The Ocean Food Chain

Understanding and Preserving the Marine Trophic Web of the Coral Reef

Covering 71 percent of the Earth’s surface, the ocean provides us with a magnificent variety of creatures. Each of these creatures occupies a unique position on the food web, or trophic web, which is composed of producers, consumers, and decomposers. For an environment to remain healthy, the food chain must remain unbroken. If one link in the chain is broken, all creatures on the chain may be endangered.

Coral reefs provide an excellent example of the trophic web since they are a biodiversity hotspot. Every link of the food web is represented in a healthy coral reef. You may observe how the organisms are in balance or not when you dive on a coral reef and wonder what humans can do to preserve the ocean's health.

Level 1: Producers

trophic web
Green algae is a producer in the ocean's food web. © NOAA

Photosynthetic organisms, like seaweed, zooxanthellae (algae living in coral tissue), and turf algae, make up this group. Turf algae is opportunistic, meaning it will claim any available reef real estate. A reef covered in turf is probably in poor health.

Level 2: Primary Consumers

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Parrotfish are primary consumers in the ocean's food web. © NOAA

Herbivores eat the first level organisms and are included in the primary consumers group. Sea urchins, some crab species, sponges, and even the large green sea turtle are primary consumers. The surgeonfish, a member of this group, mows down the turf algae to a healthy level. If surgeonfish are absent from a reef, divers can count on seeing an algae invasion.

Ever wonder where sand comes from? Parrotfish are algae eaters that use powerful fused beaks to remove algae from dead coral. The Stoplight and Queen parrotfishes even take nips of coral. The parrotfish gut then processes the coral calcium carbonate skeleton. The final product, sand, is then sprinkled over the reef. This is where most reef and beach sand comes from.

Level 3: Secondary Consumers

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Butterflyfish are secondary consumers in the ocean's food web. © NOAA

Dining on primary consumers, these animals are carnivorous. Goatfish and wrasses eat everything from snails and worms to crustaceans. This group also includes many species of coral eaters such as butterflyfish, filefish, triggerfish, and damselfish. Their specialized, elongated mouths enable them to chow down on the tiny individual polyps of the coral. Their absence paints a picture of a reef with few corals.

Level 4: Tertiary Consumers

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Goldentail moray eels are tertiary consumers in the ocean's food web. © NOAA

These are the large fish that excite divers. Barracuda, groupers, snappers, sharks, moray eels, and dolphins are at the top of the food chain. Their feast includes other fish, crustaceans, and even octopi. At-risk reefs have low numbers of these top-level (apex) predators. They help to keep other fish populations at bay. Considering that tertiary consumers are commercially fished, their absence is a possibility and even a reality in many regions.

Level 5: Decomposers

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Decomposers help to keep the ocean clean. ©

The little-glorified job of decomposing dead sea animals and plants is left to bacteria. Animal and plant wastes are converted to a food form subsequently used by animals throughout the food chain.

Human Impact on the Ocean Food Web

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Shark finning threatens the health of the entire ocean.

As in any chain, when a link is missing or weakened, the chain as a whole is also weakened and no longer functions correctly.

Fish stocks are being depleted to worrying levels. Many species are listed as endangered or threatened. This is primarily due to pressure from human consumption. Fish populations are not given the necessary time to replenish.

These problems have solutions. Humans must understand that we are part of the complex and dynamic food chain—not at the top of it. Taking care of marine food resources is critical to preserving them. Fishing methods can be adapted to be less damaging to ocean habitats and the animals they support. National and international programs must promote long-term sustainability.

How You Can Support the Health of the Marine Food Web

  • Join a Monitoring Program: Conservation programs such as Global Vision International (GVI) offer divers the opportunity to participate in fish monitoring research. For example, dives can contribute to scientific research by collecting census statistics on fish.
  • Eat Responsibly: Get a pocket guide or app to choose sustainable fish such as the Seafood Watch guide from the Monterey Bay Aquarium and chow down on only those that are ocean-friendly. Don't patronize restaurants and seafood retailers that offer products that threaten the ocean and ocean life. Effect change by spending your money at businesses and restaurants that support ocean conservation.

A healthy reef is filled with a member of each level of the trophic web. When creatures from one level are threatened, the health of the entire reef is in jeopardy. To ensure that coral reefs are present for the next generations to enjoy, humans must take steps to protect plants and animals at every level of the food chain.