Chapter 12: Plankton

Phytoplankton, image from Wikipedia


Phytoplankton are microscopic algae that form an essential component of the marine food chain.  Put simply, these single-celled plant-like organisms are the foundation of nearly all marine food webs.  Phytoplankton are photosynthetic. They can convert the energy of the sun into organic material that is eaten by everything else. Once more, we now know that the shear size and scale of their photosynthetic actives plays an important role in regulating weather climate.

Phytoplankton literally means ‘phyto’ = plant and ‘planktos’ = made to wander. Like terrestrial plants, plankton contain chlorophyll and need sunlight as well as inorganic nutrients to grow. They are different from the large algae’s and sea grasses because of their small size and their tendency to drift in the ocean currents.  Virtually all marine phytoplankton small particles of oil to increase their buoyancy and live in the upper part of the water column (the epipelagic zone.)

A plankton bloom off the coast of Southern England, image from Wikipedia

Phytoplankton reproduce asexually and if conditions are right, they can increase their populations very quickly.  Because their are very short-lived their populations may crash just as quickly when conditions change. Click on the link to watch an animation of phytoplankton’s life cycle.  

In most place parts of the ocean their abundance is highly variable based on the availability of critical nutrients and sunlight.  Because they are the basis of marine food webs, we refer to the abundance of phytoplankton as primary production. Click on the link to see a satellite animation of the Earth’s primary production.

There are three main types of oceanic phytoplankton; diatoms, dinoflagellates, and cocolithophores.  The diatoms, are common in both freshwater and oceanic habitats.  They have a rigid exoskeleton known as a frustule.  The shell is made of silica (the same material used to make glass) and has two matching halves that fit together like the two halves of a shoe box.   Diatoms can form into circles, spirals, or into long chains near the surface and may have sharp spines.  Diatoms  keep droplets of oil inside their shells to help keep them afloat.  Click to see a series of really striking photos of Arranged Diatoms on Microscope Slides in the California Academy of Sciences Diatom Collection.

Dinoflagellates have two tails (or flagella) that they use to propel them through the water.   They are composed of complex calcium carbonate outer shells or armor plating and come in a variety of shapes and sizes. Some species, called zooxanthellae, live in symbiotic relationships with corals (and several other cnidarian species,) and play an important part in the biology of coral reefs.

Some dinoflagellate species produce bio-toxins that can be poisonous to fish and mammals.  When they bloom in large numbers and eventually die off these toxins may be released. These events are termed “harmful algal blooms” or HABs. These blooms can be dangerous both to aquatic life and to those people who depend on aquatic life for subsistence. How and why these blooms occur is a complex and just beginning to be fully understood.  What is known is that they depend upon on oceanographic currents, winds, and nutrients.  We also know that lots of human pollution, particularly the run off from city sewers and farm waste can increase their frequency and severity.

A cocolithophore, image from Wikipedia

Cocolithophores are very small plankton measuring less than one one thousandth of an inch. They are distinguished by special calcium carbonate plates (or scales) called coccoliths.   Coccolithophores have two golden-brown shaded pigment marks in their cell with the nucleus located between them.


Zooplankton get their names from “zoo” meaning animal and “planktos” meaning wanderer.  The term zooplankton, loosely describes an incredible variety of animals and protozoans ranging in size from microscopic to over one hundred feet long.  What they all share in common is that they are heterotrophic eaters of phytoplankton and they float at the mercy of the oceans currents.  Zooplankton are generally found drifting in the upper reaches of the water column but many will make daily vertical migrations to depths of several hundred meters.

Daily Commute

In what might be the largest migration on the planet, zooplankton make a daily commute between the lower reaches of the epipelagic during the day and the surface at night. Zooplankton need to spend much of their time near the surface feeding on the phytoplankton that float on or near the surface. However, they prefer to do so between dusk and dawn while they are less visible to their predators.  As day light approaches they descend into the depths trying to stay just below the level of visible light.

A sonar image shows zooplankton migrating to the depths at dawn, image from NOAA

Marine biologists had known for years that trawl samples taken at night produced more animals than those taken during the day.  However, the extent of the migration wasn’t realized until World War II when, ships sonar’s looking for enemy submarines detected a puzzling layer on their sonar screens. This “deep scattering layer,” or “false bottom” as it was called, rose toward the surface each evening, and sank again the next morning. In fact, submarines hid within this layer, using the sonar echo it produced to disguise their own.  Click to read more about this.

Keeping Afloat

All species of plankton (both phyto and zoo) have certain structural adaptations help to keep them afloat in the water column. These adaptations include: flat bodies, lateral spines, long thin appendages, which increase the amount of their body surface area in contact with the water.  Water tends to stick to these surfaces and keep the plankton from sinking.  Another approach is to increase ones buoyancy. This can be done by storing oil droplets, gas filled floats, or a variety of other  buoyant materials.

