A Global Map of Human Sewage in Coastal Ecosystems
Nutrient-rich agricultural runoff is a well-known scourge of coastal marine ecosystems. But another major source of nutrient contamination—human wastewater—has received relatively scant attention in the literature. Now, researchers have constructed a high-resolution geospatial model to map and quantify the pathogens and nitrogen from human sewage that enter roughly 135,000 watersheds draining into coastal ecosystems around the world.
When a marine environment becomes too enriched in nitrogen or other nutrients—a state known as eutrophication—a slew of problems can erupt. For instance, algae can consume the nutrients and then rapidly multiply (“bloom”), effectively choking out other life forms and creating coastal dead zones. And those problems can rapidly lead to others, said Cascade Tuholske, a geographer at Columbia University in New York City. “What we’re most concerned with are tipping points, whereby eutrophication can cascade into complete ecosystem collapse.”
Nutrient-rich fertilizers and livestock waste are major sources of nitrogen, but so is human sewage. And as diets worldwide shift to include more nitrogen-rich animal protein, our waste products are destined to excrete more and more of this nutrient, said Tuholske, who coled the new research while working at the University of California, Santa Barbara. “We excrete most of the nitrogen that we ingest.”
Human sewage can enter coastal ecosystems in many ways. In communities that have either sewer or septic systems, processed wastewater drains into watersheds that eventually empty into the ocean. In regions in which open defection occurs, untreated human sewage directly enters watersheds.
But it’s often difficult to get a handle on this type of pollution, said Stephanie Wear, a marine ecologist at The Nature Conservancy in North Carolina not involved in the research. “The thing about wastewater is that it’s generally invisible. A lot of the pollution occurs unseen.”
A Whole Lot of Nitrogen
Tuholske and his colleagues compiled a variety of data sets—including estimates of population, protein consumption, and sanitation practices—to assemble a high-resolution, global map of how much human wastewater–derived nitrogen entered coastal waters in 2015. They focused on roughly 135,000 watersheds that emptied into, or were adjacent to, the ocean. The team estimated that overall, human wastewater contributed more than 6 million metric tons of nitrogen to coastal waters. That’s roughly 45% of the nitrogen delivered by agriculture.
The team furthermore found that 58% of the world’s coral reefs and 88% of its seagrass beds—particularly vulnerable marine ecosystems known for their biodiversity and ability to sequester carbon, respectively—were exposed to nitrogen runoff from human wastewater.
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