Marine Food Web: A Simple Guide

All you need to know about “who eats who” in the water. We explain the basic predator/prey relationships in the marine food chain, the different trophic levels, from primary producers to apex predators. We look at how humans are damaging the marine food web by polluting the ocean with heavy metals and other toxic chemicals, and by burning fossil fuels.
Killer whale jumping out of water, Alsaka
Killer whales (Orcinus orca) are the ocean’s apex predators. Image: Robert Pittman/ NOAA

The marine food web is a highly complex network of predator/prey relationships, that connects all of the 226,000 known aquatic species in the marine environment. 1 Above all, it illustrates the interdependency of oceanic life, in that every aquatic plant and animal species is somehow dependent on another plant or animal species for its survival.

Marine food webs are fundamental to all ocean biomes and ecosystems, and also play an important part in the way marine ecosystems respond to natural and human-induced changes. In addition, food webs are closely linked to climate change, since marine organisms capture a huge amount of carbon dioxide as they photosynthesize, which would otherwise boost the greenhouse effect and add to present levels of warming. It’s another example of how oceans influence climate around the world.

What’s a Food Chain and How Does it Differ from a Food Web?

A food chain describes how smaller organisms are fed upon by larger organisms, who in turn feed even larger organisms, and so on. Predators and prey are directly connected in sequence: in the sea, for example, phytoplankton are eaten by zooplankton who are devoured by sardines, who are preyed on by tuna who are eaten by sharks, and so on.

A food web doesn’t just follow one path, because sometimes marine animals don’t respond how they should. Instead, it follows all the eating habits that exist, while also examining the impact of tiny microorganisms (like bacteria) on the food chain, in mechanisms such as the microbial loop.

That said, when we talk in general terms about ‘who eats what’ and ‘who gets eaten by whom’, the terms food chain and food web are pretty much interchangeable.

The Marine Biome

The ocean occupies more than 70 percent of the surface of Planet Earth, contains 50-80 percent of its biodiversity and 99 percent of its living space. Of the 34,300 known species of fish, 70 percent live in the Pacific Ocean, 20 percent in the Atlantic, and 8 percent in the Indian Ocean.

The ocean environment as a whole can be divided into a number of distinct ecozones and ecosystems, including: the intertidal zone, kelp forests, mangrove swamps, the continental shelf, coral reefs, the open ocean or pelagic zone, the demersal zone, and the sea-bed or benthic zone. Each of these ecosystems has its own unique biodiversity, its own microclimate, and its own food web. 2

Trophic Levels of the Marine Food Chain

The standard marine food chain has 5 basic levels – known as trophic levels.

  1. Primary producers (phytoplankton; e.g. diatom) make food from sunlight/chemicals.
  2. Primary consumers (zooplankton & other herbivores; e.g. copepods) eat phytoplankton.
  3. Secondary consumers (medium-sized carnivores; e.g. sardines) eat herbivores.
  4. Tertiary consumers (large marine carnivores; e.g. tuna) eat other carnivores.
  5. Apex predators (e.g. killer whales, saltwater crocodiles) have no natural enemies.

Most food chains don’t take into account decomposers (detritivores) – the organisms that break down dead organic material (plant or animal remains) and animal waste, before releasing it again into the food chain, as energy and nutrients. In the sea, much of this work is done by microscopic bacteria, who gather up waste and dead matter, and break it down into inorganic chemicals that can be reused as mineral nutrients, by zooplankton and other creatures.

A Simple Guide to the Marine Food Web

Here is a very simple explanation of the aquatic food web and is only one of several differing versions, depending on which animals and organisms are included. For a comparison with the terrestrial food chain, see: Food Web on Land: Who Eats Who?

Level 1: Phytoplankton (Primary Producer)

The foundation of the ocean food web is occupied by single-celled algae and other tiny plant-like organisms, who create their own energy out of sunlight. Collectively known as phytoplankton (Greek for “drifting plant”) these microorganisms are the starting point for the entire marine food.

In fact, not only do other marine creatures depend on them but, due to their oxygen producing skills, all terrestrial creatures depend upon them, too – including humans.

