Plankton Varieties

(importance of plankton in marine life)

Marine life is classified into three groups: benthos, nekton, and plankton.

Benthos refers to the plants and animals living on the sea bottom; such as, the permanently fixed or immobile forms (sponges and corals), the various creeping forms (crabs, snails), and others that burrow. Barnacles, the larger seaweeds, and sea squirts are also members of this benthos group.

Nekton are swimming animals that can move freely and that are capable of migration from one place to another.

Plankton are the floating and drifting small animals known as (zooplankton) and plants (phytoplankton) capable of very limited locomotion, if any.

  • Animal life of one kind or another exists at all depths of the oceans and in great abundance; indeed, nearly half of all classes of animals are marine.
  • Plant life is much less abundant and consists chiefly of the phytoplankton and the large seaweeds and algae.
  • The animal life is almost entirely dependent on the phytoplankton for its existence and these minute forms, in turn, have the same requirements for growth as other green plants.
  • These minute forms, that constitute the primary source of food in the oceans, are restricted to a rather shallow layer of water generally no more than 450 feet (137 meters) deep [under even the most favorable conditions] called the photic zone, the zone in which there is sufficient light for plant growth.
  • The phytoplankton are consumed chiefly by their animal counterparts, the zooplankton, as well as the larvae of most of the larger forms that occupy the seas.
  • These in turn, constitute the food source for still larger animals and, in general, each form tends to feed on smaller organisms and is itself the food supply for somewhat larger organisms, the whole resting on the existence of great amounts of the simple plant forms of the phytoplankton.
  • All of the animals of the sea do not live directly on phytoplankton; many of them are carnivores, whose food is entirely animal.
  • An important intermediary is the zooplankton that preys upon the phytoplankton and in turn forms the food supply for the larger species.
—Compiled from information located in
Van Nostrand’s Scientific Encyclopedia, Edited by Douglas M. Considine;
Van Nostrand Reinhold Publisher; New York; 1989; pages 2070-2071.

The Invisible Forests of the Oceans

Every drop of water in the top 100 meters of the ocean contains thousands of free-floating, microscopic flora called phytoplankton.

  • These single-celled organisms, including diatoms and other algae, inhabit three quarters of the earth’s surface, and yet they account for less than one percent of the 600 billion metric tons of carbon contained within its photosynthetic biomass.
  • One of the most consequential activities of marine phytoplankton is their influence on climate.
  • New satellite observations and extensive oceanographic research projects are finally revealing how sensitive these organism are to changes in global temperatures, ocean circulation, and nutrient availability.
  • Phytoplankton draw nearly as much carbon dioxide out of the atmosphere and oceans through photosynthesis as do trees, grasses, and all other land plants combined.
  • Because phytoplankton direct virtually all the energy they harvest from the sun toward photosynthesis and reproduction, the entire marine population can replace itself every week.
  • In contrast, land plants must invest copious energy to build wood, leaves, and roots and take an average of twenty years to replace themselves.
  • As phytoplankton cells divide, every six days on average, half the daughter cells die or are eaten by zooplankton, miniature animals that in turn provide food for shrimp, fish, and larger carnivores.
  • Most influential to climate is the organic matter that sinks into the deep ocean before it decays.
—Compiled from “The Ocean’s Invisible Forest” by Paul G. Falkowski;
Scientific American; August, 2002; pages 38-45.

“Japan Blames Whales for Lower Fish Catch, Claim Seeks to Defend Whale Hunting” as seen in the July 28-29, 2001 (page 2) issue of the International Herald Tribune:

  • This is Japan’s latest argument for resuming its whale hunt: Whales eat too much.
  • As part of its effort to resume commercial whaling and justify its annual catch of about 500 whales for “research”, Japan now argues that whales consume more than their share of fish; fish that should be eaten by humans.
  • Richard Mott, vice president of the World Wildlife Fund, said, “It’s undoubtedly true that whales eat tons and tons of fish, but it’s not true that every fish the whale eats is a fish humans can’t catch.”
  • Mott went on to say that whales may eat fish that prey on smaller fish, and thus keep the overall fish population higher, not lower.
  • Despite the international ban, Japan kills hundreds of whales under an exception allowing whales to be hunted for “scientific research”, though the meat is then sold for food.
  • In the European edition of Time (August 6, 2001) there was a similar article (page 21) titled: “Whale of A Fight” by Maryann Bird.

There are Plankton and Then There Are Other Plankton

Plankton are organisms that are unable to maintain their position or distribution independent of the movement of water or air masses.

The term plankton is a collective term for the wide variety of plant and animal organisms, often microscopic in size, that float or drift freely in water because they have little or no ability to determine their own movement; found worldwide in both aquatic and marine environments and representing the basic level of many feeding relationships.

Plankton refers to small animal and plant organisms that float in water and came into English as a borrowing of German Plankton, from a Greek element meaning “wandering, drifting”. The German Plankton was coined in 1887 by the German physiologist and marine biologist Viktor Hensen, 1835-1924.

  • As with the animal kingdom, the plant kingdom is divided systematically into groups and subgroups.
  • A grand division has two subkingdoms, with the more primitive being Thallophyta.
  • These include all the one-celled plants, plus related multi-celled plants in which the individual cells have comparatively little specialization.
  • The largest of these are the seaweeds which consist of undifferentiated shoots (Greek, “thallos”) and lack roots, leaves, or true stems.
  • The Greek phyton means “plant” so Thallophyta means “shoot-plants”.
  • The Thallophyta are divided into a number of phyla which in turn fall into two groups, those that include plants with chlorophyll and those which include plants with no chlorophyll.
  • The chlorophyll plants are called algae (singular, alga) which is the Latin word for seaweed, an alga that can be seen with the naked eye.
  • The non-chlorophyll plants are fungi (singular, fungus), which is the Latin word for musroom, a fungus that can be seen without the use of a microscope.
  • The plant life of the oceans belongs to the Thallophyta and it is said that it makes up about 85 per cent of all the greenery on earth.
  • The algae of the oceans manufacture their food with the aid of sunlight and so must exist only in the layers of the ocean where light can penetrate.
  • There they float, drifting with the current and serving as food, directly or indirectly, for all the animal life of the sea.
  • The algae are phytoplankton.
  • The Greek “planktos” means “wandering”, so phytoplankton are “plant-wanderers” because they must drift (“wander”) with the ocean currents.
  • Small animal life that also drifts in the currents makes up the “zooplankton”.
  • Together, phytoplankton and zooplankton are simply referred to as plankton.
  • There are also animals in the surface waters that are independent of the currents and swim as they please. These are nekton, from the Greek “nektos” (swimming).
—From Words of Science by Isaac Asimov;
Houghton Mifflin Company; Boston; 1959; page 190.

Plankton word unit.Here is a unit about plankto-, -plankton words.