Zooplankton and the Marine Food Chain

Zooplankton form a critical link in the ocean food web between phytoplankton and other marine species, and are essential to all life on Earth. We explain the different types of zooplankton, like krill, copepods and dinoflagellates. We look at their predator and prey relationships, as well as the wider risks they face. The latter include loss of habitat (under polar ice) ocean warming, acidification, deoxygenation - all caused by global warming - and pollution.
ciliate
A ciliate zooplankton digesting phytoplankton. See mouth bottom right. Image: Wiedehopf20. CC-SA-4.0

Zooplankton (from the Greek for “drifting animal”) is a collective term for a wide range of aquatic animal plankton with little or no swimming ability, who mostly drift along with the surrounding currents. Most zooplankton are microscopic organisms – such as single-celled protozoa, or tiny crustaceans, or the larvae of certain aquatic animals – although there are larger soft-bodied species, such as jellyfish, that can grow to several meters in length.

They appear all year round, in both marine and freshwater environments, though numbers tend to increase in late spring and early autumn. They live in all aquatic biomes and throughout the ocean, but the largest number inhabit the near-surface zone, where there is enough sunlight to support phytoplankton, who are the first link in the marine food web and the main prey for zooplankton.

Unlike phytoplankton, who are autotrophs – able to create their own food from sunlight (using photosynthesis) or from inorganic chemicals (using chemosynthesis) – most zooplankton are heterotrophs, meaning they have to find things to eat. They do this mostly by filter-feeding as they drift through the water grazing on phytoplankton, bacteria and other small zooplankton.

Zooplankton should also be distinguished from two other planktonic creatures: bacterioplankton and mycoplankton. The former, as the name suggests, serve as bacteria; the latter are fungi. Both are members of the oceanic ‘clean-up’ team, dealing with and recycling the organic remains of dead plankton and other organisms, as part of the microbial loop.

One thing to remember about zooplankton: they are extremely diverse. A majority may be microscopic but many are larger (up to 2cm or .66 inch – about 1,000 times bigger, or more) and some (eggs and larvae zooplankton) grow into full-size fish or crustaceans.

This is because there are 2 types of zooplankton: holoplanktonic and meroplanktonic. Holoplankton are those (like copepods or jellyfish) that spend their whole life-cycle as plankton. Whereas, meroplankton (like krill) are planktonic only during their larva stage, before they grow up, or before becoming a nekton (a proper swimmer) or one of the benthos (a creature who lives on the ocean floor – the so-called benthic zone). Remember, zooplankton are defined only by motility – their movement ability, not size or habits.

Also, while most zooplankton are heterotrophs, some – like dinoflagellates – are mixotrophs, meaning they can photosynthesize as well as eat things. What’s more, while most zooplankton are largely vegetarian grazers since they usually eat only plant-like phytoplankton, some are carnivorous and predatory and eat only zooplankton – albeit smaller species. So, there’s a huge diversity of organisms within the zooplankton kingdom.

Why Are Zooplankton Important?

Zooplankton are a key building block in the marine food web and play a critically important role in the marine biosphere as a whole. By eating phytoplankton, the tiny primary producers who create food from sunlight, they turn themselves into convenient food parcels for larger species, passing on the solar-based energy to the rest of the marine ecosystem.

By doing this, they serve as a vital food bridge between the microscopic primary producers of energy in aquatic biomes – and consumers such as herrings, sardines, squid, smelt and even whales. Blue whales, for example, can feast on almost 5 tons of krill a day. So, if the abundance of zooplankton should fall in any significant way, the consequences for larger open-ocean animals would be severe.

Zooplankton are also extremely sensitive to changes in their habitat, so a change in zooplankton concentration or behavior can indicate a subtle change in the aquatic environment. So, in addition to acting as a key link between phytoplankton (the “grass of the sea”) and open-sea species, in the marine food chain, the biodiversity and abundance of zooplankton communities can be monitored to determine the health of an ecosystem.

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What Are the Most Important Types of Zooplankton?

There are around 10,000 or so species of zooplankton in the hydrosphere, which can be divided as follows: 1

— Crustaceans: such as, copepods, crabs, krill, shrimp.
— Dinoflagellates: can be mixotrophic and grow up to 2mm in size.
— Cnidarians: like jellyfish, with hanging tentacles.
— Worms: such as, segmented pelagic worms, arrow worms.
— With Shells: such as, molluscs, sea snails, pelagic snails.
— Squid: creatures with large eyes, 8 arms, 2 tentacles, large eyes.
— Larvae: like krill, larvae of barnacles, mussels, annelids & fish.

We’ll take a closer look at four: krill, copepods, dinoflagellates and jellyfish.

