Wastewater pollution undermines coastal marine protection: Implications for 30x30 and effective conservation

Marine Protected Areas (MPAs) are a cornerstone of global ocean conservation strategies. However, they face multiple pressures from anthropogenic land-based pollution. Untreated and poorly treated domestic wastewater represents a widespread, under-recognized threat to tropical coastal ecosystems and adjacent coastal human populations. We present the first global assessment of total nitrogen (TN) loads from wastewater pollution within coastal MPAs associated with tropical coastal ecosystems: coral reefs, seagrass meadows, and mangrove forests. Using modelled spatial data, we quantified TN exposure across 1855 MPAs located within 50 km from the coast, and assessed both distribution and magnitude of exposure across six tropical regions associated with the three ecosystem types. The results revealed great variability in the level of exposure to wastewater pollution across the different regions. The East Africa and the Middle East & North Africa regions had the highest mean, median, maximum, and standard deviation of pollution loads. Overall, across all regions, mean TN loads were consistently higher than median values, highlighting the disproportionate levels of pollution some MPAs are exposed to compared to the rest. Additionally, pixel-level analysis revealed that in four regions, MPAs showed higher median pollution than their non-MPA counterparts, suggesting that protection status does not guarantee benefits in reduced pollution exposure. This research highlights that wastewater pollution reduction should be prioritized as part of global biodiversity objectives around effective area-based conservation, simultaneously benefiting coastal ecosystem health and resilience to climate change, and human health and wellbeing in adjacent local communities.