Chapter 11: Plankton

Phytoplankton

Phytoplankton are microscopic algae that form an essential component of the marine food chain. These single-celled plant-like organisms provide the base to most marine food webs. The shear size and scale of their photosynthetic actives means that phytoplankton plays an important role in regulating weather the amount of carbon in the ocean and atmosphere.

Phytoplankton literally means 'phyto' = plant and 'planktos' = made to wander. Like terrestrial plants, these organisms contain chlorophyll and need sunlight and inorganic nutrients to grow. They are different from the large algaes and sea grasses because of their small size and their tendency to drift in the ocean currents. Virtually all marine phytoplankton are buoyant and live in the upper part of the water column called the photic zone.

Because phytoplankton reproduce asexually 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 here to watch an animation of a 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 here to see a satellite animation of the Earth's primary production.

There are three main types of oceanic phytoplankton, diatoms, dinoflagellates, and cocolithophores. One type, the diatoms, are common in both freshwater and oceanic habitats. Diatoms 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.

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, which live in symbiotic relationships with coral animals, and play an important part in the biology of coral reefs.

Some dinoflagellate species produce biotoxins. When they bloom in large numbers and then die these toxins may be released. These events are termed harmful algal blooms or HABs. These blooms can be dangerous both to aquatic life and on those (people) who depend on that water 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.

Cocolithophores are very small plankton 15-100 micrometers across (that is 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

Zooplankton get their names from "zoo" meaning animal and "planktos" meaning wanderer. The term zooplankton, loosely describes an incredible variety of animal and protista organisms 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 are at the mercy of the strong directional forces of the currents they make a living in. Zooplankton float in the upper reaches of the water column and are at the mercy of the ocean currents, being able to change their position in the water column, but not able to swim against the current or migrate in the normal sense.

Daily Commute

Most zooplankton take part in a daily movement up and down in the water column. In what has been described by some as the largest migration on the planet zooplankton move up and down in the water column as much as one thousand meters and they do so every day. 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.

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 sonars 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 here 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.

There are so many species of zooplankton you could spend years just trying to learn them all. Listed below are just a few examples.

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)

Claudocera (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. Click on the picture to learn more about antarctic krill.

Atlantic Zooplankton Slide Show - click to see lots of zooplankton species.

Jelly plankton

Jelly plankton are really a type of zooplankton. They are animals, or if you prefer multi-celled, heterotrophs, that move.

We are mentioning them separately here because they have such unique adaptations to their marine existence. The jellatenous form is described by some as nothing more than organized sea water. Reality is more complex than that, but being made of up to 99% water has its advantages. These organisms are largely transparent which makes them hard to sea. They are neutrally buoyant (they neither sink nor float.) In an environment with not hard surfaces they do not waste precious energy on building hard shells or exoskeletons that would only cause them to sink. All of that being said, they are a tremendously important food resource for many marine organisms.

Like zooplankton there are a lot of species of jelly plankton. Here are just a few examples:

Pteropods (Phylum Mulluska)

Pteropods are sometimes called sea butterflies because their foot looks like a pair of wings. (see picture at the top of the page) They make up a large part of the diets of many juvenile fish species (including salmon.) These free-swimming planktonic mollusks 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 (see picture on the left) are named for their eight rows of cilia that move in harmonious rhythm to propel them through the water. Unlike the true jellies (phylum cnidarians), comb jellies lack nematocysts (stinging cells). 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. To learn more click here.

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 neumatocyst cells. These cells are high pressure, water powered, harpoon guns.
To learn more about jellyfish click here.

To learn more about all types of jelly plankton click on the Jellieszone

Questions for Research

Watch a satellite animation of the Earth's primary production. Based on what you see describe some of the places on Earth that seem to always have high levels of primary production (phytoplankton growth) and some of the places that seem to always have low levels of primary production.
Now watch the animation again. Describe some of the places on Earth where primary production is highly seasonal (meaning periods of high abundance and periods of low abundance).
Explain what upwelling is and how it affects primary production.
Explain plankton can play a role in the creation of weather.
Explain the daily commute of zooplankton and what the term deep scattering layer refers to.
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.
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.
Watch the jelly plankton movie and write a description of one of the jellies described in the movie.
Unlike most groups of marine species, jellyfish are increasing. Click on the link and describe why this increase in jellyfish populations concerns scientists.
Draw into your notes the chart below, then use what you have learned to fill in the blanks.

Groups/ Adaptations	Buoyancy	Feeding	Avoiding predation	Movement
Diatoms		autotrophic (they use photosynthesis)		nothing
Copepods	long body appendages		spines
Krill	long appendages, high oil content		schooling
Comb jellies	gelatenous body			eight rows of cillia
Pteropods		mouth	aragonite shell
Jellyfish		Neumatocist stinging cells		a pulsating bell