A marine food web shows how marine life depends on one another for food. At the bottom of the ocean food web are single-celled algae and other plant-like organisms called phytoplankton. The next level includes zooplankton, which eat phytoplankton. Higher levels of the food web include animals that eat zooplankton and other organisms. Scientists have become more aware of the role marine microorganisms play in recent years.

Different habitats can cause food webs to change. The connections between organisms in a food web can help scientists understand how marine ecosystems work.

In marine environments, the way energy moves through food webs is different from land environments. Marine food webs have inverted biomass pyramids, meaning the base has less total mass than higher levels. This happens because the main producers in the ocean, such as phytoplankton, are tiny and reproduce quickly. A small amount of phytoplankton can create a lot of energy quickly. On land, large plants like forests grow slowly and need more mass to produce the same amount of energy. Because of this, zooplankton, which eat phytoplankton, make up most of the animal biomass in the ocean.

Food chains and trophic levels

Food webs are made up of food chains. Every living thing in the ocean can be food for another living thing. A food chain in the ocean usually begins with energy from the sun helping phytoplankton grow. A typical food chain might look like this:

phytoplankton → herbivorous zooplankton → carnivorous zooplankton → filter feeder → predatory vertebrate

Phytoplankton do not need to eat other organisms because they can make their own food using sunlight and inorganic carbon. This process is called photosynthesis, and it turns carbon into protoplasm. Because of this, phytoplankton are called primary producers and are at the first level of the marine food chain. They are assigned a trophic level of 1 (from the Greek word trophē, meaning "food"). Phytoplankton are then eaten by microscopic animals called zooplankton, which are at the second trophic level.

Zooplankton include tiny single-celled organisms called protozoa, small crustaceans like copepods and krill, and the larvae of fish, squid, lobsters, and crabs. These organisms are called primary consumers.

Herbivorous zooplankton are eaten by larger carnivorous zooplankton, such as predatory protozoa and krill, as well as by forage fish. These forage fish are small, schooling fish that filter food from the water. This group forms the third trophic level.

The fourth trophic level includes predatory fish, marine mammals, and seabirds that eat forage fish. Examples are swordfish, seals, and gannets.

Apex predators, such as orcas (which eat seals) and shortfin mako sharks (which eat swordfish), form the fifth trophic level. Baleen whales, which eat zooplankton and krill directly, may have food chains with only three or four trophic levels.

In reality, trophic levels are not always simple whole numbers because many animals eat food from multiple levels. For example, a large marine animal might eat both predatory fish and filter feeders. A stingray eats crustaceans, while a hammerhead shark eats both crustaceans and stingrays. Some animals even eat each other, such as cod eating smaller cod and crayfish, and crayfish eating cod larvae. The trophic level of an animal can change as it grows.

Fisheries scientist Daniel Pauly assigns a trophic level of 1 to primary producers and detritus, 2 to herbivores and detritivores (primary consumers), and 3 to secondary consumers, and so on. For any consumer species, the trophic level is calculated by considering the trophic levels of its prey and the proportion of each prey in its diet. In marine ecosystems, most fish and other marine consumers have trophic levels between 2.0 and 5.0. The highest value, 5.0, is rare but occurs in apex predators like polar bears and killer whales. Humans have an average trophic level of about 2.21, similar to pigs and anchovies.

By taxon

At the bottom of the ocean food web are tiny single-celled plants and other plant-like organisms called phytoplankton. These tiny plants are divided into many groups based on their shape, size, and the colors of their pigments. Most marine phytoplankton live in the sunlit surface waters of the ocean, where they use sunlight and nutrients like nitrogen and phosphorus to make food and produce oxygen. Some phytoplankton live in the deep ocean near underwater vents, where they use chemicals like hydrogen sulfide and ammonia instead of sunlight to make food.

