Marine food web

A marine food web is a food web of marine life. It shows how different ocean animals and plants are connected through what they eat and what eats them. At the very bottom of this web are tiny, single-celled plants called phytoplankton. These tiny plants are eaten by small animals known as zooplankton, which are the primary consumers.

As you move up the food web, bigger animals that eat the zooplankton come next, and then even larger animals that eat those bigger animals. This creates a chain of who eats whom in the ocean. Scientists have recently paid more attention to tiny marine microorganisms, which also play important roles in this web.

One interesting thing about ocean food webs is that they look different from land food webs. In the ocean, the animals that eat the plants, like tiny crustaceans and small fish, actually weigh more overall than the plants themselves. This is because ocean plants, the phytoplankton, are very small but grow and reproduce very quickly. On land, plants like mature forests grow more slowly, so it takes a lot more of them to weigh as much as the animals that eat them. Because of this, in the ocean it is the zooplankton and other small animals that make up most of the weight of marine life.

Food chains and trophic levels

Main articles: food chain and trophic levels

Food webs in the ocean are made from food chains. Every sea creature can become food for another. A simple food chain in the ocean starts with energy from the sun that helps phytoplankton grow. Phytoplankton make their own food using sunlight in a process called photosynthesis. Because they make their own food, they are the first step, or the first trophic level, in the ocean’s food chain.

Next, tiny animals called zooplankton eat the phytoplankton. Zooplankton are the second step in the chain. After that, bigger animals eat the smaller zooplankton, and so the chain continues, with bigger fish and marine animals eating the smaller ones. This passing of energy from one group of animals to another is what we call a food chain.

By taxon

Primary producers

At the base of the ocean food web are tiny plants called phytoplankton. These are very small plants that float in the water and need sunlight to grow. They are the first step in the ocean’s food chain because they make their own food using sunlight, just like plants on land.

Primary consumers

The next level in the ocean food web is made up of tiny animals called zooplankton. These small creatures eat the phytoplankton. Many zooplankton are very small, like tiny shrimp, and they float in the water just like the plants they eat. Some bigger zooplankton can eat the smaller ones, too. They are an important food source for many fish and other sea animals.

Higher order consumers

Larger animals in the ocean, like fish and whales, eat the zooplankton. Some fish, called forage fish, are important because they eat zooplankton and are then eaten by bigger fish, birds, and even whales. Whales, for example, help bring nutrients from deep water back to the surface when they eat and then release waste that stays near the top of the water.

Microorganisms

Very tiny living things, called microorganisms, also play big roles in the ocean. These include tiny plants, animals, and even viruses that help keep the ocean’s food web balanced. They break down dead things and recycle nutrients so that plants can grow again.

By habitat

Pelagic webs

For pelagic ecosystems, scientists proposed a way to understand how energy moves through the system. In one path, small plants called phytoplankton are eaten by tiny animals called zooplankton. These zooplankton can then be eaten by bigger animals. This simple path can lead to more or fewer phytoplankton depending on how many bigger animals there are.

Another path involves tiny plants and a type of carbon called dissolved organic carbon. This carbon is used by tiny bacteria, which are then eaten by bigger zooplankton. This creates a loop where carbon moves between different tiny organisms.

These two paths connect in many ways. Very small phytoplankton can be eaten directly by even smaller zooplankton.

Scientists are learning more about a deep part of the ocean called the twilight zone, which is about 200 to 1,000 metres deep. This area helps remove a lot of carbon dioxide from the air each year. It is home to many fish and other sea animals.

At the ocean surface

Ocean surface habitats are where the ocean meets the air. These areas have special tiny organisms called neuston that live at the surface. These organisms help move gases between the ocean and the air, and they can also create substances that affect weather and climate.

At the ocean floor

Ocean floor habitats are where the ocean meets the Earth. These areas include places with special warm spots called vents and seeps.

Coastal webs

Coastal waters are near land and include areas like estuaries and continental shelves. These areas are very productive and support many plants and animals. Seabirds that live near these areas add nutrients to the ocean through their waste, which can help plants and animals grow.

Polar webs

The Arctic and Antarctic have very different landscapes, which lead to different food webs. Both have food chains that depend on a few key species. Changes in one part of the chain can affect many other animals.

Polar regions are also home to many tiny organisms, including bacteria and viruses, which play important roles in the food web and in cycling nutrients.

