- A global modeling study found that 73% of marine protected areas are exposed to nitrogen pollution from wastewater — often at higher levels than in nearby unprotected waters.
- The findings indicate that marine conservation planning often fails to adequately account for land-based wastewater pollution or to integrate land-sea management.
- Wastewater pollution, largely untreated globally, introduces nitrogen and phosphorous nutrients and other contaminants that degrade marine ecosystems, with impacts on coral reefs, seagrass and water quality, and can undermine the effectiveness of MPAs.
- Addressing the issue is considered feasible but constrained by fragmented governance, underfunding and limited monitoring, despite solutions that include existing technologies, various policy proposals and examples of improved wastewater management.
Wastewater pollution levels are higher inside many marine protected areas than in nearby unprotected areas, according to a new study published in the journal Ocean & Coastal Management. The researchers also found that nearly three-quarters of all MPAs — more than 12,000 globally — are exposed to wastewater nitrogen pollution from sewage and agricultural runoff.
“[O]ur results expose that we are not systematically incorporating information about pollution into marine spatial planning or implementing integrated land-sea management,” study co-author Amelia Wenger, a senior research fellow at the University of Queensland in Australia and global water pollution program lead with the U.S.-headquartered NGO Wildlife Conservation Society (WCS), told Mongabay.
“Land and sea are deeply connected, but the way we manage them is not,” she added.
The researchers modeled nitrogen pollution within MPAs globally and looked at levels both inside and outside MPAs throughout a 50-kilometer-wide (30-mile) coastal zone in six tropical regions with high biodiversity.
Median pollution levels were up to 10 times higher inside MPAs across four of the study’s six focus regions, including the Caribbean and Bahamas, the Middle East and North Africa, the Coral Triangle in the Western Pacific, and the Indian Ocean.
“In regions such as East Africa and the Middle East and North Africa, around 60% of MPAs are exposed to [nitrogen] loads higher than the global median,” Wenger said.
The findings raise questions about the effectiveness of current MPA strategy, according to Jasmine Fournier, executive director of the Ocean Sewage Alliance (OSA), an international organization focused on reducing sewage pollution that partners with WCS. “We have been drawing lines on maps and saying, ‘This area is now protected,’ without considering land-based pollution,” said Fournier, who wasn’t affiliated with the study.
“MPAs are an excellent tool for conservation, but wastewater pollution is actively undermining their effectiveness,” she added, describing the phenomenon as “a blind spot in efforts to reach 30×30,” the United Nations target to protect 30% of Earth’s land and sea by 2030. These findings will help policymakers “see it, name it and change how we set MPA designations,” she said.
Water quality underpins marine protection efforts
Marine conservation is hard-baked into U.N. Sustainable Development Goal 14, “Life below water.” And developing sustainable approaches to how nations interact with ocean ecosystems is crucial to global food security, with around 3 billion people depending on oceans as a primary protein source, the U.N. says.
So far 10.01% of the world’s oceans have been designated protected, an increase of 5 million square kilometers (nearly 2 million square miles) since 2024, the U.N. Environment Programme reported in early April.
However, it’s unproductive to invest in restoring ecosystems that suffer wastewater pollution, because they “lack the environmental conditions needed to sustain recovery,” Guilherme Longo, a marine ecologist and assistant professor at the University of Massachusetts Boston, in the U.S., told Mongabay.
Longo co-coordinates the research network IntegraMAR, dedicated to integrated marine ecosystem management in Brazil. His own research unexpectedly found “a nitrogen ‘fingerprint’ consistent with sewage” hidden in the seaweeds of the Fernando de Noronha archipelago, a protected UNESCO World Heritage Site off Brazil’s coast.
Wastewater pollution impacts frequently go unrecognized and overlooked, Longo said, in favor of conservation focused on more obvious, easier-to-communicate initiatives such as designating new protected areas, and the planting of mangroves, coral or seagrass. “These are valuable and inspiring initiatives that play a key role in raising awareness and mobilizing support. However, their effectiveness depends on something less visible but equally critical — water quality,” Longo said.
The new study “is a clear call for MPAs and other actors to place water quality at the center of conservation and management strategies,” he added.
The study used geospatial modeling developed in 2021 to establish total nitrogen exposure levels. Using total nitrogen as a direct proxy for wastewater pollution has limitations recognized by the authors, including not distinguishing between natural and human nitrogen sources, how organisms process different forms of nitrogen, and dynamic coastal fluctuations like tides and rainfall.
The research is therefore “best viewed as a strong first step,” Longo said, noting that “more detailed, local studies are needed to confirm sources, understand ecological consequences and inform effective mitigation strategies.”
How does wastewater impact marine environments?