Understanding an ecosystem requires knowing how its food web moves materials and energy. Phytoplankton make food by changing inorganic compounds into organic ones. This process supports all other life in the ocean, making phytoplankton the base of the marine food web. Another important process in the ocean is the microbial loop, which breaks down bacteria and archaea, recycles nutrients, and either returns them to the water or deposits them as sediment on the ocean floor.

Phytoplankton are the foundation of the marine food web and help produce about half of the world’s oxygen and fix about half of the carbon dioxide through photosynthesis. Like land plants, they use chlorophyll and other pigments to absorb sunlight and carbon dioxide to make sugars. The color of the ocean changes based on the amount of chlorophyll, helping scientists track where phytoplankton are and how healthy the ocean is.

When phytoplankton die and are not eaten, they sink to the ocean floor as part of "marine snow," trapping about 2 billion tons of carbon dioxide in the ocean each year. This makes the ocean a major storage place for carbon dioxide, holding about 90% of all carbon dioxide that is trapped in the ocean. The ocean also produces about half of the world’s oxygen and stores 50 times more carbon dioxide than the atmosphere.

Some phytoplankton are tiny bacteria called cyanobacteria. The smallest of these, called Prochlorococcus, is about 0.5 to 0.8 micrometers wide. A single milliliter of seawater can contain 100,000 or more Prochlorococcus cells, and there are estimated to be several octillion of them worldwide. These bacteria are found in most oceans and help produce about 20% of the oxygen in Earth’s atmosphere.

In the ocean, most primary production is done by algae, unlike on land, where most plants are vascular plants. Algae can be single-celled or attached to surfaces, while vascular plants in the ocean include seagrasses and mangroves. Larger algae like seagrasses and seaweeds grow near the shore in shallow, sunny areas. However, most ocean primary production is done by phytoplankton.

The first level of the ocean food chain is occupied by phytoplankton, tiny drifting organisms that float in the sunlit surface layer of the ocean. Most phytoplankton are too small to see without a microscope, but they can make the water look green when they are present in large numbers. They grow mainly through photosynthesis and live near the ocean’s surface.

The most important groups of phytoplankton are diatoms and dinoflagellates. Diatoms are especially important, contributing up to 45% of the ocean’s primary production. Some diatoms are as large as 2 millimeters.

The second level of the food chain includes zooplankton, tiny animals that eat phytoplankton. Zooplankton and phytoplankton form the base of the food chain that supports many fish populations. Zooplankton include tiny crustaceans, fish larvae, and fry. Many zooplankton are filter feeders that strain phytoplankton from the water. Some zooplankton eat other zooplankton. They cannot swim strongly and instead float with ocean currents. Zooplankton can reproduce quickly, increasing their numbers by up to 30% each day under good conditions.

Oligotrichs are a type of ciliate with special cilia that help them eat. They are common in the ocean and are important herbivores that eat phytoplankton.

Other important zooplankton include copepods and krill. Copepods are tiny crustaceans that are a major food source for many fish. Krill are larger zooplankton that eat smaller zooplankton, placing them in the third level of the food chain.

Phytoplankton and zooplankton make up most of the plankton in the ocean. Plankton are small drifting organisms that cannot swim against ocean currents. In the ocean, the first two levels of the food chain are mostly made up of plankton, which includes producers (phytoplankton) and consumers (zooplankton).

Jellyfish are slow swimmers and are part of the plankton. They were once thought to have little impact on the ocean, but recent studies show they are an important part of many animals’ diets, including tuna, fish, birds, and invertebrates.

By habitat

In pelagic ecosystems, Legendre and Rassoulzadagan proposed in 1995 that food chains and the microbial loop are two main types of food webs. The classical food chain includes zooplankton eating large phytoplankton, and then zooplankton being eaten by larger zooplankton or other predators. In this chain, a predator can either increase or decrease the amount of phytoplankton depending on the number of levels in the system. Changes in predator numbers can cause changes throughout the food chain. The microbial loop includes not only phytoplankton but also dissolved organic carbon (DOC). DOC is used by bacteria for growth, and then bacteria are eaten by larger zooplankton. This process transforms DOC into zooplankton through a bacterial-microzooplankton loop. These two pathways are connected in many ways. Small phytoplankton can be eaten directly by microzooplankton.

