Researchers have drawn up a newly granular map of the inputs and impacts of human sewage on the world’s coastlines. The results are not pretty, but they are enlightening.
The motivation behind the research “was a desire to have a fine-grain understanding of how wastewater is impacting coastal waters worldwide,” said Cascade Tuholske, the lead author of a new paper that appears in the journal PLOS One. He said that research on coastal marine ecosystems often focuses on agricultural runoff from fertilizer and livestock waste, but few studies focus on human sewage.
This is not the first study to produce a global wastewater model, but it is the first to map inputs of nitrogen and pathogens from wastewater across 130,000 watersheds across the planet. “This is important because there are trade-offs in the intervention space,” said Tuholske. Information from this model, he added, could make those trade-offs clearer and management decisions easier to make.
Tuholske did the research along with colleagues as a graduate student at the University of California, Santa Barbara. He is now a postdoctoral researcher at Columbia University’s Center for International Earth Science Information Network.
The majority of human wastewater is discharged into the ocean around the world in a variety of treated and untreated states from sewage, septic and direct input sources. Not surprisingly, major wastewater sources host dense human populations, which tend to aggregate around major watersheds.
The researchers estimate that just 25 watersheds contribute 46 percent of global nitrogen inputs from wastewater into the ocean. Nearly half as much nitrogen comes from wastewater as from agricultural runoff globally, they say. Coastlines around the world are affected by increased nitrogen, according to the paper.
Tuholske and an interdisciplinary group of fellow scientists at UC Santa Barbara created a data visualization that maps the sources and destinations of nitrogen. It causes eutrophication, a phenomenon in which excessive nutrients create phytoplankton blooms just offshore that produce toxins and deprive the waters of oxygen. These so-called dead zones not only suffocate sea life, but also can cause problems in the food chain, including for humans.
“Many coastal ecosystems, such as coral reefs and sea-grass beds, are particularly sensitive to excess nutrients, even if you don’t have a dead zone,” said study coauthor Ben Halpern, a professor at UC Santa Barbara. “The whole ecosystem can tip into a highly degraded state when nutrient levels are too high. Coral reefs can be converted into fields of algae that overgrow and kill the corals below them. Our work here helps map where nutrients from wastewater are likely putting these ecosystems at greatest risk.”
For Tuholske, whose research focuses on food systems, the model puts into stark relief the impact of modern diets on coastal ecosystems.
“What was really surprising is how diets shifting to animal-based proteins are impacting marine ecology,” he said. As countries get wealthier and consume more meat, he said, the more nitrogen shows up in the wastewater, in addition to the already high levels generated by agriculture. “The more burgers people are eating, the more nitrogen is getting into the ocean,” he said.
Excessive nitrogen isn’t the only concern; where wastewater goes, so too go pathogens. But the removal of nitrogen or pathogens can require very different methods, which can make it difficult for decision makers with finite resources and varying priorities to weigh their options between improving public health and protecting coastal ecosystems. With the fine-scale estimates of nutrient and pathogen inputs provided by the new model, the aim is to provide information that can lead to local solutions to a complex global problem, say the researchers.
“These top-down, fine-resolution hot-spot maps can be matched with bottom-up approaches,” Tuholske said. “Adaptation and mitigation really come from the bottom up, and having a global map helps to target priorities and share knowledge.”
Adapted from a press release by the University of California Santa Barbara.