Plankton Reading


Plankton is derived from a Greek word that means "that which is made to wander or drift". The name plankton applies to aquatic organisms living unattached and having swimming power insufficient to resist most water currents. The term plankton is unusual because it classifies organisms according to their locomotion rather than genetic kinships.

Plankton includes bacteria, plants and animals. Phytoplankton are plankton which make their own food using energy from sunlight. Planktonic animals are called zooplankton and represent an extremely diverse range of organisms; nearly every major group of animals has representatives in zooplankton. Some of those organisms are plankton for only part of their life cycle. Crabs, starfish, clams and some fishes have a larval stage that live as plankton.

Most plankton are small, few are larger than a house fly. With limited mobility, small size is important. Small size reduces the rate at which a plankter will sink. Small size also gives a large surface area to volume ratio so nutrients can be absorbed efficiently.

Why are Plankton Important?

Virtually all marine animals are ultimately dependent on plankton. Phytoplankton are the breadbasket of the sea. They are food for zooplankton and zooplankton are food for fish. Fish are food for birds, mammals and humans. Dead plankton and planktonic organic by-products sink and sustain animals on the sea bottom.

Phytoplankton produce about 95% of the oxygen produced in the sea and a third of total oxygen production on the planet. Biologists can tell a great deal about water bodies from their plankton communities. Water quality is revealed by the mix of plankton species.

Some species of phytoplankton form toxins that can accumulate in the tissues of mussels, clams and oysters that feed on the phytoplankton. Sometimes these numbers of these plankton grow enough to tint the water red, hence the name red tides.

To Know Plankton is to Love Plankton

A community of similar plankton species inhabits much of the Pacific coast of North America, from California to Alaska. A rich productive plankton community lives in sheltered Puget Sound. Although hundreds of species of plankton may populate the Sound at one time or over the course of a year, most of the plankton assemblage is comprised of a relatively few important organisms.

Phytoplankton can be lumped into three broad categories. The phytoflagellates are single cell organisms with a whiplike swimming appendages called flagella. Dinoflagellates have two flagella and a distinctive cell shape. During the day, dinoflagellates stay near the surface of the water photosynthesizing. At night they sink a few meters down, but swim back up before dawn. The third category, the diatoms, have unique cell walls hardened with silica. Marine deposits of diatom shells are mined as diatomaceous earth and the shells are useful as indicator fossils.

Some zooplankton are protozoans, single-celled animals. Most zooplankton are crustaceans including a large group of shrimp-like animals called copepods. Other larger, shrimp-like planktonic crustaceans form 'krill', the staple diet of baleen whales. Zooplankton also include jellyfish, comb-jellies, and small mollusks related to clams and squid. About 3/4 of the animals in Puget Sound have larvae that live as plankton, among them are shipworms, barnacles, mussels, sponges, sea urchins, starfish, sand dollars, smelt, herring, cod, rockfish and flatfish.

And this One is Just Right

Plant production in the waters of Puget Sound is among the highest of any saltwater environment. The biomass produced rivals land forests and farms. Growth of phytoplankton depends on the same factors as land plants. Unlike the land, Puget Sound is not at rest and the water motion affects the availability of both light and nutrients. Water motion is in turn affected by wind, tides, sea floor features and water density. This creates an extremely patchy distribution and variable quantity of phytoplankton production. At the heart of this patchy variability is vertical mixing of water. Phytoplankton need the sunlight of waters close to the surface. They also need nutrients which tend to sink to the deeper waters. Too much mixing keeps the phytoplankton away from the surface and productivity decreases. Too little mixing and the plants suffer from lack of nutrients. Coastal waters are more productive than ocean because of the better mixing of shallow waters. Puget Sound has just the right conditions of mixing to encourage phytoplankton growth.

The Puget Sound was gouged by at least four major glaciations ending just 10,000 years ago. These glaciers created four major basins in Puget Sound (Main, Southern, Whidbey and Hood Canal). The basins are areas of deep water (275 meters maximum) and are separated by shallow ridges. One ridge is found in the waters of Admiralty Inlet, between Whidbey Island and Port Townsend. The water depth here is a shallow 64 meters. There are two high tides and two low tides every 25 hours. Tidal flow over the shallow ridges produces intense mixing of deeper nutrient-rich waters and surface waters. The mixing would be too much if it weren't for the layering of surface waters caused by fresh river water flowing into the Sound. The less-dense fresh water sits atop the denser sea water and stabilizes the water mixing. This allows the plankton to bask in sunny surface waters and use the nutrients provided by the mixing.

Understanding even the smallest creatures around us require us to examine interactions in the studies of geology, oceanography, chemistry, physics, weather and biology.


  1. The Fertile Fjord - Plankton in Puget Sound. Strickland, Richard M. 1983. University of Washington Press. Seattle.
  2. The Shape and Form of Puget Sound. Burns, Robert. 1985. University of Washington Press. Seattle.

Plankton Reading: Answer these questions in complete sentences:

  1. Name two conditions needed to call an aquatic organism a plankter ? (singular form of plankton)
  2. How are organisms usually classified?
  3. The radio has just announced a "red tide" warning. What does that mean and how might it affect people in Oak Harbor?
  4. Compare and contrast dinoflagellates and diatoms.
  5. Describe how water motion makes Puget Sound a good place for plankton.
  6. Oil is extremely toxic to plankton. Describe how water motion in Puget Sound would complicate an oil spill cleanup.