Dissolved organic carbon is produced by many organisms, including primary producers and consumers. Primary producers release DOC passively through leakage or actively during unbalanced growth when nutrients are limited. Another way phytoplankton contribute to DOC is through viral lysis, where viruses kill phytoplankton, especially in warm, low-latitude waters. Herbivores and other consumers also release DOC through sloppy feeding and incomplete digestion. Heterotrophic microbes use enzymes to break down particulate organic carbon and use it for growth. Some of this microbial production is eaten by microzooplankton, while other microbes are killed by viruses, releasing DOC again. The efficiency of the microbial loop depends on factors like how much predation and viral lysis affect microbes.

Scientists are studying the mesopelagic zone, which is between 200 and 1,000 meters deep. This layer removes about 4 billion tonnes of carbon dioxide from the atmosphere each year and is home to most marine fish. A 2017 study found that narcomedusae eat the most diverse range of mesopelagic prey, followed by physonect siphonophores, ctenophores, and cephalopods. These gelatinous predators, like jellyfish, may play key roles in deep ocean food webs, similar to fish and squid. Diel vertical migration helps mesozooplankton move carbon from the surface to deeper waters, supporting other organisms.

A 2020 study suggests that by 2050, global warming could increase the speed of warming in the deep ocean seven times faster than it is now, even if greenhouse gas emissions are reduced. This could disrupt deep ocean food webs as species move to survive.

Ocean surface habitats are where the ocean meets the atmosphere. The surface layer, called the neuston, supports microorganisms that live at the air-water boundary. This area covers more than 70% of Earth’s surface and is important for gas exchange and climate processes. Bacteria in the surface microlayer, called bacterioneuston, are studied for their role in gas exchange, aerosol production, and remote sensing. Surfactants, which are surface-active materials, are produced and broken down by microbes. Sources of surfactants include phytoplankton, land runoff, and atmospheric deposition.

Surfactant-associated bacteria may not be visible in satellite images of ocean color. However, synthetic aperture radar (SAR) can detect these bacteria in all weather conditions, including high winds, when gas exchange and aerosol production are strongest. SAR imagery can provide additional information about ocean-atmosphere interactions.

Ocean floor (benthic) habitats are where the ocean meets Earth’s interior. Coastal waters, including estuaries and continental shelves, cover about 8% of the ocean and support half of all ocean productivity. Nitrogen and phosphorus are key nutrients in coastal waters and lakes, respectively. Guano (seabird feces) is a major source of these nutrients. Uric acid in guano breaks down into different nitrogen forms.

Ecosystems often depend on each other for energy and nutrients. Seabirds bring nutrients to islands through guano, which can change how ecosystems function. While many studies show how guano affects land ecosystems, fewer have studied how it affects marine ecosystems, especially in tropical regions. In the tropics, coral reefs near islands with seabirds may be affected by nutrient-rich guano. Studies suggest guano nitrogen enriches seawater and supports reef life.

Coral reefs need nitrogen to grow, but they live in nutrient-poor tropical waters. They form partnerships with Symbiodinium (zooxanthellae), which take up nitrogen from water and recycle waste back to the coral as amino acids, ammonium, or urea.

Foundation and keystone species

In 1972, Paul K. Dayton introduced the concept of a foundation species. He applied this idea to certain marine invertebrates and algae communities. Studies in many places showed that a few species had a much bigger effect on the rest of the marine community than their numbers suggested. These species were important for helping the community stay strong after problems like pollution. Dayton believed that focusing on foundation species could help scientists understand how an entire community might react to changes more quickly, instead of trying to study every species at the same time.

