Coral reef predators get 70% of their energy from the open ocean

Food webs anchored by coral reefs extend much farther into open waters than scientists previously thought.

According to a new study, published Friday in the journal Science Advances, more than 70% of the caloric energy consumed by reef predators is sourced from the open ocean.

Open surface waters in the tropics are low in nutrients and typically thought to be relatively unproductive, but the latest research these environments are more ecologically valuable than scientists previously estimated.

For the study, scientists used isotopic analysis to examine the diets of four grouper species captured near coral reefs in the Maldives, an island chain a few hundred miles southwest of India.

The analysis showed all four predatory fish rely on open ocean resources. Scientists found evidence of offshore resource consumption among groupers captured both outside of an atoll, a ring-shaped reef, as well as grouper found in the lagoon within.

Scientists suspect grouper are feeding on plankton-eating fish that rely on coral reef for shelter instead of food.

Researchers estimate upwelling from the deep ocean is responsible for the delivering of nutrient-dense water to the surface, fueling the plankton that feed many reef fish -- fish that often end up in the stomachs of grouper.

"The study provides key insights into the nutrition of coral reef ecosystems, especially their dependence on offshore production," ecologist and study lead author Christina Skinner said in a news release.

"Detailed knowledge of food web dynamics is crucial to understand the impacts of anthropogenic and climate-induced change in marine ecosystems."

Skinner led the research while working at Newcastle University, but now works as a postdoctoral researcher at the Hong Kong University of Science and Technology.

"The results force us to reconsider how we view coral reefs, and they highlight the extent of the connectivity with the surrounding ocean," she said.

"If these groupers are mostly reliant on offshore energy to support their feeding, then maybe they won't be so impacted by the loss of live coral, as many fishery studies have predicted; they may be more resilient."

But coral reefs aren't the only ecosystems affected by climate change. Studies suggest global warming is already altering the makeup of plankton communities and other groups of microorganisms in the open ocean, and some models predict open ocean productivity will decline as the planet heats up.

"If that is the case, and these groupers are reliant on that open ocean energy, they will be impacted by those changes," Skinner said.

Researchers suggest their study's biggest takeaway is that open water and coral reef ecosystems are inextricably linked.

The effects of warming trends and coastal pollution on coral reef health has been well documented, and the latest findings suggests are a reminder that what's bad for reefs may also be bad for neighboring ecosystems.

"Coral reefs are really suffering across the tropics from climate-related disturbances, particularly oceanic warming," said co-author Nick Polunin, professor of environmental sciences at Newcastle.

"In spite of its tiny area, this ecosystem is a massive contributor to marine biodiversity and this study highlights how little we know about the food web sources sustaining that exceptional wealth of species it sustains."

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