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Plankton is a general term that includes plants, animals, and bacteria that drift through lakes and the oceans. Plankton are the foundation for all life in the ocean and produce much of the oxygen that sustains life on Earth. Plankton represents a diverse and large group of organisms. Often, the only factor that the nearly ten thousand different species of plankton have in common is their poor swimming ability. Rather than control where they are moving, like fish, whales, and turtles, plankton simply float wherever the water currents take them. In fact, the word plankton is derived from the Greek word planktos, meaning "to wander."

Studying and classifying plankton

Biologists identify and count plankton found in water samples. Several different methods are used to collect plankton from water samples, the most common of which includes the use of plankton nets. Most plankton nets are made of nylon or synthetic material that is produced so that the size of the holes between the fibers is uniform. The most common shape for a plankton net is cone-shaped, and the large end of the cone is attached to a metal net ring. The diameter of the net ring is usually 39 inches (1 meter). The small end of the cone is fitted with a plastic bottle, called a net bucket. The net is pulled slowly behind a boat for a specific distance and plankton larger than the holes in the net are trapped in the net bucket. The net is then reeled into the boat and the plankton trapped in the net bucket are removed for study.

Once the plankton have been removed from the net bucket, scientists identify and count the different species of animals and plants. Once the number of a certain type of plankton found in the net bucket has been counted, biologists calculate the concentration of that organism. The volume of water sampled can be calculated by multiplying the distance the net was towed by the area spanned, by the net ring. The concentration of different types of plankton gives biologists information about the water quality and ecology (study of the relationships between organisms and their environment). Biologists also use some of the larger and sturdier plankton removed from the net bucket for experiments involving nutrition, reproduction, and different processes in the human body.

Many plankton are extremely small and pass through even the smallest holes in plankton nets. In order to study these plankton, biologists filter a specified amount of sample water through membranes (tissue) that have very small holes. The water will pass through the membranes, but any plankton larger than the holes in the membranes will be concentrated on top. Researchers can then attach the membranes to slides and view them under a microscope. Special dyes are often used to stain the plankton in order to see them more clearly. In other cases, the membranes with the plankton on them are ground up and are analyzed chemically, which helps researchers determine the types of plankton in the water.

Because there are so many different types of plankton, counting and identifying them is time-consuming and often difficult. Instead, biologists often classify plankton into broad groups that simplify the process, but still provide important information about the ecology of the water sample. Three criteria often used for plankton classification are size, cell structure, and life history.

Size of plankton There are six major size categories of plankton. They range in size from plankton far too tiny to be seen with the naked eye to organisms that are many feet (meters) long.

  • Net plankton: These plankton include species that are large enough to be caught in plankton nets.
  • Macroplankton: Plankton larger than 0.79 inches (more than 20 millimeters) are called macroplankton. Macro is the prefix meaning "large." Macroplankton include the larval (immature worm-like stage) forms of many fish, some marine worms, many different types of crustaceans (water animals with an outside skeleton, including shrimp, crabs, lobsters) and jellyfish that can have tentacles stretching 25 feet (8 meters). Some plants are also classified as macroplankton, such as the giant seaweed Sargassum.
  • Mesoplankton: Plankton between the sizes of 0.79 to 0.0079 inches (20 to 0.2 millimeters) are called mesoplankton. The prefix meso means "medium." Examples of mesoplankton are shrimp-like creatures called euphausids and many types of larval fish.
  • Microplankton: These plankton range between 0.0079 to 0.000079 of an inch (between 0.2 and 0.02 millimeters. The prefix micro means "small."
  • Nannoplankton: Plankton between 79 ten-thousandths to 79 millionths of an inch (between 0.02 to 0.002 millimeter) are called nannoplankton. These plankton are so small they must be concentrated on filters in order to be identified. The prefix nanno means "very small." These plankton include many different types of protozoans (a type of one-celled animal), single-celled plants, and the larvae of crabs, sea urchins, and mollusks.
  • Picoplankton: The smallest group of plankton is the picoplankton, which are less than 79 millionths of an inch (2 thousandths of a millimeter or 0.0002 millimeters) wide. Picoplankton are the smallest and most numerous plankton in the ocean. The prefix pico means "extremely small." Picoplankton include bacteria that ingest organisms for food organisms, as well as a type of bacteria that can gather energy from the Sun as do plants. Picoplankton also include many different species of single-celled protozoans and single-celled plants.

