the marine food chain, are the primary food source for zooplankton and larger organisms. These microscopic plants use the sun’s energy to absorb minerals to make basic nutrients and are eaten by herbivores, or plant eaters. Herbivores are a food source for carnivores, them eat eaters.

In temperate zones, phytoplankton increase greatly in the spring, decline in the summer, and increase again in the fall. Zooplankton (animal plankton) are at their maximum abundance after the spring increase, and their grazing on the phytoplankton causes a decrease in phytoplankton population in the summer.

Fish and invertebrates that eat zooplankton become more abundant and so on, up the food chain. Krill, planktonic crustaceans, and larvae commonly eaten by whales, fish, seals, penguins, and seabirds feed on diatom phytoplankton.

Red Tides

Red tides

The term red tide is applied to red, orange, brown, or bright-green phytoplankton blooms, or even to blooms that do not discolor the water. Red tides are poorly understood and unpredictable.

No one is certain what causes the rapid growth of a single species of phytoplankton, although they can blossom where sunlight, dissolved nutrient salts, and carbon dioxide are available to trigger photosynthesis.

Dense phytoplankton blooms occur in stable water where lots of nutrients from sewage and run off are available. Natural events, such as storms and hurricanes, may remobilize populations buried in the sediment.

These nuisance blooms, usually caused by dinoflagellates, which turn the water a reddish brown, and cyanobacteria, are becoming more frequent in coastal waters, possibly because of increased human populations and sewage. In shallower bodies of water, such as bays and estuaries, nutrients from winter snow runoffs, spring rains, tributaries, and sewage bring about spring and summer blooms.

Some of the poisons produced during red tides are the most powerful toxins known. The release of toxins by dinoflagellates may poison the higher levels of the food chain as well as suppress other phytoplankton species. These toxins cause high mortality in fish and other marine vertebrates.

They can kill the whales and seabirds that eat contaminated fish. Dinoflagellates produce a deadly neurotoxin called saxitoxin, which is fifty times more lethal than strychnine or curare. Commercial shellfish, such as mussels, clams, and crabs, can store certain levels of the toxin in their bodies.

People who eat contaminated shellfish may experience minor symptoms, such as nausea, diarrhea, and vomiting, or more severe symptoms such as loss of balance, coordination, and memory, tingling, numbness, slurred speech, shooting pains, and paralysis. In severe cases, death results from cardiac arrest. When the toxins are blown ashore in sea spray, they can cause sore throats or eye and skin irritations.

Toxic blooms costs millions of dollars in economic losses, especially for fisheries which cannot harvest some species of shellfish. Smaller fish farms can be devastated. Additionally, coastal fish deaths foul beaches and shore water with decaying bodies, which can cripple tourism in the coastal regions.

Not all blooms are harmful, but they do affect the marine environment. Even when no toxins are released, massive fish kills can result when the large blooms of phytoplankton die.

When the blooming phytoplankton population crashes, bacterial decomposition depletes the oxygen in the water, which in turn reduces water quality and conditions, andfish and other marine animals suffocate.