Human activity lowers coral reef resilience by accumulating contaminants

Human activities are fundamentally altering the chemical composition of local coral reefs, according to research led by the University of Hawai‘i at Mānoa and published in Nature Communications. An international research team discovered that 25 contaminants from agricultural, industrial, and pharmaceutical sources have accumulated in the soft tissues of coral colonies surrounding Maui, Hawai‘i. The study warns that in areas heavily impacted by human activity, coral tissues experience a depletion of vital energy and nutrient reserves, leaving these organisms significantly less resilient to environmental stressors like ocean warming and acidification.

The researchers examined the metabolomes—the pool of chemicals within biological tissues—of 380 individual corals. The sample set included two distinct species, lobe corals (Porites lobata) and rice corals (Montipora capitata), collected from 16 different sites located off the west and south coasts of Maui. By analyzing these samples, the team found that human activities, occurring both within the immediate marine ecosystem and in the adjacent watersheds, resulted in altered chemical profiles within the coral tissues.

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"Our findings suggest that monitoring the pool of chemicals within the coral tissues, called metabolomes, can serve as a powerful tool for tracking the hidden impacts of anthropogenic disturbance on marine life," said Zach Quinlan, lead author and research biologist at the Hawai‘i Institute of Marine Biology in the UH Mānoa School of Ocean and Earth Science and Technology.

A notable aspect of the study was the consistency of the findings across species. "It was extremely surprising that the metabolomes of both coral species had almost identical trends," Quinlan said. "These corals have very different life strategies, and we wouldnʻt normally expect them to accumulate contaminants the same or even necessarily respond the same to disturbances. This demonstrates how strong of a forcing these anthropogenic activities really are."

The research team linked these chemical shifts to historical environmental data. By examining coral cover trends at five of the sites they sampled, the team found that sites that had the most severe declines in coral cover after the 2016 bleaching event are the sites with the most impacted metabolomes. At these disturbed sites, stress-related chemicals were found to be enriched, while the essential nitrogen and energy stores required for coral health were significantly reduced. The research team proposed two potential mechanisms by which human activities and contamination of marine environments lead to decreased coral resilience: 1) accumulation of anthropogenic molecules such as pharmaceuticals and industrial byproducts, and 2) increased pressure from anthropogenic activities requires coral to utilize portions of their nitrogen and energetic reserves.

"This response to anthropogenic pressure makes the coral less resilient to stressors," Quinlan noted, adding that the relationship between human disturbance and coral health appears consistent across species. Megan Donahue, the senior author of the paper and director of the Hawai‘i Institute of Marine Biology, highlighted the broader implications of the findings. "Beyond the implications for coral health and resilience, this study demonstrates how many anthropogenic contaminants are escaping into marine ecosystems," Donahue said. "We see increasing evidence that anthropogenic contaminants have broad cumulative impacts, undermining the resilience of coastal ecosystems."

Looking ahead, the research team is initiating controlled experiments to determine if enriching nitrogen and energy reserves in coral tissue can improve their ability to withstand external pressures. The scientists suggest that measuring the metabolomes of corals and other seafloor species could become a standardized, powerful method for tracking the long-term impact of human activity on natural marine environments.