Foundation and keystone species

The ochre starfish is a key predator that helps control the number of California mussels, which would otherwise take over and push out other species.

A foundation species is one that plays a big role in shaping its environment and supporting the whole ecosystem. Examples include red mangroves, which give young fish like snapper a safe place to grow, and kelp forests. These species can be found at any level of the food web but are often producers.

Sea otters are a keystone species because they eat sea urchins, which would otherwise destroy kelp forests by chewing through the kelp's base. When sea otters were hunted for their fur, sea urchin numbers exploded and kelp forests disappeared. Bringing back sea otters helped restore these important kelp ecosystems.

Topological position

See also: Marine coastal ecosystem § Network ecology

Networks of eating habits can tell us a lot about how sea ecosystems work. Besides what animals eat, three more traits (how they move, their size, and where they live) of different sea creatures can add to our understanding.

To keep ecosystems healthy, scientists ask important questions about what roles different animals play. Since the jobs animals do and where they fit in the food chain are connected, we need to know which animals are in which spots. Researchers have long been interested in finding special animals called keystone species and where they fit in the food chain. At first, they thought these were top animals that eat others, but later found out plants, animals that eat plants, and even parasites can be important too.

One way scientists study this is by looking at diagrams based on real data from sea life. These diagrams show how the positions of animals in the food chain relate to how they move. The animals are shown in different shapes based on how they move and in different colors to show their place in the food web.

The importance of different animals can change over time and place. By studying large collections of data, scientists can learn more about which animals are most important. Comparing different ways to measure importance in the food web helps scientists understand better which animals play key roles. Adding information about animal traits to food web groups helps explain the results in biological terms. Scientists have also studied these importance measures in other types of networks, like ones showing habitats. With more data and better analysis methods, these questions can be studied again to update our knowledge.

Cryptic interactions

Further information: Auxotrophy and Mixotrophy

Cryptic interactions are hidden connections that happen in the ocean's tiny plant and animal life, but we don't see them easily. These hidden connections might change how food and nutrients move in the ocean. Including them in science models is very important, especially when they involve moving nutrients or energy.

The diagram shows how five hidden interactions affect the ocean's tiny plants and animals. These interactions can work together in surprising ways. For example, how tiny plants share carbon can affect both what other tiny animals eat and how they share nutrients. Simple ideas like "tiny animals eat tiny plants" have helped scientists understand the ocean, but new tools show there are many more hidden connections than we thought.

Complexity and stability

See also: Ecological complexity and Ecological stability

Food webs help us understand how different animals and plants are connected through what they eat. Each food chain starts with a plant or alga that can make its own food, called a primary producer. Other animals then eat these producers, and some of those animals are eaten by others. This creates a chain of who eats whom.

Scientists have studied these chains and found that more complex food webs might be less stable. This means that when many different kinds of animals and plants are connected, small changes can sometimes cause big effects in the whole system. Removing important animals, like top predators, can change the whole balance of the food web. For example, when large fish like Atlantic cod are caught too much, it can cause an increase in smaller fish and animals that they used to eat. This shows how connected everything is in the ocean.

Terrestrial comparisons

Marine life has some big differences compared to life on land. In the ocean, tiny plants called phytoplankton make food for many animals. These tiny plants grow and reproduce very quickly, which helps them feed lots of other creatures.

In the ocean, animals called zooplankton eat the phytoplankton. These animals also grow quickly but live longer than the plants. Because they live longer, there is more of them overall. This is different from land, where big plants like trees live for many years and store a lot of material, but do not make food as quickly. In the ocean, the animals can end up weighing more than the plants they eat!

Anthropogenic effects

Further information: Human impact on marine life

Changes made by humans are affecting marine life in many ways. Overfishing and ocean acidification are two big problems. Acidification is hurting tiny sea creatures like pteropods and brittle stars. Pteropods have shells that dissolve in more acidic water, and brittle stars lose strength when they grow new parts. These creatures are important because many bigger animals, like fish and whales, eat them.

Climate change is also changing the ocean. As the water gets warmer, many fish are moving to new places. This can break the balance between animals that eat others and the ones they eat. Scientists are studying how these changes will affect the whole ocean community. Some bigger animals might not move as fast as smaller ones, which can create new problems for feeding and finding food. These changes will affect fishing and the rules about where fish can be caught.