Researchers map impact of human wastewater on coasts around the world
Researchers have mapped the release of global wastewater – treated, septic and raw sewage – into coastal ecosystems in what they say is the smallest detail to date.
And the numbers are daunting.
- Researchers have found that only 25 watersheds contribute half of the total nitrogen in wastewater in the world’s oceans
- Meat-rich diets are a big factor in nitrogen levels
- High nitrogen levels can lead to algae blooms
Scientists at the University of California and Columbia University have calculated the volume of nitrogen indicator organisms and fecal matter entering the ocean from approximately 135,000 watersheds around the world.
A watershed is an area of land, often bounded by hills or mountains, that drains all of that land’s water to a common outlet such as a river.
They found that just 25 watersheds contribute almost half of all nitrogen in wastewater, with the Yangtze River in China contributing 11% of the global total.
The researchers calculated that in 2015, around 6.2 million tonnes of nitrogen entered coastal waters from human wastewater.
This was about 40 percent of the nitrogen that agricultural runoff would normally supply in coastal waters.
The watersheds that released the most nitrogen from the wastewater were in Korea, India and China.
However, in terms of the overall nitrogen contribution to coastal ecosystems, the United States was responsible for the third highest level behind China and India.
Just under a third of the nitrogen generated by wastewater reaching our oceans globally came from untreated wastewater, with the rest being treated and septic.
While wastewater treatment removes solids and some organic matter, nitrogen is still present in the treated wastewater.
The research is published today in the scientific journal PLOS ONE.
A diet high in meat equals a high nitrogen content
To obtain these figures, the researchers combined data on population density, sewage treatment and water supply, down to a grid resolution of one square kilometer, for watersheds around the world.
The most surprising variable that altered the volume of nitrogen releases was the amount of meat in local diets, said senior author and postdoctoral researcher Cascade Tuholske of Columbia University.
“As a person who studies food systems, the most surprising result for me has been the contrast between the Yangtze [China] and the Brahmaputra river [Tibet, India, Bangladesh] in terms of nitrogen inputs, ”said Dr Tuholske.
One possible solution to reducing nitrogen loadings in wastewater would be to reduce our consumption of meat, he said.
“The more burgers we eat, the more nitrogen we poop, the worse the outcomes for coastal habitats.
“This suggests to me that diets in China have switched to animal protein much faster than diets in India.”
Why nitrogen levels in the oceans matter
According to Megan Huggett, a marine and coastal ecosystem ecologist at Newcastle University, high nitrogen levels can lead to algal blooms, which in turn can deplete oxygen levels in the water as they grow. algae break down.
“This can lead to the breakdown of algal cells, which can deplete oxygen and lead to the death of fish,” said Dr Huggett, who was not involved in the study.
“[Wastewater] also brings things like herbicides and pesticides into the system, and plastics of course. “
The researchers also estimated the potential exposure of coral reefs and seagrass beds to high nitrogen content from sewage.
They found that more than half of the world’s coral reefs and almost 90 percent of seagrass beds experience some amount of human nitrogen.
According to Dr Huggett, some research has actually shown that seagrass beds can help mitigate the impacts of sewage and pathogen loads on nearby reefs.
It is an example of ecosystem service and explains why these ecosystems should be conserved, she said.
The Oceania region, which includes Australia, Papua New Guinea and our Pacific neighbors, had the lowest overall nitrogen inputs to coastal ecosystems, the research found.
Part of this is due to lower population densities, but Dr Huggett says Australia has fairly good wastewater treatment.
“I think Australia is doing a very good job,” she said.
“A new report on the state of the environment [which includes wastewater data] coming out later this year or early next year, so that will be one to watch. “
Today’s research has revealed that about half of all watersheds in the world have not experienced any human sewage incursion.
The researchers hope their findings can help inform conservation measures, but Dr Tuholske says there’s one thing anyone can do to fix the problem.
“A shift to plant-based diets can lead to healthier reefs and seagrass beds, not only by reducing fertilizer and runoff from feedlots, but also from our own droppings.”