Microscopically small (about 1-5mm in length), phytoplankton saturate the sunlit near-surface waters around the world in their billions. Like their terrestrial counterparts, these marine plants use photosynthesis to convert sunlight, carbon dioxide (CO2) and water to create complex carbohydrates, like sugars.

Phytoplankton are the “primary producers” of the organic carbon needed by all marine animals in order to live. Known as autotrophs, they create their own energy (food) without needing to eat. The most common type of primary producer (photoautotrophs) uses the sun’s energy to build carbohydrates. Another type (chemoautotrophs) creates energy out of chemicals. These organisms use chemosynthesis to metabolize chemical compounds emitted from hydrothermal vents, methane seeps, and other geological features.

Amazingly, phytoplankton also produce half the world’s oxygen. 3

One particular family of phytoplankton is called cyanobacteria (also called blue green algae). It includes the world’s smallest known photosynthetic microbe, known as Prochlorococcus, which measures less than one millionth of a meter, but is one of the most plentiful species on Earth. It is estimated to produce 20 percent of the world’s oxygen.

Other common phytoplankton are diatoms, green algae, and dinoflagellates. In total, there are around 5,000 known species.

Due to their use of the green pigment chlorophyll to carry out photosynthesis, phytoplankton often appear as a green stain in the water when they are present in high numbers, allowing scientists to map their location and approximate population.

There are several larger forms of phytoplankton algae – including seaweed, seagrasses and kelp – which photosynthesize in coastal waters and along the shore.

Level 2: Zooplankton (Primary Consumer)

The next level in the food chain is occupied by zooplankton (Greek for “drifting animal”) – an umbrella term for numerous microscopic species of aquatic animals that float in the water column, drifting with the currents. Most of these animals are minute, though a few species can reach lengths of eight feet. There are two general types of zooplankton: holoplankton – those that remain planktonic throughout their entire life, like jellyfish; and meroplankton – those who are planktonic only while they are in the larvae stages of larger fish types.

Zooplankton do not make their own food; they have to find things to eat. Mostly filter-feeders, they drift through the water grazing on phytoplankton and bacteria that form the base of the food web, and in turn, are preyed upon by fish, insects and other predatory zooplankton.

Of the 10,000 or so species of zooplankton, two of the most important and abundant are copepods and krill. In fact, they are the two biggest sources of protein in the sea.

Copepods, the biggest source, are crustaceans with a resilient exoskeleton made of calcium carbonate. They feed on phytoplankton. A single copepod can devour more than 370,000 phytoplankton per day.

Krill are a particularly large type of predatory zooplankton which feeds on phytoplankton and also (to a lesser extent) zooplankton. They are an important element of the aquatic food chain, because they convert the primary production of their phytoplankton prey, into a form suitable for consumption by larger marine species, that are unable to feed directly on the microscopic phytoplankton.

One species of krill in Antarctica, for example, has an estimated biomass of around 379 million tonnes – the largest on the planet. Every year, a significant proportion of this species is consumed by whales, seals and penguins. 4

Northern Krill - North Atlantic Ocean
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

Zooplankton and Krill

Zooplankton are the primary consumers of the marine food web. They serve as the crucial link between the phytoplankton (primary producers) and the rest of the marine food web (secondary consumers), serving as a nutritious food source for larger creatures higher up in the food chain. Krill are one of the largest zooplankton (most adults are actually one step up in the food chain), so, when they consume plankton, they turn themselves into an even bigger and more convenient meal for larger fish. In this sense, krill serve as a nutrient bridge from microscopic phytoplankton to larger fish and mammals.

What is the Role of Plankton in the Marine Food Chain?

To recap: plankton occupy the base of the ocean food chain, meaning they play a critical role in supporting marine and freshwater food webs. Phytoplankton convert sunlight into food. A proportion of this food passes up the food chain when zooplankton eat the phytoplankton and are then consumed by fiercer zooplankton and larger animals, and so on. Very large animals can eat plankton, too. Blue whales, for example, can feast on almost 5 tons of krill every day.

What’s the Difference Between Phytoplankton and Zooplankton?