Cell structure of plankton. Plankton are classified into three major groups according to cell type: phytoplankton, zooplankton, and bacterioplankton.

Phytoplankton: Phytoplankton are plants and oxygen-like bacteria. The prefix phyto means "plant." Most phytoplankton are single-celled organisms, although there are some phytoplankton that form colonies (groups) and others that are multicellular, such as seaweed. In the open oceans, about three-fourths of phytoplankton are nannoplankton. In coastal waters and lakes, phytoplankton tend to be larger, in the microplankton size range.

Diatoms are the most numerous group of phytoplankton. Diatoms are single-celled plants that can be shaped like rods, spools of thread, or pillboxes (round boxes with a top and bottom of equal height). They secrete two shells made of silicon, the same substance that makes up glass. The plant cell lives inside the silicon shells and produces threads that protrude through perforations in the shells. In some species, the threads join with threads on other diatoms and form long chains. Diatoms are usually found in the surface waters and when conditions are right, they can reach high concentrations that can make the water appear green.

After diatoms, dinoflagellates are the next most common group of phytoplankton. Dinoflagellates come in many different shapes, but commonly look like a chocolate candy kiss placed bottom to bottom with another candy kiss that has two peaks instead of one. Most have two flagella (whip-like appendages) that they use like propellers to spin through the water. Some are covered with protective plates composed of cellulose, the material that makes up the woody part of trees. A few species of dinoflagellates contain chemicals that poison fish and, occasionally, people. Other dinoflagellates contain pigments that make the ocean appear red.

Zooplankton: Zooplankton are animal plankton that wander in the water currents. Because they are poor swimmers, most zooplankton have special feeding structures that allow them to capture food that they bump into as they drift. These structures come in all forms, from sticky hairs to nets made out of mucous, to brush-like appendages that sweep food particles toward the mouth. Most zooplankton diet on phytoplankton.

Red Tides

Red tides are actually not tides at all, but rather the common name for patches of ocean or lake water that are given their color by a type of reddish-brown phytoplankton called a dinoflagellate. Under certain environmental conditions (such as after a winter rainstorm), these single-celled plants can quickly grow to extremely high concentrations. When dinoflagellates occur in these dense "blooms," they actually make the water appear red. Some of these blooms become so large they can be observed from satellites in space. One of the common dinoflagellates that cause red tides in the waters off of California is called Gymnodinium breve. This species can also produce sparks of light called bioluminescence. During a red tide, these phytoplankton cause the tops of waves to glow at night.

Most red tides are harmless, but some do have negative impacts. As phytoplankton grow very quickly, they consume most of the nutrients in the water. After the nutrients are gone, the phytoplankton cells begin to die and bacteria grow on them. These bacteria consume oxygen as they break down the dead plant cells. This can cause the water to become depleted in oxygen. When this occurs, fish and other invertebrates that need oxygen die. As a result, red tides are often associated with large fish kills. Some dinoflagellates contain harmful chemicals that are poisonous to fish and humans who consume the fish. These chemicals can cause aches, stomach pains, dizziness, and tingling in the fingers.

The most common zooplankton in the ocean are the crustaceans, which include the crabs, shrimp, and lobsters, and account for about 70% of all zooplankton. In particular, a small shrimp-like animal called the copepod is the most numerous type of animal in the plankton family. In fact, if all the copepods in the world were divided equally among all the people in the world, each person would receive one billion copepods. Another common zooplankton is the shrimp-like crustacean called the euphausid. Euphausids, also called krill, are slightly bigger than copepods, around 2 inches (5 centimeters) long. They are so numerous in the waters around Antarctica that the diet of many whales consists entirely of krill. In fresh water, the tiny crustacean Daphnia is the most numerous zooplankton. Daphnia are particularly interesting zooplankton because they can reproduce without mating in the spring and summer using a process called parthenogenesis, where female Daphnia produce exact copies of female offspring.

Other important animal groups found among zooplankton are the jellyfish, the worms, the mollusks (squid and snails), and the echinoderms (sea cucumbers and sea urchins). Some zooplankton are single-celled organisms called protozoans. For example, the foraminifera are a type of amoeba (a one-celled animal) that has a shell with holes through it. Foraminifera produce sticky spines that extend through the holes, where animals that bump into them are captured and eaten.

