The Bottom of the Food Chain: Plankton, Shrimp and Krill

The Bottom of the Food Chain: Plankton, Shrimp and Krill

The ocean is such a massive expanse that it might seem that there is not much order or system underwater. Surprisingly though, it’s completely the opposite! Even though there are so many different types of fish, mammals, plants and smaller creatures that make their home within the ocean, they all depend on each other very heavily. There are several levels of the oceanic food chain. At the very bottom are many tiny creatures known as phytoplankton and zooplankton; some of them are small enough to be made up of only one cell! However, these tiny organisms are not to be taken lightly. They serve an extremely important role in the food chain since they make up the bottom level, which is also the largest amount of biomass. In this way, these tiny creatures essentially support all of the life forms that are above them on the food chain. If they were to be killed, diseased or endangered, it would mean that the fish that normally feed on them would have less food and die off. Think of it as a domino effect. Each level would be affected in turn and eventually it would even reach humans and land animals that depend on the ocean for food. Without a healthy ecosystem in the ocean, we would not have fish, crabs, squid, lobster or other such foods that make up a very healthy part of our diet.


Phytoplankton are so small that we need a microscope to properly see them. Many phytoplankton are actually types of bacteria, while others are plants that are made up of single cells. The main way that phytoplankton move is by drifting along with the water. They can also move by themselves, although their own movements do not allow them to go very far. Some of the ways that phytoplankton can move is by modifying the quantity of gas in their bodies, moving the tiny hairs (cilia) or a type of tail (flagella) that are attached to their bodies.

Phytoplankton are too small to eat other animals but they do love sunlight! Instead of using food for energy, they use chlorophyll to convert light into energy. If this process sounds familiar, you’re absolutely correct. Plants on land function in the same way. Another function that phytoplankton performs as part of this process is that they constantly convert carbon dioxide into oxygen, which is very important since fish need oxygen to stay alive. The temperature of the water, the amount of light entering the water and minerals are some of the factors that affect how well phytoplankton can grow and thrive. If there is an extremely large amount of phytoplankton in the water, we still won’t be able to see individual organisms, but observers will notice a slight color (usually greenish) in the water.


Zooplankton are tiny creatures that range from microscopic to tiny but visible to the naked eye. They are slightly more advanced than phytoplankton. There are many different types of creatures that make up the zooplankton category. Some of these are babies (larvae) of fish, snails and other underwater species. However, over half of zooplankton is actually very small crustaceans and invertebrates, such as krill. Crustaceans are creatures that have their skeletal systems on the outside of their bodies rather than the inside and invertebrates are creatures that do not have a backbone. Since there are so many incredibly different types of zooplankton, they all have very different ways of moving. For example, krill move by pushing with their tiny legs, while other zooplankton gather together in a group and drift with the water.

Zooplankton eat phytoplankton, bacteria and dead matter that is found in the water or on the ocean floor. In this way, zooplankton help to clean the water and in turn they end up as food for larger creatures. If there is too much bacteria or phytoplankton in the water, the zooplankton can help to control the overgrowth and maintain a happy balance.

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