How Each Organism Works in the Ocean Food Chain

Understanding and Preserving the Marine Trophic Web of the Coral Reef

underwater scenery

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Covering 71 percent of the Earth’s surface, the ocean provides 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

Parrotfish on algae bed, Thailand

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

Level 2: Primary Consumers

Male Parrot Fish

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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.

Parrotfish are algae eaters that use powerful fused beaks to remove algae from dead coral. The Stoplight and Queen parrotfish 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

A yellow blue and white butterfly fish

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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

Moray looking out his pipe house. Ose, Shizuoka, Japan. Depth 18m

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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

tuna fin is cut on the seabed, Maldives

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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

Rockfish trapped in lost Fishing Net, Scorpaena scrofa, Cap de Creus, Costa Brava, Spain

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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, with many species listed as endangered or threatened—primarily due to pressure from human consumption. Fish populations are not given the necessary time to replenish.

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. 

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.