Plankton are microscopic plants and small animals that live in the surface waters of oceans, lakes, and rivers and drift with the currents. They include bacteria, fungi, algae, protozoa, invertebrates, and some vertebrates.

Phytoplankton are photosynthetic, meaning that they use sunlight to convert carbon dioxide and water into organic molecules such as glucose to use as food. Phytoplankton include microscopic algae, blue-green bacteria, and some true bacteria. These organisms exist in waters where light is able to penetrate. Phytoplankton form the base of nearly all aquatic food chains, directly or indirectly supplying the energy needed by most aquatic protozoa and animals.

Words to Know

Algae: Single-celled or multicellular plants or plantlike organisms that contain chlorophyll.

Bacteria: Single-celled microorganisms that live in soil, water, plants, and animals, and some of which are agents of disease.

Consumer: Organisms that cannot make their own food and consume other organisms to obtain the nutrients they need for growth.

Ecosystem: A community of organisms—plants, animals, and microorganisms—together with their environment.

Food chain: A series of organisms, each dependent on the organism at the level below it for food.

Food web: An interconnected set of all the food chains in the same ecosystem.

Fungi: Kingdom of various single-celled or multicellular organisms, including mushrooms, molds, yeasts, and mildews, that do not contain chlorophyll.

Invertebrates: Animals that lack backbones.

Photosynthesis: Chemical process by which plants containing chlorophyll use sunlight to manufacture their own food by converting carbon dioxide and water to carbohydrates, releasing oxygen as a by-product.

Primary producer: Organisms such as plants, algae, and certain bacteria that make organic molecules from inorganic substances.

Protozoa: Single-celled animal-like microscopic organisms that must live in the presence of water.

Vertebrates: Animals with backbones.

Zooplankton are small grazing animals that live in surface waters and feed on phytoplankton. The amount of zooplankton present in a given area depends on the amount of the microscopic algae present. Zooplankton are a diverse group mostly made up of crustaceans (animals with external skeletons) such as water fleas and shrimps but also include jellyfish, protozoa, and insects. Some species of zooplankton are predators, feeding on other species of zooplankton, and some spend part of their lives as parasites of larger animals, such as fish.

Zooplankton are very important in open-water marine and freshwater food webs. They are eaten by relatively small fish that are then eaten by larger fish. Zooplankton are an important link in the transfer of energy from the algae (the primary producers) to the ecologically and economically important fish community (the consumers).

Species of zooplankton react differently to factors that place stress on aquatic ecosystems. Toxic chemicals, acidity of the water, decreased oxygen, or changes in temperature may kill some zooplankton while others survive. As a result, the health of a body of water or a change in its physical or chemical makeup can be determined in part by the species of zooplankton that are present.

[ See also Crustaceans ; Protozoa ]

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