Plastic waste has infiltrated every corner of our planet, sparing even the delicate coral reefs, and a recent scientific revelation underscores a troubling correlation: the deeper the reef, the greater the accumulation of plastic debris.

Published in the journal Nature, a comprehensive study titled “Plastic Pollution on the World’s Coral Reefs” exposes a disconcerting reality – every coral reef bears the burden of plastic contamination. Astonishingly, nearly three-quarters of the larger plastic items found on these reefs originate from “ghost gear,” discarded fishing equipment such as nets, ropes, and lines, alongside ubiquitous items like food wrappers and plastic bottles. This plastic incursion poses an “emerging threat” to reefs that are already grappling with the combined pressures of climate change and overfishing.

Conducted by a collaborative team from the California Academy of Sciences, the Nekton foundation and charity, and the universities of São Paulo, Oxford, and Exeter, the study meticulously surveyed 85 reefs across various depths and locations in the Indian, Pacific, and Atlantic Oceans. The researchers ventured into the twilight depths, known as “mesophotic” zones, between 30 to 150 meters (100 to 490 feet) below the surface, revealing that deeper sites were more heavily plagued by plastic contamination compared to shallower counterparts.

“It was surprising to find that debris increased with depth, since deeper reefs in general are farther from sources of plastic pollution,” said Luiz Rocha, the co-director of the Hope for Reefs initiative at the California Academy of Sciences.

“We are almost always the first humans to set eyes on these deeper reefs, and yet we see human-produced trash on every dive.”

A disturbing pattern emerged – reefs situated closer to densely populated urban centres and bustling markets, as well as marine protected areas, were particularly afflicted. These zones experience frequent encroachments from fishing activities, exacerbating the plastic influx.

Beyond the visible pollution, the study emphasised the insidious impact of plastic on coral health, including the spread of diseases and structural damage. Fishing lines and nets, typical components of plastic debris, inflict harm on reef structure, thereby undermining fish diversity and abundance.

“Our findings reveal some of the complex collective challenges we face when dealing with plastic pollution,” said Hudson Pinheiro, the lead author, from the University of São Paulo and a research fellow at the academy. “As marine resources around the world dwindle, humans that rely on those resources are turning to deeper habitats and those closer to marine protected areas where fish remain abundant.”

Among the locations surveyed, Comoros, an island chain off Africa’s southeastern coast, endured the most severe plastic pollution, with a staggering 84,500 plastic items per square kilometre. In stark contrast, the Marshall Islands boasted the least contamination, with around 580 pieces per square kilometre.

Multiple factors contribute to the higher plastic density at greater depths, including heightened wave action and turbulence at the surface, which could carry debris to lower depths. Potential interactions involving recreational divers cleaning shallower reefs or the growth of shallow corals over debris also come into play.

The study underscores the pressing need for transformative action. The findings underscore the urgency of expanding marine protected areas to encompass mesophotic reefs, reinforcing international agreements to combat plastic pollution at its source, particularly targeting fishing gear, and promoting the development of eco-friendly and biodegradable alternatives.

The revelation of plastic’s deep-seated invasion within our precious coral reefs serves as a poignant call to address this crisis before further irreversible damage is wrought upon these vital marine ecosystems.