Krill

Northern Krill - North Atlantic Ocean - one of the largest zooplankton
Northern krill (Meganyctiphanes norvegica) is a species of krill that live in the North Atlantic Ocean. They are an important source of food for whales, fish and birds. Photo: © Øystein Paulsen/CC BY-SA 3.0

No longer than a thumb, and weighing less than an ounce, krill are small crustaceans and are found in all the world’s oceans. They prey on phytoplankton and some zooplankton, and in turn are preyed upon by many larger animals. They live for up to five years. Krill are one of the most abundant species in the world; their combined biomass (nearly 400 million tonnes) is calculated to exceed that of all the people on the planet. In the Southern Ocean, the most abundant species is Euphausia superba, half of whom are consumed by whales (they are the main prey of baleen and blue whales), seals, penguins, squid, and fish each year.

Krill are most heavily fished in the Southern Ocean around Antarctica, where they swarm in dense shoals, kilometers long, containing up to 10,000 krill per cubic meter of water. Most krill are used for aquaculture and aquarium feeds, or in the pharmaceutical industry, but krill oil is becoming popular for human consumption. Overall, the world catch of Antarctic krill is up by more than 50 percent, over the last decade.

Most krill species are filter feeders and consume tiny phytoplankton known as diatoms, small marine bacteria and some small zooplankton. They tend to migrate to the surface at night to feed, and retreat to deep water during the day. Krill reproduce during the spring, spawning up to 8,000 eggs. They can release eggs several times during the breeding season, which can last as long as 5 months.

Copepods

Copepod with big eyes
Corycaeus sp., a copepod with two big eyes. Copepods belong to the same group as crabs and lobsters, but are much smaller. They are the most abundant multicellular animals in the sea and possibly outnumber all other animals in the world. You can often see them hopping on the water surface. Photo: Otto Larink/CC BY-SA 3.0

Known as the “insects of the sea”, copepods – small aquatic crustaceans – are the most abundant multicellular animals in the ocean and are estimated to outnumber all the other animals in the world. They can be found almost anywhere there is water, irrespective of salinity: from underground caves to pools and puddles on the ground, from mountain lakes, streams and rivers, to the open ocean and the deepest ocean trenches. In freshwater habitats copepods consume mosquito larvae, thus acting as a control mechanism for the spread of malaria.

Tropical areas of the Indian and Pacific Oceans (notably coral reefs, tidal flats and mangrove swamps) teem with copepod life and the number of known species around the world exceed 13,000. 2 At least one third of these species live as parasites on other sea animals. See also: Marine Microbes Drive the Aquatic Food Web.

Copepods range in size from 500 micrometers to over 16 mm in length. Parasitic copepods on large vertebrate hosts may exceed 20 cm in length. They boast an armoured but usually transparent exoskeleton; a head, thorax and abdomen; two pairs of antennae that are used for swimming; and a single eye. Most are transparent or grey/brown in color, although bright red and orange copepods are not uncommon. The number of segments and appendages varies considerably and no further generalization is possible.

Female copepods produce anywhere between 1 and several dozen eggs a day during the breeding season. 3 4

Copepods feed on microscopic algae, bacteria and other small zooplankton. A single copepod can consume up to 373,000 phytoplankton per day. 5 Some copepods (cyclopoida) are strong enough to tear pieces out of the body of their victims (such as mosquito larvae, small fishes) with their powerful mandibles. Parasitic copepods (Siphonostomatoida and Poecilostomatoida) eat the skin of their hosts, while others suck blood. In turn, copepods are preyed upon by a variety of forage fish and are an important food source for many reef fish, as well as whales. 6

Dinoflagellates

Dinoflagellate
Dinoflagellate (Ceratium hirundinella). Photo: Dr. Ralf Wagner under the GFDL. Creative Commons.

Dinoflagellates are microscopic, unicellular algae, who typically have two flagella (lash-like appendages), and vary in size from 15 to 40 micrometers. The largest (Noctiluca), can grow to 2 mm in diameter. Dinoflagellates are mixotrophic, meaning they are photosynthetically active, but are also heterotrophic. 7 They are vital for the health of coral reefs.

There are about 2,000 species of dinoflagellates 8 Reproduction is asexual. Child cells (genetically identical to that of the original cell) form by simple mitosis and division of the cell.

Many dinoflagellates are photosynthetic, making their own food out of sunlight. Some photosynthetic dinoflagellates are symbiotic, inhabiting the cells of their hosts, such as corals. Known as zooxanthellae, they are found in many marine invertebrates, including corals, sponges, jellyfish, and flatworms, as well as within other protists, such as ciliates, foraminiferans, and radiolarians.