A copepod, image from Wikipedia

There are so many species of zooplankton you could spend years just trying to learn them all.  Listed below are just a few examples.  Click on the link to watch a slide show of zooplankton species.
Copepods (Phylum Arthropoda)

Copepods are incredibly numerous in the worlds oceans.  They can also be found in many freshwater environments. Copepods swim using two long antenna and frontal structures on their bodies. They eat phytoplankton and detritus, and occasionally other zooplankton smaller in size.  They are an important food source for fish and marine mammals.

Cladocerans (Phylum Arthropoda)

Claudoceran (also known as daphnia or water fleas) are planktonic crustaceans found in coastal waters. They swim using an antenna, like copepods, but instead of using their first antenna—they use the second antenna. They appear to have two sections to their body but it’s only an illusion caused by a folded outer shell. Cladocerans eat phytoplankton and other zooplankton. Like many species of zooplankton, cladocerans migrate to the surface at night.

Krill (Phylum Arthropoda)

Krill means whale food in Norwegian.  Its an appropriate name as they make up a huge portion of the diet for many of the worlds largest whale species.  Krill are shrimp-like organisms that feed on phytoplankton.  They may grow up to two inches long and can be found in many of the worlds ecosystems.  They are particularly abundant in Antarctica where they have been seen in swarms as large as 450 square kilometers.  

A Pteropod, image from NOAA

Pteropods (Phylum Mulluska)

There are two main groups of pteropods (wing-foot), one with shells  and one without. They are closely related to snails, but are free swimming with a gelatinous form. They make up a large part of the diets of many juvenile fish species (including salmon.) Shelled pteropods form a calcium carbonate shell made of aragonite and are considered particularly vulnerable to the effects of ocean acidification.

Comb jellies (Phylum Ctenophora)

Comb jellies  are named for their eight rows of cilia that move in harmonious rhythm to propel them through the water.  Unlike the true jelly fish, comb jellies lack  stinging cells and the radial symmetry of the cnidarians.  The ctenophore body exhibits bi-radial symmetry instead of the radial symmetry typical of true jellyfish.  To learn more click here.

Siphonophores (Phylum Cnidaria)

Siphonophores, like the Portuguese man-o-war, are often confused with jelly fish.  Although they belong to the same phylum  as jellyfish (Cnidaria), siphonophores are colonial species. This means they are a group of related but independent cells living together.  There are about 175 described species.  The majority of siphonophores are long and thin, consisting mostly of a clear gelatinous material. Some deep water species have dark orange or red digestive systems that can be seen inside their transparent tissues.  Many siphonophores are bioluminescent, glowing green or blue when disturbed. All siphonophores are predators, and use their many tentacles to capture crustaceans and small fish. Click on the link to learn more about Siphonophores.

Jellyfish  (Phylum Cnidaria)

The life cycle of a typical jellyfish is complex and involves an alteration of generations in which the animal passes through two different body forms. The dominant and conspicuous medusa is the familiar form, while the smaller polyp form is restricted to the larval stage. Jellyfish reproduce sexually, and individuals are either male or female. The reproductive organs (gonads) develop in the lining of the gut.

Jellyfish use their bell to move. They don’t move anywhere fast, and many scientists believe the pulsating rhythm of the bell is more about bringing plankton to them than it is about taking them to plankton.

Jellyfish have an advanced weapon system known as nematocyst cells.  These cells are high pressure, water powered, harpoon guns.  Click on the link to see a video of different kinds of gelatinous plankton. To learn more about all types of jelly plankton click on the Jellies zone.

Questions for Research

  1. Together, let’s watch Five Reasons To Thank Plankton (YouTube.)  Please record all five reasons.
  2. Next, let’s watch a video about the seasonal blooms of plankton.  Why is are these plankton blooms so important to the planet?
  3. For a third plankton bloom question, describe whatkinds of conditions that can lead to large plankton blooms? Here are a couple of  YouTube videos (Nasa: In the Zone or Glacier Melt Stimulates Plankton Blooms) or you can just read above.
  4. What are diatoms? Just for fun, describe your impressions of diatoms as art.
  5. When it gets to sunny, plankton make clouds.  Read about it and explain how plankton can control the weather.
  6. Go through the slide show of zooplankton species.  Select one species that seems interesting to you.  Research it, and describe what you learned.
  7. Read the linked article about ocean acidification and describe how changes in ocean pH might lead to reductions in the numbers of pteropods and potentially fish populations.
  8. Watch a video of a comb jellies swimming and then watch a video of a true jellyfish swimming.  Compare and contrast their styles of movement and the structures they use to move.
  9. Click on the link for the jelly plankton movie.  This may take you to National Geographic’s webpage where you will have to search for the “jelly plankton movie.”  Once you watch it,  write a description of one of the jellies described in the movie.
  10. Describe two surprising findings made by scientists in the creation of the first global atlas of marine plankton.
  11. (Bonus)  Write a short poem or haiku about what can be found in a drop of ocean water.