Bacterioplankton: The smallest type of plankton, bacterioplankton are microscopic single-celled organisms. Along with other forms, they play an important role in the ecology (living organisms and their environment) of aquatic systems. Bacteria are single-celled microscopic organisms. These organisms are numerous. Bacteria digest dead zooplankton and phytoplankton, producing the nutrients and other materials needed for new life to grow.

Life history of plankton Plankton are also classified according to their life history. Most species of bacterioplankton, phytoplankton, and zooplankton spend their entire life floating and drifting with the currents. These plankton are called holoplankton (holos is the Greek root meaning "entire").

Other plankton live only the early part of their life as plankton; the adult part of their life is spent in a different part of the ocean or lake. These plants and animals are called meroplankton (meros is the Greek root meaning "mixed"). Some examples of meroplankton are sea urchins, sea slugs, lobsters, worms, and some coral reef fish. Some aquatic plants are also meroplankton. Many meroplankton scatter eggs into the plankton, where they are fertilized. The fertilized eggs develop into larvae, which float in the plankton. Just as a caterpillar looks nothing like a butterfly, in general these larvae look nothing like the adults they will eventually become. When the larvae are in the plankton, they eat other plankton or survive off of the yolk that was with them in the egg. Depending on the species, the larvae remain in the plankton for varying periods of time from several days to several months. Afterwards, the larvae settle onto the seafloor or swim away from the plankton and change into their adult form.

Importance of plankton

Plankton are vital to the global climate. Phytoplankton perform photosynthesis, which is a process that uses the energy from sunlight to produce food. In the process of photosynthesis, phytoplankton take in carbon dioxide and produce oxygen. About half of the oxygen on the planet comes from phytoplankton photosynthesis. As humans burn oil and gas to keep their cars moving and their houses warm, carbon dioxide is produced. This carbon dioxide holds heat and is one of the leading causes of global warming. It is estimated that phytoplankton remove three billion tons of carbon dioxide from the atmosphere each year, as much as all the trees on land.

Plankton are also key to the ecology of the ocean. Phytoplankton are the base of the marine food chain. In other words, they are the food for zooplankton, corals, and mollusks. Even some sharks, such as basking sharks and nurse sharks, rely on phytoplankton for their diet. In turn, fish and larger predators eat the zooplankton. In fact, the giant blue whale relies entirely on shrimp-like zooplankton called euphausids for its diet. Humans also eat fish that prey on plankton.

After plankton die, they sink to the bottom of the ocean. Over millions of years, the dead plankton are buried by sediments, and then eventually converted into fossil fuels such as oil and gas.

Juli Berwald, Ph.D.

For More Information


Garrison, Tom. Oceanography. 3rd ed. Belmont, CA: Wadsworth, 1999.

Miller, Charles B. Biological Oceanography. Malden, MA: Blackwell Science, 2004.


Carlson, Shawn. "The Amateur Scientist: How to Rear a Plankton Menagerie." Scientific American (August 2000).


"Beyond the Reef: Meroplankton." The Australian Museum Online. (accessed on August 12, 2004).

Herring, David. "What Are Phytoplankton?" NASA's Earth Observatory. (accessed on August 12, 2004).

Perry, Robert. "A Guide to Marine Plankton of Southern California." UCLA Marine Science Center Ocean GLOBE. (2003). (accessed on August 12, 2004).

"Phytoplankton Lab." Smithsonian Environmental Research Center. (accessed on August 12, 2004).

"The Red Tide and Harmful Algae Page." National Atmospheric and Atmospheric Administration and Woods Hole Oceanographic Institution. (accessed on August 12, 2004).

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Awareness is growing regarding the importance of the oceans and the variety of life they support. Research in many branches of oceanography is discovering the vast unknown of the marine world, and has expanded interest in the understanding of the marine environment and the role each member plays in a complex community.

The free-floating organisms known as plankton, from the Greek "wandering," are the drifters of the ocean. Although most of these organisms are motile (moving), they cannot swim or move against currents, but they can move vertically in the water column.

Many marine plankton are found in the deep waters of the outer ocean, or pelagic waters, whereas others are found in the shallow waters known as the neritic zone. Many of the neritic plankton are known as meroplankton, and spend only a brief period of their life cycle in the planktonic category. Many pelagic forms, such as the holoplankton, are planktonic during their entire lifespan.

