Plankton facts for kids

Phytoplankton include tiny one-celled eukaryotes which do photosynthesis

Noctiluca scintillans is a dinoflagellate which eats up other tiny plankton

Northern krill, the favourite food of the baleen whales

Plankton are drifting organisms that live in the surface layers of the ocean. They live in the top layer of the ocean, called the epipelagic zone. They are not strong enough to swim against ocean currents. The term is in contrast to nekton, who can control their movements. There are three groups:

Phytoplankton: which live at the surface of the ocean and photosynthesise (use light to make sugars and other molecules).
- Eukaryote algae: diatoms, coccolithophores, some dinoflagellates.
- Bacteria: cyanobacteria.
Zooplankton: small protozoans or metazoans: Ctenophores; jellyfish; rotifers; foraminifera; tiny crustacea and other animals. Some eggs and larvae of larger animals such as fish, crustaceans, and annelids. Apart from the eggs, they all feed on other plankton.
Some groups fall into both categories. Dinoflagellates can be either photosynthetic producers or heterotroph consumers; many species are mixotrophic depending upon their circumstances. It is also hard to fit viruses into this scheme; yet they are present in great numbers.

Plankton are important in the ocean's food chain.

Ecological significance
Biomass variability
Importance to fish
Images for kids
See also

Ecological significance

Food chain

Aside from representing the bottom few levels of a food chain that supports commercially important fisheries, plankton ecosystems play a role in the biogeochemical cycles of many important chemical elements, including the ocean's carbon cycle.

Carbon cycle

Primarily by grazing on phytoplankton, zooplankton provide carbon to the planktic foodweb, either respiring it to provide metabolic energy, or upon death as biomass or detritus. Organic material tends to be denser than seawater, so it sinks into open ocean ecosystems away from the coastlines, transporting carbon along with it. This process, called the biological pump, is one reason that oceans constitute the largest carbon sink on Earth. However, it has been shown to be influenced by increments of temperature.

It might be possible to increase the ocean's uptake of carbon dioxide (CO₂) generated through human activities by increasing plankton production through seeding, primarily with the micronutrient iron. However, this technique may not be practical at a large scale. Ocean oxygen depletion and resultant methane production (caused by the excess production remineralising at depth) is one potential drawback.

Oxygen production

Phytoplankton absorb energy from the Sun and nutrients from the water to produce their own nourishment or energy. In the process of photosynthesis, phytoplankton release molecular oxygen (O₂) into the water as a waste biproduct. It is estimated that about 50% of the world's oxygen is produced via phytoplankton photosynthesis. The rest is produced via photosynthesis on land by plants. Furthermore, phytoplankton photosynthesis has controlled the atmospheric CO₂/O₂ balance since the early Precambrian Eon.

Biomass variability

Amphipod with curved exoskeleton and two long and two short antennae

The growth of phytoplankton populations is dependent on light levels and nutrient availability. The chief factor limiting growth varies from region to region in the world's oceans. On a broad scale, growth of phytoplankton in the oligotrophic tropical and subtropical gyres is generally limited by nutrient supply, while light often limits phytoplankton growth in subarctic gyres. Environmental variability at multiple scales influences the nutrient and light available for phytoplankton, and as these organisms form the base of the marine food web, this variability in phytoplankton growth influences higher trophic levels. For example, at interannual scales phytoplankton levels temporarily plummet during El Niño periods, influencing populations of zooplankton, fishes, sea birds, and marine mammals.

The effects of anthropogenic warming on the global population of phytoplankton is an area of active research. Changes in the vertical stratification of the water column, the rate of temperature-dependent biological reactions, and the atmospheric supply of nutrients are expected to have important impacts on future phytoplankton productivity. Additionally, changes in the mortality of phytoplankton due to rates of zooplankton grazing may be significant.

Freshly hatched fish larvae are also plankton for a few days, as long as it takes before they can swim against currents.

Copepod and larvae
Copepod from Antarctica, a translucent ovoid animal with two long antennae
Herring larva imaged in situ in the typical oblique swimming position with the remains of the yolk and the long gut visible in the transparent animal
Icefish larvae from Antarctica have no haemoglobin
Siphonophora – the "conveyor belt" of the upgrowing larvae and the ovarium can be seen
Eel larva drifting with the gulf stream

Other plankton
Antarctic krill, probably the largest biomass of a single species on the planet
Northern krill: the mid gut is red. It feeds on zooplankton
Tomopteris is a genus of marine planktonic polychaete
Microzooplankton, the major grazers of the plankton: two dinoflagellates and a tintinnid ciliate).
Sea foam can be produced by plankton, photo of many, differently sized bubbles with image of photographer

Importance to fish

Zooplankton are the initial prey item for almost all fish larvae as they switch from their yolk sacs to external feeding. Fish rely on the density and distribution of zooplankton to match that of new larvae, which can otherwise starve. Natural factors (e.g., current variations) and man-made factors (e.g. river dams) can strongly affect zooplankton, which can in turn strongly affect larval survival, and therefore breeding success.

The importance of both phytoplankton and zooplankton is also well-recognized in pond fish farming. Plankton population based pond management strategies for fish rearing have been practised by traditional fish farmers for decades, illustrating the importance of plankton even in man-made environments.

Images for kids

Marine microplankton and mesoplankton Part of the contents of one dip of a hand net. The image contains diverse planktonic organisms, ranging from photosynthetic cyanobacteria and diatoms to many different types of zooplankton, including both holoplankton (permanent residents of the plankton) and meroplankton (temporary residents of the plankton, e.g., fish eggs, crab larvae, worm larvae)
Plankton (organisms that drift with water currents) can be contrasted with nekton (organisms that swim against water currents), neuston (organisms that live at the ocean surface) and benthos (organisms that live at the ocean floor)
Some marine diatoms—a key phytoplankton group
An amphipod (Hyperia macrocephala)
Plankton species diversity Diverse assemblages consist of unicellular and multicellular organisms with different sizes, shapes, feeding strategies, ecological functions, life cycle characteristics, and environmental sensitivities. Courtesy of Christian Sardet/CNRS/Tara expeditions
Jellyfish are gelatinous zooplankton.
Salmon egg hatching into a sac fry. In a few days, the sac fry will absorb the yolk sac and start feeding on smaller plankton
Larva stage of a spiny lobster
World concentrations of surface ocean chlorophyll as viewed by satellite during the northern spring, averaged from 1998 to 2004. Chlorophyll is a marker for the distribution and abundance of phytoplankton.
Marine phytoplankton cycling throughout water column.
Plankton sizes by taxonomic groups
Sea spray containing marine microorganisms can be swept high into the atmosphere and may travel the globe as aeroplankton before falling back to earth.
Diatoms have glass shells (frustules) and also produce much of the world's oxygen
The elaborate silica shells of microscopic marine radiolarians can eventually produce opal
Coccolithophores have chalk plates called coccoliths, and produced the Cliffs of Dover
Planktonic algae bloom of coccolithophores off the southern coast of England
Foraminiferans have calcium carbonate shells and produced the limestone in the Great Pyramids
Pelagibacter ubique, the most common bacteria in the ocean, plays a major role in global carbon cycles
The tiny cyanobacterium Prochlorococcus is a major contributor to atmospheric oxygen
Antarctic krill, probably the largest biomass of a single species on the planet
Sargassum seaweed drifts with currents using air bladders to stay afloat
Macroplankton: a Janthina janthina snail (with bubble float) cast up onto a beach in Maui