Foundation species are species that play a major role in shaping an ecological community. They help create the environment and define the ecosystem. These ecosystems are often named after the foundation species, such as seagrass meadows, oyster beds, coral reefs, kelp forests, and mangrove forests. For example, the red mangrove is a common foundation species in mangrove forests. Its roots provide safe places for young fish, like snapper, to grow. A foundation species can be found at any level in a food web but often acts as a producer.

In 1969, zoologist Robert T. Paine introduced the concept of a keystone species. He developed this idea based on his observations and experiments with marine invertebrates in the intertidal zone, including starfish and mussels. Some sea stars eat sea urchins, mussels, and other shellfish that have few natural predators. If sea stars are removed from an ecosystem, mussel populations can grow too large, pushing out most other species.

Keystone species are species that have a large effect on an ecosystem, even if they are not very common. An ecosystem can change greatly if a keystone species is removed, even if that species is not a major part of the ecosystem in terms of size or productivity. Sea otters help control the damage sea urchins cause to kelp forests. When sea otters were hunted for their fur on the North American west coast, their numbers dropped so low they could no longer control the sea urchin population. The urchins then ate the kelp so much that the kelp forests disappeared, along with many species that relied on them. Bringing sea otters back helped restore the kelp ecosystem.

Topological position

Networks that show how organisms eat and are eaten can help explain how marine ecosystems work. In addition to what animals eat, other traits like how they move, their size, and the places they live can add more information about their roles in these ecosystems.

To understand how ecosystems stay healthy, scientists need to answer a question first asked by Lawton in 1994: What do species do in ecosystems? Because the roles species play in food webs and their positions in these networks are connected, it is important to study which types of species are found in different parts of the network. Since the idea of keystone species was first introduced, scientists have been interested in where these important species are located in food webs. Early ideas suggested keystone species were often top predators, but later research showed they could also include plants, herbivores, and parasites. Knowing where these species are in complex food webs is helpful for both studying ecosystems and protecting them.

An example of this type of network study is shown in a diagram based on data from a marine food web. The diagram shows how the positions of organisms in the network relate to how mobile they are. Shapes and colors are used to represent these traits: shapes show how mobile the organisms are, and colors help identify (A) groups that are less mobile, such as those that stay in one place or drift, and (B) groups that are at the top of the food web.

The importance of different organisms in ecosystems changes over time and in different areas. Studying large sets of data can help scientists understand these changes better. If different types of organisms are found in different parts of the network, using this information in food web models can lead to more accurate predictions. Scientists have compared different measures of importance, such as how connected an organism is to others in the network and how central it is to the food web, to understand which species are most important. They have also compared these measures to the trophic level of species, finding that some highly important species are found in the middle of the food web. Adding information about traits like mobility to these studies helps scientists better explain their findings. These types of comparisons have also been studied in other networks, such as those that show how habitats are connected. With large data sets and new statistical tools, scientists can revisit these questions and improve their understanding.

Cryptic interactions

Hidden interactions, which are not easily noticed even though they are common, happen throughout the marine plankton food web. However, current methods used to study these interactions are not good enough to collect large amounts of data about them. Even so, some evidence shows that these hidden interactions might affect how the food web works and the results of models used to study it. Including these hidden interactions in models is especially important when they involve the movement of nutrients or energy.

The diagram shows how five hidden interactions—mixotrophy, differences between species and life stages, microbial cross-feeding, auxotrophy, and how cells divide their carbon—affect the flow of materials, populations, and molecular pools in the food web. These interactions can work together because the parts of the food web they influence often overlap. For example, how phytoplankton divide their carbon can affect the amount of organic matter used in microbial cross-feeding, the exchange of materials during auxotrophy, and how prey is chosen based on differences between species and life stages.

Simplifications, such as "zooplankton eat phytoplankton," "phytoplankton take in nutrients from the environment," and "the amount of carbon produced by plants determines how much carbon is available in the food web," have helped scientists explain and model general interactions in the ocean. Traditional methods have focused on measuring and describing these general ideas. However, recent advances in technology, such as better genomics tools, sensors, and experiments, have shown that these generalizations might be too simple. These improvements have revealed many interactions that are hard to study because common sampling methods and experiments are not well suited to find them.