The size of plankton can also determine its general name.

  • Picoplankton: Smaller than 2 μm ; includes bacteria, prochlorophytes, and viruses
  • Nanoplankton: 2 to 20 μm; includes diatoms, coccoliths, and silicoflagellates
  • Microplankton: 20 to 200 μm; includes large diatoms, dinoflagellates, and small zooplankton, such as ciliates
  • Macroplankton: 200 to 2,000 μm; includes large zooplankton, copepods, and invertebrate larvae
  • Megaplankton: Larger than 2,000 μm; includes fish larvae and gelatinous zooplankton


Many kinds of marine and fresh-water organisms utilize inorganic carbon (as carbon dioxide) and fix it into organic compounds by photosynthesis . The principal taxa of microscopic planktonic producers, primary producers , are found over most of the world's oceans, lakes, rivers, and estuaries, and comprise the base of the food web . Phytoplankton consist primarily of diatoms, dinoflagellates, coccolithophorids, silicoflagellates, bacteria, and viruses. All of the organisms discussed below are key players in the microbial food web.


Diatoms have cell walls of silica and pectin, and float in the water column or attach to surfaces as single cells or chains. They are one of the major contributors to primary production in coastal waters, and occur everywhere in the ocean, but are most abundant in colder, nutrient-rich, nearshore waters. Cell division occurs by fission, which is accompanied by a reduction in cell size. They are one of the principal groups that fix carbon through photosynthesis, and this production is prominent during seasonal blooms of short duration.


Dinoflagellates occur as single cells, either naked or within a cellulose cell wall, and many species use flagella to move. These organisms are sometimes classified as protozoa and algae because of their ability to photosynthesize and also absorb nutrients by being parasitic, or by ingesting organic particles. They are second to diatoms in contributing to primary production, and are widespread in the oceans, but are most abundant in nutrient-poor waters offshore. Reproduction is by cell division. Some species are bioluminescent (emitting a pale blue glow seen at night). Dinoflagellates often are the cause of red and brown tides, so named because the algal pigments give the water a colored tint.


Coccolithophorids are single-celled organisms. Many are flagellated, and are protected by ornate calcareous plates, called coccoliths, embedded in a gelatinous sheath that surrounds the cell. These organisms may form cysts that produce spores to produce new individuals. They are most abundant in warm, open-ocean waters, and are sometimes found nearshore.* Coccolithophores can photosynthesize (autotrophic) and may also absorb organic matter (heterotrophic).


Silicoflagellates occur as single flagellated cells and typically secrete a silicious outer skeleton. Like coccoliths, these organisms are both autotrophic and heterotrophic , and are most abundant in cold, nutrient-rich waters.


Bacteria are prokaryotes with cell walls made of chitin , and occur as single coccoid cells or long filaments. They often are restricted to waters with low oxygen, and are important in the metabolism of aquatic ecosystems. To support their metabolism, they obtain nutrients by the uptake of organic matter and the release of exoenzymes to lyse (distintegrate or dissolve) particulate organic matter, and attack diatoms, dinoflagellates, and flagellates. Blue-green algae, or cyanobacteria, are photosynthetic. Bacterial activity in marine waters is strongly affected by availability of nutrients and organic matter. Their productivity increases as phytoplankton productivity increases.


Viruses play an important role in marine food webs. They infect a wide range of hosts, including bacteria and phytoplankton. They can potentially reduce phytoplankton and bacterial production by viral lysing of their cells and the releasing of dissolved organic carbon. This dissolved carbon can than be utilized by other phytoplankton cells.


Prochlorophytes are a recently discovered group of extremely abundant producers that are barely visible by microscopy. They are most abundant at the lower layers of the illuminated region of the water column, and are now considered to be another major player in primary production.


Nanoflagellates are both autotrophic and heterotrophic. They feed on viruses, bacteria, and some picoplankton and nanoplankton. Nanoflagellates are major consumers of bacteria; some experiments show that they may be able control their abundances when larger predators, such as dinoflagellates, are not present. However, this is less likely to occur in nature.


Nanoplanktonic and microplanktonic protozoan groups are mainly ciliates and heterotrophic dinoflagellates. They consume bacteria, nanoplankton, and microplankton. While these groups engulf their prey, they also release nutrients that stimulate the growth of these same prey.


