A new global study has found that many Marine Protected Areas (MPAs), which are meant to safeguard coral reefs and coastal ecosystems, are facing higher levels of domestic wastewater pollution than nearby unprotected waters, raising serious concerns about their ability to protect marine biodiversity.
The report, published in Ocean and Coastal Management, found that four out of six major tropical coral regions recorded higher wastewater pollution levels inside MPAs than outside them. Researchers warned that unless wastewater control becomes part of marine conservation planning, international biodiversity goals may fail.
Wastewater Threatening Coral Survival
The study analysed 1,855 Marine Protected Areas located within 50 kilometres of coastlines and assessed total nitrogen loads, which were used as an indicator of untreated or poorly treated domestic sewage entering marine ecosystems.
The findings showed that 55 percent of the world’s coral reefs and 88 percent of seagrass ecosystems are exposed to wastewater pollution.
Researchers said excessive nutrient loading from sewage is severely weakening marine ecosystems. For coral reefs, wastewater pollution reduces reproductive success, slows growth, weakens survival during early life stages, and increases the spread of coral diseases.
The pollution also accelerates bioerosion, allowing coral structures to break down faster.
Climate Change Making the Crisis Worse
The study identified wastewater as a major “threat multiplier” for climate change.
Scientists said constant nutrient loading makes coral reefs much more vulnerable to marine heatwaves and coral bleaching. It also slows the recovery process after environmental disturbances.
Marine heatwaves can worsen eutrophication and trigger low-oxygen events in the water, further damaging fish populations and marine biodiversity.
The researchers noted that wastewater pollution is also harming other coastal ecosystems such as seagrass meadows and mangrove forests.
In seagrass habitats, wastewater reduces water clarity and blocks sunlight, lowering photosynthesis. It also introduces harmful pathogens and encourages the growth of competing algae.
For mangrove forests, pollution increases vulnerability to erosion and weakens their capacity to store carbon.
Protected Areas More Polluted Than Open Waters
The study found that in four of the six tropical marine regions, protected marine zones had higher median pollution levels than nearby non-protected areas.
East Africa and the Middle East and North Africa recorded some of the highest wastewater pollution levels.
In East Africa, 53 percent of MPAs were above the 50th percentile for pollution exposure, while the Middle East and North Africa recorded 57 percent.
These regions also reported the highest average and peak nitrogen loads, suggesting that several protected areas are exposed to extremely high pollution.
The Caribbean and Bahamas, along with the Coral Triangle, showed somewhat lower levels, with 57 percent of MPAs recording pollution below the 50th percentile.
Conservation Targets at Risk
The findings raise concerns about the effectiveness of the global 30×30 biodiversity target under the Kunming-Montreal Global Biodiversity Framework, which aims to protect 30 percent of the world’s oceans by 2030.
Researchers warned that simply increasing the size of protected marine areas will not guarantee biodiversity protection if pollution sources are not controlled.
The study said many MPAs are already located in areas with intense human activity, making them highly vulnerable to pollution pressure.
The authors stressed that protected status alone is not enough to deliver ecological benefits if wastewater continues to enter these ecosystems.
Call for Better Wastewater Management
Researchers called for urgent reforms to integrate wastewater management into marine conservation strategies.
They recommended aligning sewage infrastructure, sanitation planning, and wastewater treatment projects with coral reef conservation goals.
The report also called for new policy reforms, stronger financing systems, and the use of location-based monitoring tools to identify pollution hotspots.
The study concluded that for Marine Protected Areas to truly protect coral reefs and biodiversity, governments must tackle land-based pollution sources, especially untreated domestic wastewater.
Without this, the global effort to protect marine ecosystems may fall short of its promise, leaving coral reefs, seagrass, and mangrove ecosystems increasingly exposed to damage.