Wastewater can include home and commercial sewage, contaminants from industrial processes, plus stormwater and agricultural runoff. Globally, an estimated 80% is discharged back into waterways and estuaries without adequate treatment, according to the 2017 “World Water Development Report,” the U.N.’s most recent to focus on wastewater.
Often, sewage and agricultural runoff are loaded with high levels of nitrogen and phosphorous nutrients, while sewage can also contain pathogens found in urine and feces. Other wastewater contaminants include microplastics, toxic chemicals and pharmaceuticals.
Wastewater releases containing high nutrient levels can cause harmful algal blooms that deplete ocean oxygen, stressing marine life. Algal blooms can also deprive crucial seagrass beds of light, while some types of algae can produce toxins that imperil human health.
“The risks are visceral: cholera, E. coli, and the secondary rise of harmful algal blooms, which can turn a local food source [such as shellfish] into a neurotoxin-laced hazard,” Mehrnaz Ghojeh, the U.K.-based CEO of Okhtapus, an internet platform that aims to scale up and integrate climate -resilient solutions, told Mongabay in an email.
Who is responsible?
The world is designed in silos and most projects are funded in silos, Ghojeh said. “The Ministry of Water builds pipes; the Ministry of Environment manages reefs; the Ministry of Finance writes the checks. They rarely share a lunch, let alone a strategy.”
This departmentalization could, for example, result in an NGO getting a grant to protect coral reefs, “but they aren’t allowed to spend it on toilets for the village uphill, even though the toilets are what the reef actually needs to survive,” she said.
“Currently, responsibility usually stops at the end of the pipe. We need to shift that so that upstream polluters are legally and financially linked to the health of the downstream MPA,” Ghojeh added.
Fournier calls this fragmented wastewater governance “a critical gap.” What’s missing is the political will and accountability mechanisms, she said, not the solutions. “The most important thing to recognize is that wastewater pollution is solvable. The technology exists, the financing exists and the solutions are within reach.”
Managing wastewater sustainably
The study flags key barriers to sustainable management: chronic underfunding, insufficient MPA water quality monitoring, and neglect of wastewater in global assessments of marine environments and conservation agendas.
However, upgrading wastewater treatment infrastructure to drive environmental recovery, even in low-income coastal zones, is “absolutely feasible,” Fournier said.
In Roatán, Honduras, for example, a rapidly growing tourism sector drove unplanned development, resulting in the sewage of nearly 2 million visitors annually impacting the second-biggest barrier reef on Earth. In response, the local water board partnered with the Coral Reef Alliance (CRA) to improve wastewater treatment and disposal by connecting 99% of homes and businesses to a water treatment plant. Seven years later, fecal bacteria levels in the water were down by 95%, according to the CRA.
There are large-scale solutions too: Singapore’s state-owned NEWater reclaims wastewater, using microfiltration, reverse osmosis and ultraviolet disinfection, so that no wastewater contaminants enter the environment. This system recycles up to 800,000 cubic meters (211 million gallons) of clean water daily, serving around 40% of the city-state’s 6.11 million population, according to the NEWater website.
On the policy side, the Ocean Sewage Alliance is lobbying the U.N. for a global sewage treaty that would establish international wastewater standards, and unlock funding and support for building infrastructure in developing countries.
If there’s one takeaway from this study, it’s that “[A]n MPA is actually a land-management project disguised as an ocean-management project,” Ghojeh said. “We spend so much time talking about fishing quotas and patrol boats, but if the nitrogen levels stay high, the reef will die regardless … We have to stop treating ‘sanitation’ as a boring utility issue and start seeing it as the front line of ocean conservation.”
Banner image: A man on the Philippine island of Boracay walks beside a drainage pipe discharging untreated sewage into the ocean in 2018. Image by AP Photo/Aaron Favila.
A French city cut its marine pollution — and its seagrass bounced back
Citations:
Carrasco Rivera, D. E., & Wenger, A. S. (2026). Wastewater pollution undermines coastal marine protection: Implications for 30×30 and effective conservation. Ocean & Coastal Management, 276, 108150. doi:10.1016/j.ocecoaman.2026.108150
Mello, T. J., Longhini, C. M., Wanderley, B. M. S., Silva, C. A. da, Lehrback, B. D., Bom, F. C., … Longo, G. O. (2024). Pollution affects even oceanic marine protected areas in Southwestern Atlantic. Environmental Pollution, 366, 125485. doi:10.1016/j.envpol.2024.125485
Tuholske, C., Halpern, B. S., Blasco, G., Villasenor, J. C., Frazier, M., & Caylor, K. (2021). Mapping global inputs and impacts of human sewage in coastal ecosystems. PLOS ONE, 16(11), e0258898. doi:10.1371/journal.pone.0258898
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