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Marine Food Web

Plants and animals must have organic carbon to survive. Organic carbon has high-energy chemical bonds which are broken to provide metabolic energy. For example, humans eat organic carbon in the form of green beans and chicken, but we couldn't survive if we only had diamonds to eat. Diamonds are made from carbon, but it is the wrong kind - inorganic carbon. Just like humans, plants and animals in the ocean require organic carbon.

How do they get it? There are only two ways. They either produce their own or make use of organic carbon produced by others. Species that make their own organic carbon are called "primary producers". They are the base of the marine food web because all other species depend on their productivity either directly or indirectly to survive. In most parts of the ocean, primary producers are marine plants, typically algae. These are the phytoplankton (phyto = plant; plankton = floating). They live in the sunlit portions of the ocean and use energy from the sun to convert inorganic carbon into organic carbon.

Marine food web for the Arctic.
Phytoplankton generate organic carbon through photosynthetic reactions.
Phytoplankton are eaten by the zooplankton, who are in turn eaten by fish, on up to large marine mammals. Organic carbon that settles to the sea floor is usually consumed by marine organisms there.
(Credit: Christopher Krembs, NOAA, US Department of Commerce)

How do primary producers generate organic carbon? The most common way by far is through a chemical reaction called photosynthesis. Phytoplankton, benthic plants or bacteria use energy from the sun to convert inorganic CO2 and water to organic carbon and free oxygen. Alternatively, organic carbon may be created through a process called chemosynthesis. However this reaction produces only a small portion of the organic carbon available in the ocean. During chemosynthesis, bacteria use chemical energy stored in methane, hydrogen sulfide and ammonia (rather than light energy) to convert inorganic CO2 to organic carbon. Chemosynthetic reactions are the basis for life at hot ridge vents and cold seeps, but photosynthesis is the basis for life in the rest of the ocean.

What comes next in the marine food web? Small animals named zooplankton (zoo = animal; plankton = floating) eat the primary producers. These zooplankton are small herbivores or bactivores (plant or bacteria eaters) since primary production comes from marine plants and bacteria. They are in turn consumed by small marine carnivores (that may also be zooplankton), and these are consumed by larger marine carnivores and so on, up to the apex predators. These animals are the highest level within the marine food web.

Often marine organisms eat plants, animals, or whatever source of organic carbon they can find. This makes them omnivores because they eat a variety of things. Other sources of organic carbon include fecal pellets (animal excrement) and dead bodies or body parts of marine plants and animals. If these particles aren't consumed in shallow water, they sink into the deep where they are consumed by "deposit" feeders. In this way, almost all of the produced organic carbon is recycled to other organisms.

So you see, the ocean really is a highly
interconnected marine food web!

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Related Links

Learn more about marine food webs

Learn more about mid-ocean ridge vents and cold seeps


Lalli, C. M. & Parsons, T. R. (2001). Biological Oceanography - An Introduction (2nd ed.). Oxford: Butterworth-Heinemann.

Revised on: January 2, 2005