For the first time, high-resolution mapping analysis is being used to estimate the amounts of nitrogen and pathogens released into coastal ecosystems from human wastewater sources around the world.

Human sewage can introduce disease-causing pathogens and nitrogen into the ocean, potentially impacting human health as well as coastal ecosystems and the communities that depend on them. First reported in the journal PLOS ONE, researchers found that wastewater from human sewage introduces 6.2 million metric tonnes of nitrogen into coastal ecosystems per year.

The team employed newly available, high-resolution data on global human populations and modelled how wastewater entering the ocean would overlap with different ecosystems. The analysis suggests of the watersheds that appear to release the most nitrogen from sewage, most are located in India, Korea, and China, with the Chang Jiang (Yangtze) River contributing 11% of the global total.

The researchers also identified hotspots for coral reef exposure to nitrogen in China, Kenya, Haiti, India, and Yemen. Seagrass exposure hotspots were found in Ghana, Kuwait, India, Nigeria, and China. The Chang Jiang and Brahmaputra Rivers have the highest input of pathogens.

Further research will be needed to refine the model and its estimates. Nonetheless, this work provides a new resource that could play a key role in efforts to mitigate harm to ecosystems and human health.

The authors add: "The sheer scale of how much wastewater is impacting coastal ecosystems worldwide is staggering. But because we map wastewater inputs to the ocean across more than 130,000 watersheds, our results identify target priority areas to help marine conservation groups and public health officials to work together and reduce the impacts of wastewater on coastal waters across the planet."