Capturing the impact of human sewage on Earth’s coastal ecosystems
New worldwide mapping analysis identifies key exposure hotspots in unprecedented resolution
IMAGE: A) GLOBAL MAP OF THE TERRESTRIAL SOURCES (GREEN TO BLUE) AND COASTAL DIFFUSION OF INPUTS (YELLOW TO PURPLE) OF TOTAL WASTEWATER N, MEASURED IN LOG10(GN) IN BOTH. COASTAL PLUMES HAVE BEEN BUFFERED TO LINE SEGMENTS TO EXAGGERATE PATTERNS TO BE VISIBLE AT THE GLOBAL SCALE. INSETS SHOW ZOOMED-IN VIEWS OF THE B) GANGES, C) DANUBE, AND D) CHANG JIANG (YANGTZE) RIVERS, SHOWING WASTEWATER PLUMES AT HIGH RESOLUTION. view more
CREDIT: TUHOLSKE ET AL., 2021, PLOS ONE, CC-BY 4.0 (HTTPS://CREATIVECOMMONS.ORG/LICENSES/BY/4.0/)
A first-of-its-kind, high-resolution mapping analysis estimates the amounts of nitrogen and pathogens released into coastal ecosystems from human wastewater sources around the world. Cascade Tuholske (now affiliated with the Columbia Climate School) and colleagues at the University of California, Santa Barbara, present this research in the open-access journal PLOS ONE on November 10, 2021. The researchers have created a visual representation of this, available here.
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 for such purposes as fishing. However, most research into humans’ impact on coastal ecosystems has focused on agricultural runoff, while investigations on human sewage have been limited.
To better capture the impact of sewage on coastal ecosystems, Tuholske and colleagues conducted a novel analysis in which they estimated and mapped nitrogen and pathogen inputs into the ocean from sewage for about 135,000 watersheds around the world at a resolution of 1 kilometer. The assessment employed newly available, high-resolution data on global human populations and modeled how wastewater plumes entering the ocean would overlap with different ecosystems.
The analysis suggests that wastewater from human sewage introduces 6.2 teragrams of nitrogen into coastal ecosystems per year—for comparison, that is about 40 percent of estimated inputs from agriculture. Sixty-three percent of the nitrogen is from sewage systems, 5 percent from septic systems, and 32 percent from untreated, direct input.
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 percent 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—such as by highlighting locations where tradeoffs between managing nitrogen and pathogen levels are particularly important to consider.
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."
CAPTION
The global total wastewater input is 6.2Tg N, with 3.9Tg from sewers, 0.3Tg from septic, and 2Tg from direct input. The top 40 countries are shown in the horizontal bar chart; remaining countries are in the pinwheel, grouped by continent or larger geographical region. Values for all countries are also reported in S5 Table in S1 File. Note that the Netherlands is shown in both places (in red) to help connect the scale of the two parts of the figure.
CAPTION
Maps show where A) coral reefs and B) seagrass beds are heavily impacted (raster cells in top 2.5% of exposure; red dots), not impacted (no exposure to wastewater N; dark blue dots), or impacted but not in the top 2.5% (yellow dots). Raster cells are represented as points which visually over-represents the habitat; red is overlaid on top which makes it visually dominant; blue points are transparent and overlaid on green/yellow points such that higher densities of unimpacted areas are brighter blue.
METHOD OF RESEARCH
Computational simulation/modeling
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
Mapping global inputs and impacts from of human sewage in coastal ecosystems
ARTICLE PUBLICATION DATE
10-Nov-2021
COI STATEMENT
The authors have declared that no competing interests exist.
What’s in the water?
Researchers take a granular look at global inputs and impacts of human wastewater in coastal ecosystems
Peer-Reviewed PublicationIMAGE: EXCESS NITROGEN AND OTHER NUTRIENTS CONTRIBUTE TO HARMFUL ALGAL BLOOMS view more
CREDIT: NICHOLAS AUMEN/UCGS
The tendency for most of us when it comes to human wastewater is out of sight, out of mind. Rarely do we consider what happens after we flush that toilet or turn off that tap.
However, researchers at UC Santa Barbara have turned their attention and considerable computational power to the subject and its impacts on global coastal ecosystems. The results aren’t pretty, but they are enlightening.
“The motivation behind this 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 paper that appears in the journal PLOS One. While research on terrestrial threats to coastal marine ecosystems often focuses on agricultural runoff and what happens when fertilizer and livestock waste winds up in the ocean, he said, few studies investigate what happens when human sewage does the same.
“This isn’t the first study to produce a global wastewater model, but it is the first study to map the inputs of nitrogen and pathogens from wastewater across 130,000 watersheds across the planet,” Tuholske said. “And this is important because there are trade-offs in the intervention space.” Information from this model, he added, could make those trade-offs clearer and management decisions easier to make.
The Scale of the Problem
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 human wastewater sources are also places with dense human populations, which tend to aggregate around major watersheds.
“We estimate that 25 watersheds contribute approximately 46% of global nitrogen inputs from wastewater into the ocean,” said Tuholske, a postdoctoral researcher at Columbia University who conducted this study as a graduate student at UC Santa Barbara. “Nearly half as much nitrogen comes from wastewater as agricultural runoff globally,” he added, “which is a huge fraction.” Coastlines all around the world are affected by increased nitrogen, according to the paper.
Tuholske and an interdisciplinary group of fellow UCSB scientists — Ben Halpern, Gordon Blasco, Juan Carlos Villasenor, Melanie Frazier and Kelly Caylor — have created a data visualization that maps globally the sources and destinations of nitrogen, a common element in both agricultural and human wastewater that causes eutrophication. It’s a phenomenon in which excessive nutrients create phytoplankton blooms just offshore that produce toxins and deprive the waters in the area of oxygen. These so-called “dead zones” not only suffocate the sea life unfortunate enough to be trapped in them, but also can cause problems in the food chain, including for humans.
“Many coastal ecosystems, such as coral reefs and seagrass beds, are particularly sensitive to excess nutrients, even if you don’t have a dead zone,” said Halpern, a professor in the Bren School of Environmental Science & Management and the director of the National Center for Ecological Analysis & Synthesis at UCSB. “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 through this research is how diets shifting to animal-based proteins are impacting marine ecology,” he said. As countries get wealthier and incorporate more meat into their food systems, he explained, 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.
Two Targets
Excessive nitrogen isn’t the only concern with the growing amount of human wastewater being discharged into the ocean; 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 decisionmakers 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 this model, the aim is to provide information that can lead to local solutions that together can tackle a complex global problem.
“These top-down, fine resolution hotspot maps can be matched with bottom-up approaches, and we can transfer knowledge across geographies,” Tuholske said. “Adaptation and mitigation really come from the bottom up, and having a global map helps to target priorities and share knowledge.
“While we map the scale of this problem, we can do something about it,” he added. “We can protect both public health and coastal ecosystems.”
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JOURNAL
PLoS ONE
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