Approximately half of all dinoflagellate species are heterotrophic, preying upon other plankton, and sometimes each other, by snaring or stinging their victims. These species feed on diatoms or other protists including other dinoflagellates.

Occasionally, a dinoflagellate population becomes so large (as high as 20 million cells per liter) that it turns the water red. This “red tide” – often luminescent – may be caused by nutrient or hydrographic conditions, although scientists are still unsure as to the exact cause. Some, though not all, red tides are toxic. Toxic conditions occur when the zooplankton release a chemical that acts as a neurotoxin in other animals. If this neurotoxin accumulates at high enough concentrations inside a shellfish predator, any human eating the shellfish would also be affected.

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Jellyfish

Jellyfish are a type of soft-bodied, transparent zooplankton that drifts in the sea but also has some swimming ability. They are the largest type of holoplankton (lifetime zooplankton) – growing up to 8 feet in length, with tentacles up to 200 feet long – and can be found in every ocean. They belong to the same animal group (Cnidaria) as corals and sea anemones. There are more than 2,000 known species of jellyfish around the world, and some have been around for 500 million years, or more.

Jellyfish are predators that prey on plankton and larval fish using stinging cells on their tentacles. Most (such as the surface-living Portuguese Man-o’-War) are painful but not dangerous to humans but a few species of box jellyfish can be deadly marine stingers. They include: the “sea wasp” species (Chironex fleckeri), and two Irukandji jellyfish (Carukia barnesi and Malo kingi).

Jellyfish are believed to have relatively few predators – although new evidence suggests they are a more important food source than hitherto imagined. Predators include swordfish, tuna, sharks, sea turtles and penguins. 9 If they do lack predators, this might be extremely destabilizing for the marine balance. Because, if we accidentally overfish those species that feed on jellyfish larvae, for instance, then those fish stocks may be doomed. Because the jellyfish will feast on fish eggs and young fish, and compete with adult fish for food, thus preventing fish stocks from recovering. 10 Meantime, a single breeding jellyfish can spawn 45,000 eggs a day. 11

Excessive levels of nutrients in the water, from agricultural and urban runoff of nitrogen and phosphorus chemicals, lead to a surge in phytoplankton growth, and the appearance of algal blooms. When the phytoplankton die, large amounts of oxygen are used up, leading to the creation of hypoxic or dead zones that are fatal to most fish and other sea animals, but not jellyfish. This allows jellyfish to dominate the area. Indeed, jellyfish populations may be increasing globally due to this type of toxic runoff and overfishing of their natural predators. 12

Scientists are already warning that the proliferation of jellyfish could lead to a “jellification” of the oceans, which are facing profound changes, according to a UN report. Rising ocean temperatures and overfishing are enabling jellyfish populations to grow at explosive rates. 13

How Zooplankton Avoid Predators

It’s not easy being a zooplankton when almost every other marine animal wants to eat you. Not only do you need to eat, but also you need to avoid being eaten.

One characteristic of many zooplankton is a daily habit of diurnal vertical migration. Numerous species, though weak swimmers, descend hundreds of meters into the depths during the day to hide from predators, and then return to surface waters to feed on microzooplankton at night.

Zooplankton have also adapted to floating in the water column and protecting themselves from predation. Most do their best to hide in plain sight. But developing effective camouflage when living in clear, blue water is not easy. The solution chosen by most zooplankton is to be as transparent as possible. In addition, some zooplankton, have spikes that protect them and allow more surface area for better flotation. 14

What’s more, some zooplankton, while bad swimmers, have developed techniques for making sudden movements with the least disturbance in the water body in order to foil attacks by predators. Copepods, for example, are noted for their characteristic ‘hop’ through the water.

Effect of Climate Change on Zooplankton

Climate change may have profound impacts on the zooplankton. As climate change, combined with pollution and overfishing, creates unprecedented stresses for ocean life, scientists are closely monitoring zooplankton (the “canary in the marine cage”) to see how the entire marine biosphere is responding. 15

Sea Ice

Krill need sea ice and cold water to survive. But rising temperatures reduce the abundance of plankton on which krill feed, while the loss of sea ice removes the vital habitat that shelters both krill and the plankton they eat.

Not surprisingly, therefore, as Antarctic sea ice declines, so do krill. Antarctic krill populations have dropped an estimated 80 percent since the 1970s. 16 Scientists have yet to determine the exact reason for this, but loss of sea ice is thought to be a major factor.