Complexity and stability

Food webs help organize the many predator-prey relationships in an ecosystem. A food web is a network of food chains. Each food chain starts with a primary producer, such as a plant or alga, which can make its own food. The next step in the chain is an organism that eats the primary producer, and this pattern continues with each organism eating the one before it. These organisms are grouped into trophic levels based on how far they are from the primary producers. The length of a food chain, or its trophic level, shows how many species are involved as energy moves from plants to top predators. Energy flows from one organism to the next, but some energy is lost at each level. At each trophic level, there may be one species or a group of species that share the same predators and prey.

In 1927, Charles Elton published an important summary about food webs, which made them a key idea in the study of ecosystems. In 1966, interest in food webs grew after Robert Paine studied intertidal shores, showing that the complexity of food webs helps keep species diverse and ecosystems stable. Scientists like Robert May and Stuart Pimm later studied the math behind food webs. Their work suggested that complex food webs might be less stable than simple ones. This creates a puzzle: why are food webs in nature so complex if models suggest they might be fragile? Some researchers think this might be because the idea of a food web lasting over time is different from the idea of it being stable in balance.

A trophic cascade can happen in a food web if one level is reduced. For example, a top-down cascade occurs when predators reduce the number or change the behavior of their prey, allowing the next lower level to grow. In this case, the top predator controls the population of the primary consumer, letting the primary producer thrive. If the top predator is removed, the primary consumers may overgrow and use up the primary producers. Eventually, there may not be enough primary producers to support the consumers. Stability in top-down food webs depends on competition and predation in higher levels. Invasive species can change this by becoming or removing top predators. Sometimes, these changes help repair ecosystems. For example, in the northwest Atlantic during the 1980s and 1990s, overfishing removed Atlantic cod and other fish, leading to more prey species like snow crabs and shrimp. This change affected zooplankton, which are food for smaller fish and invertebrates. Top-down cascades help explain how removing top predators can change ecosystems, as humans have done through hunting and fishing.

In a bottom-up cascade, the population of primary producers controls energy levels in higher trophic levels. Primary producers include plants, phytoplankton, and zooplankton that use photosynthesis. While light is important, the amount of nutrients in the environment affects their numbers. Food webs depend on the availability of resources. If nutrients are plentiful, all populations may grow initially.

Terrestrial comparisons

Marine environments sometimes have upside-down biomass pyramids. In these cases, the total mass of consumers, such as copepods, krill, shrimp, and forage fish, is usually greater than the mass of primary producers. This means that zooplankton, which are small animals that eat phytoplankton, make up most of the marine animal biomass. Zooplankton act as the main link between primary producers (mainly phytoplankton) and other parts of the marine food web. Phytoplankton are tiny and grow and reproduce quickly, so even a small amount of them can produce a lot of energy.

In contrast, many land-based primary producers, like mature forests, grow and reproduce slowly. This means they need a much larger mass to produce the same amount of energy. The ratio of an organism’s energy production to its biomass is called the Production/Biomass (P/B) ratio. Production is measured by how much mass or energy moves per area over time. Biomass is measured by how much mass exists per area or volume. The P/B ratio uses units of time, such as 1/month. This ratio helps compare how much energy flows through different parts of the food web. Usually, the P/B ratio decreases as organisms grow larger and live longer. Small, short-lived organisms have higher P/B ratios than large, long-lived ones.

For example, the bristlecone pine tree can live for thousands of years and has a very low P/B ratio. The cyanobacterium Prochlorococcus lives only about 24 hours and has a very high P/B ratio. In oceans, most primary production is done by algae. On land, most primary production is done by vascular plants.

