Marine protected areas are designed to protect coral reefs and other marine ecosystems by restricting human activities within their boundaries. However, most do not account for one of the most serious and widespread threats to marine life that originates from land: sewage.
A new study by the Wildlife Conservation Society (WCS) and the University of Queensland in Australia has found that more than 70 per cent of marine reserves around the world are contaminated by untreated or inadequately treated wastewater.
Pollution is even more widespread in places with vast coral reefs, such as the Coral Triangle, a 2 million square mile stretch of ocean that spans six Southeast Asian countries, including Indonesia, Malaysia, the Philippines and Papua New Guinea.
More than 90 percent of the Coral Triangle’s coastal protected areas are affected by high levels of sewage pollution, up to 10 times higher than nearby unprotected waters, according to a study published this month in the journal Ocean & Coastal Management.
“What we found was shocking,” said the study’s lead author, David E. Carrasco-Rivera, a PhD candidate at the University of Queensland. “Region by region, areas set aside for conservation actually received more pollution than areas that were not protected at all.”
Many marine protected areas are established near coastlines to help restore, rebuild, and thrive fragile and overburdened ecosystems that people rely on for food, tourism, and livelihoods. But their proximity to the coast makes them especially vulnerable to pollution that could undermine that purpose, said study co-author Amelia Wenger, director of global water pollution at the Wildlife Conservation Society, a global nonprofit organization dedicated to wildlife conservation based at New York’s Bronx Zoo.
“Even a perfectly managed marine reserve will no longer benefit conservation or people if wastewater continues to flow upstream,” she says.
For this study, Wenger and Carrasco Rivera analyzed exposure to pollution in more than 16,000 marine protected areas across Australia and Melanesia, Mesoamerica and the Caribbean, the Coral Triangle, East Africa, the Indian Ocean, the Middle East, and North Africa. About 12,000 people were contaminated with sewage, Wenger said.
Wastewater reaches the ocean in a variety of ways. In some areas, the lack of toilets inevitably makes rivers and beaches the default option, leaving waste to be washed away by rain and tides. In other areas, sanitation systems exist but waste is not contained. The septic tank leaks. The pit toilet overflows. Also, not all wastewater treatment centers adequately remove harmful nutrients such as nitrogen and phosphorus, Bengel said.
“Even a perfectly managed marine reserve will no longer benefit conservation and people if wastewater continues to flow in from upstream.”
— Amelia Wenger, Wildlife Conservation Society
Excess nutrients can interfere with corals’ ability to grow properly and withstand increased pressures from climate change. These effects make corals less resistant to rising sea temperatures and more vulnerable to bleaching, a stress response caused by rising sea temperatures in which corals expel the colorful algae that live in their tissues and turn white. Contaminated water also makes it harder for corals to recover from such bleaching events and makes them more susceptible to diseases caused by pathogens in the wastewater, such as bacteria, viruses, and fungi.
Mangroves are also affected. In contaminated areas, these salt-tolerant trees, which play an important role in protecting coastlines from storms and floods, are more likely to die during periods of drought. Nutrient-rich wastewater can cause toxic algal blooms, creating oxygen-depleted “dead zones” in the ocean and causing mass die-offs of fish, seagrass, and other marine life.
Chemicals in medicines and cleaning products can have negative effects on marine ecosystems when flushed down drains or dumped directly into nearby waterways.
Aerial view of Izmir Bay after the discharge of wastewater, which led to discoloration of the water, on March 11, 2025 in Izmir, Turkiye. Credit: Berkan Cetin/Anadolu, Getty Images
“We need to proactively address and address these threats as a priority, just as we respond to climate change,” said Rachel Sapele James, who leads the World Wildlife Fund’s Coral Reef Relief Initiative.
Ocean policymakers are now pushing to expand the number and size of marine protected areas to meet the global biodiversity goal known as 30 by 30, which aims to protect 30 percent of the world’s land and oceans by 2030. These areas have been shown to help ecosystems recover from pressures such as overfishing and destructive fishing practices. But simply establishing areas where fishing is restricted or prohibited, as many marine protected areas do, is not enough to meet conservation goals, Wenger said. To effectively manage marine protected areas, she said, land-based threats such as sewage pollution that can undermine their effectiveness must also be considered.
“This study really shines a spotlight on the fact that there is a huge threat to undoing all of our hard work and investments in 30 by 30 that we haven’t even considered,” she said.
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Until now, the issue of wastewater pollution has been largely ignored in ocean policy discussions, James said. “Sewage wastewater pollution remains under-recognized, under-funded and under-addressed.”
According to the World Health Organization, more than 40 percent of the world’s population lacks access to properly managed sanitation services. And more than 80 percent of the world’s industrial and municipal wastewater is discharged without proper treatment. But people in many places don’t want to discuss the issue, James said.
Sewage is considered a “dirty” topic of discussion in many places and even a “cultural taboo,” she said. “We don’t talk about sewage wastewater pollution enough, and we need to.”
Tackling the problem also requires unprecedented collaboration between governments, conservation, public health and humanitarian organizations, which too often work in silos, James said. “We need an integrated, multidisciplinary approach that addresses land-based threats alongside active conservation.”
“Sewage wastewater pollution remains under-recognized, under-funded and under-addressed.”
— Rachel Saperi James, Coral Reef Rescue Initiative
Addressing this issue will also require significant investment. “Solutions have to happen onshore, upstream, and must be part of the way governments plan and fund ocean protection,” Wenger said. “We need to put money into this so that organizations can support governments and communities to work towards fixing sanitation systems.”
This includes building and maintaining effective wastewater treatment systems, improving sanitation infrastructure and preventing untreated waste from entering rivers and coastal waters, the study said. It also calls on governments and conservation managers to consider ways to monitor and reduce the impact of sewage and other land-based pollution sources in marine protected areas.
“This is the best local management strategy we can implement to make coral reefs more resilient to climate change,” Wenger said.
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Teresa Tomasoni
oceans correspondent
Teresa Tomasoni is an environmental journalist who covers the intersection between oceans, climate change, coastal communities, and wildlife for Inside Climate News. Her previous work has appeared in The Washington Post, NPR, NBC Latino, and Smithsonian American Indian Magazine. Theresa holds a master’s degree in journalism from the Craig Newmark Graduate School of Journalism. She is also a recipient of the Stone & Holt Weeks Social Justice Reporting Fellowship. She has taught journalism at Long Island University and the New York Times School. She is an avid scuba diver and spends much of her free time underwater.