One recent study suggests that if current global warming trends continue, Antarctic krill could lose between 20 and 55 percent of their habitat by the end of the century. 17

In addition to krill, a species of tubular, gelatinous zooplankton known as salps also feeds on the great abundance of Antarctic phytoplankton. Scientists believe that ocean warming and the amount of sea ice may regulate the balance between salp and krill populations. When there is more sea ice, krill seem to thrive but salps decline, and vice versa. Unfortunately, the gelatinous salps contain much less nutrition, which means that as sea ice declines, whales, seals, penguins, squid, and other fish in the Southern Ocean will receive a lower quality food. 18

Carbon Dioxide Absorption

There is also new research revealing that Antarctic krill play an increasingly important role in how the Southern Ocean absorbs carbon dioxide (CO2). According to a study published in Nature Communications, each year Antarctic krill absorb an amount of carbon equivalent to the carbon produced by 35 million cars. 19 20

Zooplankton and Ocean Acidification

Pteropods, a type of small mollusc, are an abundant source of food for a range of sea animals including krill, whales, salmon and many other smaller fish. Due to their sensitivity to pH levels, because of their calcium carbonate shell, pteropods have also become an important indicator of the effects of ocean acidification, a key symptom of rising CO2 levels in the ocean.

In a test, the shell of a pteropod (Limacina helicina) was submerged in ocean water with the projected pH level that the ocean is likely to reach by the year 2100. After 6 weeks, the pteropod’s shell was almost completely dissolved.

Ocean Deoxygenation

Ocean deoxygenation is becoming a growing problem for phytoplankton, and zooplankton, who are both sensitive to lack of oxygen. Some species swim deeper into cooler water to find more oxygen, but this quickly becomes counterproductive, because it gets harder to find prey or reproduce in lower temperatures. But if zooplankton numbers suffer, it is likely to have knock-on effects on krill and all the way up the food chain,

Jellyfish Threat to Fish Stocks

The rise in jellyfish populations may soon have serious effects on local fish stocks. Large numbers of jellyfish eat a considerable amount of fish larvae, including the larvae of many commercially important species. This is likely to have a crippling effect on the populations of the fish in question, as well as other fish who depend upon the same larvae for food.

References

  1. Zooplankton Guide: Taxa List” Scripps Institution of Oceanography. []
  2. Global diversity of copepods (Crustacea: Copepoda) in freshwater“. Geoff A. Boxhall; Danielle Defaye (2008). Hydrobiologia. 595 (1): 195–207. []
  3. Egg production rates of two common copepods in the Barents Sea in summer.” Vladimir G.Dvoretsky, Alexander G.Dvoretsky. []
  4. “Calanoid Copepods.” Dr. Adelaide Rhodes. WetWebMedia.com []
  5. “Small Is Beautiful, Especially for Copepods.” Suzan Bellincampi. Vineyard Gazette. Thursday, April 26, 2018. []
  6. Biology of Copepods: An Introduction.” []
  7. Mixotrophy among Dinoflagellates“. Stoecker DK (1999). The Journal of Eukaryotic Microbiology. 46 (4): 397–401. []
  8. Dinoflagellate diversity and distribution“. Taylor FR, Hoppenrath M, Saldarriaga JF (February 2008). Biodivers. Conserv. 17 (2): 407–418. []
  9. Jellyfish and other gelata as food for four penguin species – insights from predator-borne videos“. Thiebot, Jean-Baptiste, et al; (2017). Frontiers in Ecology and the Environment. 15 (8): 437–441. []
  10. “Stung! On Jellyfish Blooms and the Future of the Ocean.” Lisa-Ann Gershwin (2013). University of Chicago Press. ISBN 978-0-226-02010-5 []
  11. Jellyfish: The Next King of the Sea.” []
  12. Jellyfish overtake fish in a heavily fished ecosystem” (PDF). Lynam, C. P. et al; (2006). Current Biology. 16 (13): 492–493. []
  13. IPCC’s Special Report on the Ocean (Sept 2019) []
  14. The Australian Museum. “Australian Museum.” []
  15. MarineBio.org.” []
  16. Projected changes of Antarctic krill habitat by the end of the 21st century.” Andrea Pinones, Alexey V. Fedorov. Geophysical Research Letters. August 2016. []
  17. Potential Climate Change Effects on the Habitat of Antarctic Krill in the Weddell Quadrant of the Southern Ocean.” Simeon L. Hill, Tony Phillips, Angus Atkinson. PLoS One. 2013. []
  18. Jellyfish and Other Zooplankton.[]
  19. The importance of Antarctic krill in biogeochemical cycles.” Cavan, E.L., Belcher, A., Atkinson, A. et al. Nature Communications 10, 4742 (2019). []
  20. Tiny Antarctic Krill Play Big Role in Climate Mitigation.” October, 2019. []
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