Zooplankton are planktonic free-floating animals in fresh and marine aquatic systems, and are the major consumers of the organisms in the microbial food web. These organisms possess a wide range of feeding strategies, from the nematocysts (stinging cells) of cnidarians (e.g., jellyfish) to the complicated mouthparts of copepods. Some are carnivorous (animal-eaters), some are herbivorous (plant-eaters), and some are omnivorous (eaters of plants and animals).

These animals can move by means of cilia, flagella, jointed appendages, jet propulsion, or tailed larvae (as in tunicates to larval fish). Reproduction varies from asexual, to fission and fragmentation, to sexual reproduction where some gametes are released into the water and fertilized, yet others are retained and fertilized internally.

Zooplankton include many phylum, and not all can be discussed here. Some live their entire life cycle in the water (holoplankton), whereas only the larval stages of fish and other benthic organisms (such as starfish) live in the water column for a short time (meroplankton). All are considered zooplankton. An overview of the major zooplankton phyla follows.


Discussed previously, this group includes ciliates, dinoflagellates, foraminifera, and radiolarian.

Coelenterata (Cnidaria).

Typically known as jellyfish, the major groups are Hydrozoa, Scyphozoa, and Anthozoa. The hydrozoans medusae are the prominent members in zooplankton, and the most common forms are aurelia, pelagia, and siphonophores. These gelatinous animals are major consumers of smaller zooplankton and some of the microbial food web.


Best known as comb jellies, these possess eight "comb" rows of fused cilia. When they are abundant, these animals can consume phytoplankton and zooplankton, and can clear the water of food for other zooplankton.


Known as the arrow worm, this is a common member of deep-water plankton. Smaller species are found in coastal waters, whereas larger species are abundant offshore in blue water. They are predacious carnivores that grasp their prey and paralyze them before ingesting them.


This includes many species of marine polychaetes. Many of these organisms can be seen on the surface at night, shedding gametes for sexual reproduction. Their larvae are abundant in the zooplankton community.


This includes marine gastropod larvae, pteropods, and cephalopods (commonly known as squid and octopus).* Mollusks are consumers of larger zooplankton.


This includes starfish, brittle stars, and sea cucumber. All these animals are meroplankton. Their larvae are a major presence in the zooplankton community.


These are the major members of zooplankton and include copepods, shrimp, crabs, lobsters, amphipods, crustaceans, and euphausids, or krill, which are the major source of nutrition for some whales.* The most studied of crustacea are the copepods. These animals are found in all parts of the world's oceans, lakes, and estuaries and are considered the major consumers of most of the organisms in the microbial loop. Because they are holoplankton, spending their entire life in water, they can consume a wide range of food particles, from nanoplankton to microplankton, as they mature. Copepods are responsible for much of the carbon energy transferred from phytoplankton to larger zooplankton.


Known as the urochordates (tunicates), this includes ascidians (or sea squirts) and are found on the coast, whereas larvaceans, oikopleura, thaliaceans, salps, and doliolids are pelagic and spend their entire life cycle in the water column. Tunicates are now realized to be major consumers of phytoplankton and smaller zooplankton, and can contribute to the entire food-web dynamics as much or even more than copepods.

Larvaceans have retained their notochord and tail as adults and produce a mucus net, or "house," around their bodies to capture food particles. The house is either ingested or abandoned.

The salps and doliolids are free-swimming tunicates with a cylindrical or barrel-shaped body with up to eight muscle bands to aid in swimming by jet propulsion and feeding with an internal mucus net. These animals have a complicated life cycle that includes a sexual stage and one or two asexual stages. They are known for their ability to create "blooms," or a rapid increase in their abundance, exceeding 1,000 animals in a cubic meter of water in a short period of time. With this rapid increase in population and their ability to filter feed a wide range of food sizes, they can outcompete copepods during these bloom events. Their role in the food web is being studied more intensely because of their production of large, fast-sinking fecal pellets that can transfer organic matter produced by primary producers to fish and benthic organisms.

see also Algal Blooms, Harmful; Algal Blooms in the Ocean; Cephalopods; Crustaceans; Ecology, Fresh-Water; Ecology, Marine; El NiÑo and La NiÑa; Lakes: Biological Processes; Life in Water; Microbes in the Ocean; Ocean Biogeochemistry.