Phytoplankton are aquatic plants or bacteria, while zooplankton are tiny fish, crustaceans and other marine animals. Both types of phytoplankton make their own food by photosynthesis; zooplankton eat other organisms like phytoplankton. Phytoplankton inhabit the near-surface of the sea (the euphotic zone) in order to photosynthesize sunlight; zooplankton live in the darker and colder depths. Once darkness falls, hungry zooplankton rise to the near-surface layer to feed on phytoplankton. As sunrise approaches the now full zooplankton return to the depths to avoid predators.

Are Krill a Type of Zooplankton?

Yes and No. What determines whether a marine species is plankton or not is their self-propulsion. Plankton can’t swim against the tide, and instead drift with the currents. Adult krill are capable of swimming against currents, but juvenile krill, along with krill larvae and eggs drift with the current, and thus fall into the zooplankton category.

What Other Ocean Grazers Are There?

In addition to zooplankton, other animals that feed on the sea’s abundant plant life include surgeonfish, parrotfish, green sea turtles, the marine iguana, dugongs and manatees. Although they vary enormously in size, herbivores share a prodigious appetite for marine vegetation, as well as a common fate. Nearly all of them are destined to be eaten by the carnivorous animals further up the food chain.

What is the Role of Primary Producers in the Marine Food Web?

Phytoplankton, the ocean’s primary producers, are the foundation of the marine food chain. They “produce” energy from non-living (abiotic) stuff, like sunlight and inorganic chemicals. Almost every other living thing in the marine biosphere “consumes” the energy initially created by phytoplankton. In other words, all marine life depends on phytoplankton creating enough usable energy in their bodies.

According to a 2010 study published in Nature, the numbers of phytoplankton in the ocean had declined substantially over the past century. Concentrations in surface waters were estimated to have decreased by around 40 percent since 1950, at a rate of about 1 percent per year. Ocean warming was thought to be a culprit. 5

In a follow-up study, prompted by criticism as to sample sizes and other matters, the authors made use of a larger database of measurements and upgraded their methodology, but ultimately came to similar conclusions. 6

If changes in ocean temperature and ocean acidification are threatening plankton health, it has major implications for marine food webs as well as oxygen supply to the rest of the planet.

The Microbial Loop Within the Food Chain

Energy created by phytoplankton doesn’t always pass up the food chain, via zooplankton, krill and other larger fish. Some of this energy can also enter another pathway which is controlled by bacteria. This is the “microbial loop”, so named because the organisms involved are microscopic – around 100 times smaller than phytoplankton. These bacterial microbes absorb chemicals released by decomposing phytoplankton, and then enter the main food chain when they are eaten by zooplankton. They’re like an invisible clean-up crew – gathering up all the nutritious waste and recycling it to organisms like zooplankton. See also: Marine Microbes Drive the Aquatic Food Web.

The Whale Pump feeds the Microbial Loop

Large marine creatures also contribute to the recycling of nutrients via the microbial loop. Studies have found that marine mammals like whales enhance primary productivity through the release of fecal plumes. Whales feed on nutrients in the deeper levels of the sea, then return to the surface where they leave their feces to be absorbed by micro bacteria, who pass on the nutrients to zooplankton. The whales’ behavior is different to most other fish who defecate where they feed. 7

What About Jellyfish?

Jellyfish being slow swimmers, are a type of zooplankton. Traditionally regarded as minor players in the marine food web, due to having a gelatinous, watery body with little nutritional value, jellyfish are now thought to be a major constituent of the diets of swordfish and tuna, as well as octopus and crabs. Researchers believe that despite their low energy density, the contribution of jellyfish to the diets of predators may be significantly greater than we imagine due to rapid digestion, low capture costs and availability. But higher consumption of jellyfish may lead to greater ingestion of plastics.” 8

Level 3: Secondary Consumers

What eats Zooplankton? Answer: planktivorous, secondary consumers, commonly known as forage fish. These are species of small open-sea fish which consume plankton, typically by filter feeding, and in turn are preyed upon by larger predators, including seabirds and mammals as well as larger fish.

Species of forage fish include: anchovies, herrings, hilsa, mackerel, menhaden, sardines, shad, sprats, as well as capelin, halfbeaks, silversides, squid and smelt.

