The Food Chain of the Ocean

Although the way in which aquatic plants and animals feed can be difficult to determine, we know that all living organisms are either autotrophs or heterotrophs. This means they can sustain themselves in one of two ways:

• Autotrophs: produce their own food without the need for another organism.
• Heterotrophs: consume other organisms to survive.

With this basic understanding, we can see that some organisms produce, while others consume. There are even some rare occasions where an organism is both. This can help us to understand the difference between a food chain and a food web.

A food chain shows how matter and energy move within an ecosystem through different organisms in a linear and unidirectional way. Food chains always begin with an autotrophic being that is the primary producer of matter and energy. These organisms are capable of transforming inorganic matter into organic matter and non-assimilable energy sources into assimilable energy.

A good example of this transformation is the conversion of sunlight into adenosine triphosphate (ATP), a source of energy for living beings. This occurs in autotrophic organisms with the ability to photosynthesize. The matter and energy created by the autotrophs will pass to the rest of the heterotrophic beings or consumers. These consumers are usually animal that can be primary, secondary and tertiary consumers.

On the other hand, a food web is a set of food chains that are interconnected. It is based on the same principles, but it provides a much more complex explanation of the movement of energy and matter. The marine ecosystem food web is one of the most complex and fascinating in nature.

Although the types of organisms vary greatly between aquatic and terrestrial ecosystems, the fundamental principles apply to both. They are both created by charting the interactions between producers and consumers. The main differences can be found in the types of species involved in the food chain of the ocean, as well as the amount. The biomass of terrestrial ecosystems is significantly greater than those of the ocean.

We can understand the food chain of the ocean by looking at the trophic levels. These are the positions an organism occupies within a food chain or food web. It is thanks to trophic levels that we think of different species being at the top or bottom of their respective food chains. Here we look at some of the trophic levels within oceanic food chains.

Primary producers

In the aquatic food chain, we can find primary producers which are mainly phytoplankton. Plankton is the term used for the many organisms which float through the ocean without an ability to propel themselves through the water. Phytoplankton are the autotrophic plankton which use photosynthesis to create their own energy.

One of the most common types of phytoplankton are the various types of algae which can be found in the ocean. They can be unicellular, such as those belonging to the phyla Glaucophyta, Rhodophyta and Chlorophyta or multicellular, such as those of the superphylum Heterokonta. The latter are the algae that we can see with the naked eye washed up on beaches. In addition, we can find bacteria at this level of the food chain, such as cyanobacteria which also carry out photosynthesis.

Learn more about this type of phytoplankton in the food chain of the ocean with our sister site's guide to what are cyanobacteria?

Primary consumers

The primary consumers in the aquatic food chain are usually herbivorous animals that feed on microscopic or macroscopic algae and even bacteria. This level is usually made up of zooplankton and other herbivorous organisms. Specific empales of primary consumers in the food chain of the ocean are small crustaceans, the larvae of larger animals, various fish species and even types of coral.

Discover more about certain types of marine primary consumers with our guide to types of aquatic insects.

Secondary consumers

Secondary consumers are notable for being carnivorous animals, feeding on herbivores in the lower level. They can be fish, arthropods, waterfowl or even mammals. For example, certain species of mackerel are considered secondary consumers as they will eat other smaller fish, but can themselves be eaten by larger carnivorous fish.

Tertiary consumers

Tertiary consumers are generally considered to be supercarnivores. These are carnivorous animals that feed on other carnivores, which form the trophic level of secondary consumers. In this group we can find animals of the marine ecosystem food web such as orcas, sharks and even certain species of tuna.

Although relevant, it is important to know that size is not the only factor in determining the trophic level of an animal in a marine ecosystem food web. For example, some of the largest aquatic animals are whales which are often considered secondary consumers since they do not hunt like certain shark or orca species.

Learn more about the orcas place in the food chain of the ocean by learning what do killer whales eat?

Although we have provided the basic trophic levels of oceanic food chains, they are generally more complicated to understand. One reason for this is that difference synecologists use different systems to understand marine ecosystem food webs. Below we look at some examples of different types of food chains in the ocean:

1. The first example of an aquatic food chain consists of two links. This is the case of phytoplankton and whales. Phytoplankton is the primary producer and whales the only consumer.
   
   
2. These same whales can form a three-link chain if they feed on zooplankton instead of phytoplankton. Then the chain would look like this: phytoplankton > zooplankton > whale. The direction of the arrows indicates where energy and matter are moving.
   
   
3. In an semi-aquatic/terrestrial system such as a river, we could find a chain of four links: phytoplankton > mollusks of the genus Lymnaea > common barbel (Barbus barbus) > gray heron (Ardea cinerea).
   
   
4. An example of a five-link chain where we can see a supercarnivore is the following: Phytoplankton > krill > emperor penguin (Aptenodytes forsteri) > leopard seal (Hydrurga leptonyx) > orca (Orcinus orca).

In a natural ecosystem the relationships between different species are never as simple as the ocean food chain suggests. Food chains are used to simplify trophic relationships and help us better understand the principles of marine life. It is important to remember food chains interact with each other in a complex network of food webs.

We must also consider the fact that aquatic food webs are also influenced by terrestrial animals. This is the case with birds which dive into the water to feed, as well as large mammals that eat fish such as polar bears. There are also marine animals that will venture partway onto land to eat terrestrial animals.

If you want to read similar articles to The Food Chain of the Ocean, we recommend you visit our Facts about the animal kingdom category.