Deidre M. Gibson


Barnes, R. S. K., and K. H. Mann. Fundamentals of Aquatic Ecosystems. Oxford, U.K.: Blackwell Scientific Publications, 1980.

Parsons, T. R., M. Takahashi, and B. Hargrave. Biological Oceanographic Processes, 3rd ed. Oxford, U.K.: Pergamon Press, 1984.

Smith, Deboyd L. A Guide to Marine Coastal Plankton and Marine Invertebrate Larvae. Dubuque, IA: Kendall/Hunt, 1977.

Valiela, Ivan. Marine Ecological Processes, 2nd ed. New York: Springer Verlag, 1995.

* See "Algal Blooms in the Ocean" for a photograph of a coccolithophore bloom near the coast.

* See "Cephalopods" for a photograph of an octopus's suction-cupped tentacles, and the Nautilus.

* See "Crustaceans" for photographs of crustaceans, and "Ecology, Marine" for a photograph of a krill swarm.

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Plankton (from the Greek word planktos, which means "wandering") are communities of mostly microscopic organisms that inhabit watery environments, from oceans to muddy regions. Some plankton drift passively or swim weakly near the surfaces of oceans, ponds, and lakes, while others exist as bottom-dwellers, attaching to rocks or creeping on the ground through sand and silt.

Plankton are classified under the kingdom Protista. During the genesis of protists, a true nucleus, as well as the other components of eukaryotic cells (mitochondria, chloroplasts, endoplasmic reticulum, Golgi bodies, 9 2 flagella and cilia , the functions mitosis and meiosis ) arose. Thus these organisms are considered to be ancestral to plants, fungi and animals. While the majority of plankton are unicellular and therefore considered to be simple eukaryotic organisms, at the cellular level they are extremely complex. Plankton should be considered an organism in itself and not be compared to a single cell from a multicellular organism.

Despite their small size, plankton are the very basis for life in the earth's various ecosystems . An ecosystem is comprised of all the organisms living in a community and all abiotic factors with which the organisms interact.

The two main processes within an ecosystem are energy flow and chemical cycling. Energy enters most systems in the form of sunlight and is converted to chemical energy by autotrophs . The chemical energy is then passed to heterotrophs in organic compounds of food, and finally dissipates into the system as heat. Trophic levels are based on an organism's main source of nutrition. Autotrophs, also called primary producers, are generally photosynthetic organisms that use light energy to synthesize sugars and other organic compounds. Heterotrophs, or consumers, are supported by these photosynthetic organisms. The primary consumers are herbivores, who gain sustenance directly from autotrophs. Secondary consumers feed on the herbivores and tertiary consumers feed on the secondary ones. Those organisms that feed off of dead organisms are known as detritovores. An understanding of this pyramid within an ecosystem explains why the extent of photosynthetic activity determines the energy supply of the entire ecosystem.

Algae, as freshwater and marine phytoplankton and intertidal seaweeds, are responsible for nearly half of all photosynthetic production of organic material, rendering them extremely significant in the aquatic food webs where they support countless suspension-feeding and predatory animals. All algae, except prokaryotic cyanobacteria (formerly called blue-green algae), belong to the kingdom Protista. Algae all contain chlorophyll A, a primary pigment in cyanobacteria and plants, but differ in accessory pigments, which trap wavelengths of light to which chlorophyll A is not as sensitive. These accessory pigments include other chlorophylls (green), carotenoids (yellow-orange), xanthophylls (brown), and phycobilins (red and blue).

These differences in pigments point to different roles and effects of algae on the ecosystem. An overabundance of dinoflagellates (algae containing phycobilins) results in the blooming of red tides. When shellfish such as oysters feed on the dinoflagellates, they concentrate the algae along with toxic compounds released by the dinoflagellate cells. Because these toxins are dangerous to humans, collection of shellfish is restricted during red tides to reduce the risk of paralytic shellfish poisoning. Seaweed is the large marine algae that inhabits intertidal and subtidal zones of coastal waters. Coastal people, especially in Asia, harvest seaweed for food since it is high in iodine and other essential minerals. The brown alga laminaria is used in soups and the red alga porphyra is used to wrap sushi.

With so much dependent on the existence of plankton, the fight against water pollution aims to prevent not only the destruction of plankton, but of other species as well.

see also Food Web; Zooplankton.

Danielle Schnur


Campbell, Neil A. Biology, 3rd ed. Berkeley, CA: Benjamin/Cummings Publishing Company, Inc, 1993.