To compensate for their size and to deter predators, forage fish form large shoals which follow regular routes along coastlines, and also across the open sea, pursued by large numbers of marine predators, including whales, seals, tuna, dolphins and seabirds. Some larger species of whales rely upon forage fish for up to three quarters of their food intake.

Level 4: Tertiary Consumers

The fourth trophic level consists of predatory fish, marine mammals and seabirds that prey on forage fish. They in turn are devoured by very large fish and apex predators.

Marine tertiary consumers include: baleen, humpback and minke whales; harbour seals, cod, tuna, Chinook salmon, barracuda, dolphins and porpoises, swordfish, penguins and seals, as well as seabirds like pelicans, shearwaters, cormorants and gannets.

Level 5: Apex and Other Top Marine Predators

The large predators on top of the marine food chain, that have no (or hardly any) predators to contend with, include: killer whales (the No 1 global ocean predator), saltwater crocodiles (the No 1 predator in sub-tropical parts of SE Asia and Australia), blue and sperm whales, large sharks (great white, tiger) and leopard seals.

Like terrestrial apex predators such as lions, tigers or Nile crocodiles, top marine predators help to maintain the health of their ecosystem by controlling the populations and health of species below them. If they become targeted by humans, the health of their environment suffers.

A recent example concerns the overfishing of sharks, who are being hunted down for shark fin soup. An estimated 100-200 million sharks per year are believed to be affected. Unfortunately, removing sharks from the ocean leads to population growth among larger predators, such as groupers. But groupers eat herbivores, who eat macroalgae, who compete with coral reefs. So the more groupers, the fewer herbivores, and the more macroalgae. Which is bad news for coral reefs and the biological diversity they support. 9 10

Another study off the southeastern coast of the United States revealed that the loss of large sharks resulted in an increase of the ray population. The hungry rays ate all the coastal scallops, forcing the closure of the local shellfish industry. 11

How Human Actions Are Harming the Marine Food Web

Marine biodiversity (numbers and types of sea creatures) is being harmed by overfishing, as well as coastal land and sea use change, and pollution. First, almost 90 percent of the world’s fish stocks are now fully exploited, over-exploited or depleted. 12 Second, coastal marine habitats are being damaged by offshore development, and the resulting pollution.

Third, we are polluting the sea with heavy metals and other chemicals. Roughly 300-400 million tons of heavy metals, solvents, and other toxic slurry from industrial plants are dumped annually into the world’s oceans. 13 In addition, millions of tons of nitrogen and phosphorus from agricultural and industrial runoff arrive in the ocean every year, causing more hypoxic ‘dead zones’. For more, see our articles on the Nitrogen Cycle and the Phosphorus Cycle.

If all this wasn’t having a big enough impact on the world’s oceans, we also use them as a dumping ground for plastic, especially microplastic. Plastic pollution, which has skyrocketed since 1980, is now found in all parts of the ocean, from shallow coastal waters to the bottom of the 36,000-foot Mariana Trench. 13

An estimated 8 million tonnes of plastic enter the oceans each year. That’s one garbage truckload every minute. According to recent studies that examined marine species for pollution, pieces of plastic were found in marine turtles (100 percent), whales (59 percent), seals (36 percent) and seabirds (40 percent).

Microplastic waste is ingested by almost every creature in the sea, and so far has been found to block the digestive tracts of at least 267 different species. 14

According to a 2017 global drinking water study, an average of 83 percent of tap water samples were found to be contaminated by micro plastic fragments. United States led the field with a 94 percent rate of contamination. European countries, such France, Germany and the UK, had the lowest contamination rate (72 percent). According to researchers, people may be ingesting between 3,000 and 4,000 microparticles of plastic from tap water annually. 15

Apex Predators Are Worst Affected by Marine Pollution

Ocean contamination is a nightmare for apex predators. Take killer whales, for example. Because they are the top apex predator, they absorb all the chemical pollution absorbed by the prey they devour in their food chain – from fish, right up to seals and sharks.