Aquatic producers, like planktonic algae or aquatic plants, do not grow as large as woody trees on land. However, they reproduce quickly enough to support more biomass of animals that eat them. This creates an upside-down pyramid. Primary consumers, such as zooplankton, often live longer and grow more slowly than the phytoplankton they eat, leading to more biomass in consumers than in producers. Phytoplankton live only a few days, while zooplankton that eat them can live for weeks, and fish that eat zooplankton can live for years. Aquatic predators also tend to have lower death rates than smaller animals, which contributes to the inverted pyramid. Other factors, like population structure, migration, and where prey can hide, may also cause inverted pyramids. Energy pyramids, however, always have an upright shape because of the second law of thermodynamics.

Most organic matter created is eventually eaten or used by organisms and turned back into inorganic carbon. Only a small amount of organic matter, about 0.2 to 0.4 billion tonnes each year, is preserved in sediments. Global phytoplankton production is about 50 billion tonnes per year, and their total biomass is about one billion tonnes. This means phytoplankton are replaced every week. Marine plants have a similar total biomass but produce only one billion tonnes per year, meaning they are replaced every year. These high replacement rates (compared to land plants, which take one to two decades to replace) show that organic matter is used quickly and efficiently. Organic matter is lost through many processes, such as breathing by plants and animals, grazing, viruses breaking cells apart, and decaying material. All these processes eventually result in carbon being released back into the environment.

Anthropogenic effects

Pteropods and brittle stars are important parts of the Arctic food webs. Both are harmed by acidification. Pteropods' shells dissolve as acidification increases, and brittle stars lose muscle mass when they regrow their limbs. Also, brittle star eggs die quickly when exposed to conditions expected from Arctic acidification. Acidification could destroy Arctic food webs by harming the base. Arctic waters are changing quickly and are becoming less rich in aragonite, a type of mineral. Arctic food webs are simple, with few steps from small organisms to larger predators. For example, pteropods are an important food source for many larger animals, such as plankton, fish, seabirds, and whales.

Ocean ecosystems are more sensitive to climate change than other places on Earth. This is because of rising temperatures and ocean acidification. As ocean temperatures increase, fish species are expected to move to new areas outside their usual ranges. During this change, the number of individuals in each species may drop. Currently, many predator-prey relationships depend on each other for survival. If species move to new areas, these relationships will be affected. Scientists are still studying how these changes might affect food webs.

Using models, scientists can study how species interact in their environments. Recent models show that larger marine species may move more slowly than expected due to climate change. This could worsen predator-prey relationships because smaller species are more likely to move quickly as oceans warm. Larger predators may stay in their old areas longer, waiting for smaller species to move. As new species enter areas where larger animals live, the ecosystem changes, providing more food for them. Smaller species may have smaller ranges, while larger animals may expand their ranges. These changes could greatly affect all ocean species and impact the entire ecosystem. The movement of predators and prey may also affect the fishing industry. Fishermen may find it harder to locate fish species, increasing costs as they travel farther to find them. This could change fishing rules for certain areas.

A study from Princeton University found that marine species are moving in line with "climate velocity," which describes the speed and direction of climate change. From 1968 to 2011, 70% of changes in where animals live underwater and 74% of changes in their north-south positions matched changes in ocean temperature. These movements cause species to shift 4.5 to 40 miles farther from the equator each decade. Models can help predict where species may move. As scientists learn more about how climate affects species, models will need to change.

"Our results show how future climate change could weaken marine food webs by reducing energy flow to higher levels and shifting food webs toward systems based on dead material, which could simplify food webs and change how producers and consumers interact. These changes may affect communities living on the ocean floor."

"Increased temperatures reduce the flow of energy from primary producers, such as algae, to herbivores, and then to top predators. These disruptions could reduce food availability for top predators, harming many marine species. While climate change increased plant productivity, this was mainly due to more cyanobacteria (a type of algae). However, these cyanobacteria are not eaten by herbivores, so they do not support food webs. Understanding how ecosystems work under global warming is a challenge for scientists. Most studies on ocean warming use simple experiments focusing on only a few species."