Odum, Howard T. Systems Ecology: An Introduction. New York: Wiley, 1984.

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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 organismsplants, animals, and microorganismstogether 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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Plankton are small aquatic organisms that live in both freshwater and marine environments. The word "plankton" is derived from the Greek word planktos, which means "drifting." In general, plankton have little or no means of locomotion and their distribution is determined largely by water currents and mixing. However, some plankton can swim through less turbulent waters using flagella and other appendages .

There are several broad categories of plankton. Phytoplankton are small plantlike plankton and are commonly referred to as algae. Phytoplankton are primary producers (they use energy from the sun to make organic food molecules). Bacterioplankton are very small (only seen through a microscope) and include bacteria, fungi, and viruses. Some bacterioplankton play important roles as primary producers and others as decomposers. Zooplankton are planktonic invertebrate animals (for example, the water-flea Daphnia ). Some zooplankton consume phytoplankton, whereas others are predatory and consume smaller zooplankton. Ichthyoplankton are planktonic fish eggs and larvae. The ichthyoplankton are highly vulnerable to predation by invertebrate and vertebrate predators.

Plankton are important because they form the base of aquatic food webs . That is, plankton are a critical food resource for other aquatic organisms (such as fish) that live in freshwater and marine environments. Plankton are important to humans because they support recreational and commercial fisheries. Some humans consume plankton directly in the form of dietary supplements. For example, the phytoplankton species Spirulina has been marketed as a source of vitamins and protein .

Plankton are also important in processes that control the distribution and movement of energy and essential nutrients such as carbon, nitrogen, and phosphorus. A significant amount of the total global carbon is stored in the ocean. Some researchers have proposed that it is possible to increase the uptake of carbon dioxide generated by human combustion of fossil fuels by increasing production of ocean plankton through fertilization . Researchers debate whether this proposal is practical at a large scale.

see also Algae; Biogeochemical Cycles; Ecosystem; Estuaries; Ocean Ecosystems: Hard Bottoms; Ocean Ecosystems: Open Ocean; Ocean Ecosystems: Soft Bottoms

Janet M. Fischer


Hutchinson, G. Evelyn. A Treatise on Limnology, vol. 2. New York: John Wiley & Sons, Inc., 1967.

Smith, DeBoyd L. A Guide to Marine Coastal Plankton and Marine Invertebrate Larvae. Dubuque, IA: Kendall/Hunt Publishing Company, 1996.

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plankton (adj, planktonic) Minute aquatic organisms that drift with water movements, generally having no locomotive organs. The phytoplankton (plants) comprise mainly diatoms, which carry out photosynthesis and form the basis of the aquatic food-chains. The zooplankton (animals) which feed on the diatoms may sometimes show weak locomotory powers. They include protozoans, small crustaceans (Crustacea), and in early summer the larval stages of many larger organisms. Plankton are sometimes divided into netplankton (more than 25 μm diameter) and nanoplankton, which are too small to be caught in a plankton net (see NANO-; and NANOFOSSIL).

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plank·ton / ˈplangktən/ • n. the small and microscopic organisms drifting or floating in the sea or fresh water, consisting chiefly of diatoms, protozoans, small crustaceans, and the eggs and larval stages of larger animals. Many animals are adapted to feed on plankton, esp. by filtering the water.Compare with nekton. DERIVATIVES: plank·tic / -tik/ adj. plank·ton·ic / -ˈtänik/ adj.

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plankton Minute pelagic organisms that drift or float passively with the current in a sea or lake. Plankton includes many microscopic organisms, such as algae, protozoans, various animal larvae, and some worms. It forms an important food source for many other members of the aquatic community and is divided into zooplankton and phytoplankton. Compare benthos; nekton; neuston.

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Organisms that live in the water column, drifting with the currents. Bacteria, fungi , algae, protozoans, invertebrates, and some vertebrates are represented, some organisms spending only parts of their lives (e.g. larval stages) as members of the plankton. Plankton is a relative term since many planktonic organisms possess some means by which they may control their horizontal and/or vertical positions. For example, organisms may possess paddle-like flagella for propulsion over short distances, or they may regulate their vertical distributions in the water column by producing oil droplets or gas bubbles. Plankton comprise a major item in aquatic food chain/webs.

See also Phytoplankton; Zooplankton