A recent study published in Science magazine suggests the long-term viability of half the killer whale population around the globe is now in question, due to their ingestion of polychlorinated biphenyls, or PCBs. Some groups, such as those around the UK, Brazil, Japan and California, are almost certainly doomed. These highly toxic pollutants, now banned, damage the ovaries of female orcas, limiting their ability to breed. The chemicals also damage the immune system. 16

Effects of Climate Change on the Marine Food Web

The Ocean absorbs 25 percent of all CO2 released into the atmosphere from the burning of fossil fuels. 17 It also absorbs 93 percent of the excess heat produced by global warming, whose impact on water temperature, oxygen and pH levels, and marine life, is intensifying. The effects of global warming on the oceans are also becoming noticeable throughout the marine food chain, which acts as a barometer of change. Species are becoming disoriented and damaged by marine heatwaves, ocean warming, acidification and deoxygenation.

Here are two specific instances of how global warming is forecast to impact on the ocean food web.

• A new study published in PLoS Biology suggests that levels of commercial fish stocks could fall as rising sea temperatures affect their source of food. 18

• Predicted increases in greenhouse gas emissions might suppress oceanic biological productivity for an entire millennium. Because as the climate warms, westerly winds in the Southern Hemisphere are likely to strengthen and shift towards the pole. As a result, surface waters will warm, sea ice will disappear, and upwelling of thermohaline circulation currents will increase around Antarctica. Ultimately, the net effect of these changes will be a major decrease in marine biological productivity. By 2300, this could reduce fish-yields by more than 20 percent globally and by nearly 60 percent in the North Atlantic. 19


  1. World Register of Marine Species (WoRMS) 2014 []
  2. See also: “Marine Ecosystem.[]
  3. Marine organisms produce over half of the oxygen that land animals currently need to breathe.” NOAA []
  4. “A re-appraisal of the total biomass and annual production of Antarctic krill.” (PDF). A. Atkinson, et al; (2009). Deep-Sea Research Part I. 56 (5): 727–740. []
  5. Global phytoplankton decline over the past century“. Boyce, Daniel G.; Lewis, Marlon R.; Worm, Boris (2010). Nature. 466 (7306): 591–6. []
  6. Boyce, Daniel G.; Dowd, Michael; Lewis, Marlon R.; Worm, Boris (2014). “Estimating global chlorophyll changes over the past century“. Progress in Oceanography. 122: 163–73. []
  7. The Whale Pump: Marine Mammals Enhance Primary Productivity in a Coastal Basin.” Joe Roman, James J. McCarthy. PLOS One. October, 2010. []
  8. A Paradigm Shift in the Trophic Importance of Jellyfish?” Graeme C Hays, et al; Trends Ecol Evol. 33(11):874-884. Nov 2018. []
  9. Caught in the Middle: Combined Impacts of Shark Removal and Coral Loss on the Fish Communities of Coral Reefs.” Jonathan L. W. Ruppert, et al; PLoS ONE, 2013; 8 (9): e74648 []
  10. See also: “When Predators Become Prey: The Need For International Shark Conservation.” Holly Edwards, 12 Ocean & Coastal L.J. (2007). []
  11. Overfishing Large Sharks Impacts Entire Marine Ecosystem, Shrinks Shellfish Supply.” ScienceDaily. ScienceDaily, 29 March 2007. []
  12. Source: United Nations Conference on Trade and Development. UNCTAD []
  13. Source: IPBES 2019 Global Assessment Report. [][]
  14. Plastic Oceans.” Future Agenda. []
  15. Your tap water may contain plastic, researchers warn (Update)” []
  16. Predicting global killer whale population collapse from PCB pollution.” Jean-Pierre Desforges, et al; Science Sept 2018: Vol. 361, Issue 6409, pp. 1373-1376. []
  17. Ocean-Atmosphere CO2 Exchange.” NOAA. []
  18. Climate Change Could Drive Marine Food Web Collapse Through Altered Trophic Flows and Cyanobacterial Proliferation.” Hadayet Ullah, et al; PLoS Biol. 2018 Jan 9. []
  19. Sustained climate warming drives declining marine biological productivity.” J. Keith Moore, et al; Science 09 Mar 2018: Vol. 359, Issue 6380, pp. 1139-1143. []
Share on facebook
Share on twitter
Share on linkedin
Share on whatsapp
Share on email