What Do Plankton Eat and Why are They Important for Our Survival?

Smita Pandit Jun 22, 2019
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Plankton refers to a group of organisms that drift in water, and include phytoplankton, zooplankton, and bacterioplankton. Here we explain the importance of plankton, along with details on what plankton eat to survive.

Hey, we are colorful too!

Some types of plankton exhibit bioluminescence. Commonly known as sea sparkle, a dinoflagellate called Noctiluca can make the water appear neon blue at night. Thousands of sea sparkles can fit in a single drop of water. Zooplankton such as copepod squirt glowing chemicals in water to mislead predators.
The term 'plankton' was coined by a German zoologist named Victor Hensen (1835-1924), and refer to a group of organisms that drift with the ocean currents. These don't have the ability to swim against currents.
However, some of them exhibit vertical migration, which means that they can move long distances vertically. These are categorized into phytoplankton, zoo plankton, and bacterioplankton. Phytoplankton are called primary producers, and include tiny microorganisms such as algae, dinoflagellates, diatoms, cyanobacteria, etc.
These form the base of the marine food chain. Zoo plankton comprise primary consumers or the animal-like members of the plankton community. These include copepods, krill, daphnia, arthropods, larvae and eggs of some fish, crustaceans, etc.
Zooplankton are called primary consumers as they feed on phytoplankton. Bacterioplankton, as the name suggests, include archaea and bacteria. These are involved in the process of decomposition, and help recycle nutrients in the aquatic environment. Phytoplankton live near the surface of water, and use sunlight for photosynthesis.
In order to thrive, phytoplankton also need nutrients, which they get from decomposed plants and animals. Plankton are indispensable for the marine animals to survive, but what do plankton eat to survive? The following sections provide the answer to this question.

Types of Plankton

To be a plankton, an organism must live in the pelagic zone, and should be a drifter. The inability to swim against a current differentiates them from nekton, which refers to a group of organisms that have the ability to control their movement in water.
While a microscope is required for one to be able to spot a plankton, there are some large-sized marine animals that are classified as plankton due to their drifting habit. For example, shrimp and jellyfish can be seen with the naked eye. The size of organisms classified as plankton varies greatly.
On the basis of size, plankton are placed under the following categories:

Femtoplankton: Less than 0.2 µm
Picoplankton: 0.2-2 µm
Nanoplankton: 2-20 µm
Microplankton: 20-200 µm
Mesoplankton: 0.2-20 mm
Macroplankton: 2-20 cm
Megaplankton: Greater than 20 cm
These organisms can also be classified on the basis of the relative length of their planktonic life. The organisms that remain planktonic throughout their life cycle are called holoplanktons.
On the other hand, meroplankton includes organisms that are in a planktonic form for a short time during their development. Larvae of sea stars, sea urchins, crustaceans, marine worms, etc., are grouped under meroplankton.

Diet of Phytoplankton, Bacterioplankton, and Zooplankton

Phytoplankton include algae that live near the surface of water. These include blue-green algae, red algae, brown algae, and green algae.

Cyanobacteria

This group includes diatoms, coccolithophores, cyanobacteria (blue-green algae), and dinoflagellates. The term 'diatom' means cut in two. A common type of phytoplankton, this is a single-celled yellow algae.
Within its glass-like cell wall that is impregnated with silica, two interlocking parts are present, with one half placed over the other half, and hence the name. The living matter is present within the two halves.

Euglena with flagella

✦ Unlike diatoms that drift with the current, dinoflagellates use flagella (whip-like appendage) to move themselves through the water. Dinoflagellates produce food through photosynthesis, but some of them are predatory species. Some types of dinoflagellates exhibit bioluminescence.

Glowing Phytoplankton

✦ Zooplankton are unicellular or unicellular animals that feed on phytoplankton. On the marine food chain, zooplankton are the link between the phytoplankton and the larger animals.
Unicellular zooplankton consume phytoplankton, but might sometimes consume other zooplankton. Larger, unicellular zooplankton include jellyfish, crustaceans, and arrow worms.

Spider Crab

✦ Copepods, which are relatives of crabs and crayfish, are zooplankton that are quite abundant in the marine environment. One copepod can consume about 130,000 cells of phytoplankton in a day.
Foraminifera and radiolaria are unicellular predatory organisms that have shells made of calcium carbonate and silica, respectively. They possess stick parts that they extend through pores in their shells for catching their prey and feeding.
✦ Jellyfish is a predatory species that consumes phytoplankton, copepods (crustacean zooplankton), eggs, and larvae of fish and some other marine animals.
Large jellyfish feed on crustaceans, as well as other species of jellyfish. Zooplankton might consume bacterioplankton as well as animal feces.
✦ In the marine food chain, zooplankton form the diet of fish, squid, marine mammals, and sea birds. Plankton are also consumed by filter feeders. These are aquatic animals that catch plankton or other nutritional particles, and move them to their mouth with the help of a siphoning motion.

Whale shark seen filter feeding

✦ Clams, Antarctic krill, baleen whales, and whale shark are examples of filter feeders. The diet of bacterioplankton includes organic matter.

Importance of Plankton

✦ As mentioned earlier, phytoplankton flourish in the presence of sunlight and nutrients. At the surface, they use sunlight to convert nutrients like silicon and phosphorous into organic tissue.
They get nutrients from dead, decomposed plants and animals. However, these decomposed plants and animals sink, and they can become a source of nutrients only when they come to the surface.
✦ In summer, the water at the surface stays warm, while the nutrients stay in the dense, icy water. However, when the surface water becomes cold in winter, the nutrients come to the surface, thereby enriching the water at the surface.
✦ With the arrival of spring, phytoplankton thrive in the presence of nutrients and sunlight. This phenomenon is referred to as 'spring bloom'. During this time, both phytoplankton and zooplankton increase in number, and there's enough food for the marine animals that come higher up in the food chain.
Plankton play a very important role in the aquatic environment, as they provide food to other organisms and recycle elements. Photosynthetic bacteria decompose organic matter, thereby enriching the ocean.
In the absence of phytoplankton, zooplankton will not survive, which, will disturb the food chain, adversely affecting the marine environment. With the exception of some species that produce toxins that could have an adverse effect on our health, these are highly beneficial due to their role in regulating the amount of carbon dioxide in the atmosphere.