Marine Protected Areas Show Higher Wastewater Pollution Than Unprotected Waters

A global assessment of 1,855 tropical Marine Protected Areas found that four of six key coral regions have higher domestic wastewater pollution inside protected zones than in adjacent unprotected waters, undermining the ecological outcomes these areas are designed to deliver. The study, published in Ocean and Coastal Management, warns that the 30x30 biodiversity target will fail to protect coral reefs, seagrass meadows and mangroves unless wastewater management is integrated into conservation planning.

The study is the first global assessment of total nitrogen loads from wastewater across tropical MPAs within 50 km of coastlines. Globally, 55 per cent of coral reefs and 88 per cent of seagrass ecosystems are exposed to wastewater pollution. In the East Africa and Middle East and North Africa regions, 53 per cent and 57 per cent of MPAs respectively recorded nitrogen pollution above the 50th percentile, with the highest mean, median and maximum loads globally. The Caribbean, Bahamas and Coral Triangle showed 57 per cent of MPAs below the median, indicating more variable outcomes.

Nutrient enrichment from wastewater reduces coral reproductive rates, growth and early survival, while increasing disease prevalence and bioerosion. Wastewater also acts as a threat multiplier for climate change: chronic nutrient loading makes corals more vulnerable to bleaching events and slows recovery after marine heatwaves. Mangroves become less effective at storing carbon and more susceptible to erosion when exposed to sewage-driven nutrient stress, and seagrass meadows lose photosynthetic capacity through reduced light penetration.

The authors recommend aligning wastewater infrastructure and sanitation investment with coral conservation plans, and call for land-based pollution to be integrated into MPA effectiveness metrics and the 30x30 framework's spatial planning tools. Policy reform, targeted financing and independent monitoring of nitrogen and pathogen levels are identified as priority actions. The study concludes that expanding protected area extent while leaving wastewater pollution unaddressed will not deliver the biodiversity outcomes the Kunming-Montreal Global Biodiversity Framework requires by 2030.

Key figure — 55% of global coral reefs exposed to wastewater pollution