Researchers have mapped the discharge of global wastewater — treated, septic, and raw sewage — into coastal ecosystems in what they say is the finest detail yet.
And the numbers are intimidating.
• Researchers found just 25 watersheds contribute half of the total wastewater nitrogen to the world’s oceans
• Meat-heavy diets are a significant factor in nitrogen levels
• High nitrogen levels can lead to algal blooms
Scientists from the University of California and Columbia University calculated the volume of nitrogen and faecal indicator organisms entering the ocean from about 135,000 watersheds around the world.
A watershed is an area of land, often bounded by hills or mountains, that drains all the water from that land to a common outlet such as a river.
They found that just 25 watersheds contribute nearly half of all wastewater nitrogen, with the Yangtze River in China contributing 11 per cent of the world’s total.
The researchers calculated that, in 2015, around 6.2 million tonnes of nitrogen entered coastal waters from human wastewater.
This was about 40 per cent of the nitrogen that agricultural runoff would normally feed into coastal waters.
The watersheds that released the most nitrogen from wastewater were located in Korea, India and China.
However, in terms of the overall contribution of nitrogen to coastal ecosystems, the United States was responsible for the third-highest level behind China and India.
Just under a third of the wastewater-generated nitrogen reaching our oceans globally was from untreated sewage, with treated and septic making up the rest.
While treating sewage removes solid and some organic matter, nitrogen is still present in treated wastewater.
The research is published today in the scientific journal PLOS ONE.
High meat diets equal high nitrogen
To get these figures, the researchers combined data on population density, wastewater treatment, and diet, down to a resolution of 1-square-kilometre grids, for watersheds worldwide.
The most surprising variable that changed the volume of nitrogen discharge was the amount of meat in local diets, lead author and postdoctoral researcher Cascade Tuholske from Columbia University said.
“As someone who studies food systems, the most surprising result to me was the contrast between the Yangtze [China] and Brahmaputra River [Tibet, India, Bangladesh] in terms of nitrogen inputs,” Dr Tuholske said.
“The Yangtze added far more nitrogen to coastal waters than what would be expected.”
One possible solution to reduce nitrogen loads in wastewater would be to cut down on our meat consumption, he said.
“The more burgers we eat, the more nitrogen we poop, the worse outcomes for coastal habitats.
“This suggests to me that diets in China have shifted far faster to animal-based protein than diets in India.”
Why ocean nitrogen levels matter
Elevated nitrogen levels can lead to algal blooms, which can in turn deplete oxygen levels in water as that algae decomposes, according to Megan Huggett, a marine and coastal ecosystem ecologist from the University of Newcastle.
“It can lead to decomposition of algal cells, which can deplete oxygen and lead to fish kills,” said Dr Huggett, who wasn’t involved in the study.
“[Wastewater] also brings in 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 elevated nitrogen due to wastewater.
They found that more than half of the world’s coral reefs and nearly 90 per cent of seagrass beds experience some elevated nitrogen from human sources.
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.
That’s an example of an ecosystem service, and makes a case for why these ecosystems should be conserved, she said.
The Oceania region, which includes Australia, Papua New Guinea and our Pacific neighbours, had the lowest overall nitrogen inputs to coastal ecosystems, according to the research.
That’s partly to do with lower population densities, but Dr Huggett says Australia has quite good wastewater treatment.
“I think Australia does a pretty good job,” she said.
“A new State of the Environment report [which includes wastewater data] will be released later this year or early next year, so that’ll be one to watch.”
Today’s research found that around half of all watersheds globally had no human wastewater incursion.
The researchers hope their findings can help inform conservation measures, but Dr Tuholske says there’s one thing everyone can do to help the problem.
“A shift to plant-based diets can lead to healthier reefs and seagrasses, not just by reducing fertiliser and runoff from feedlots, but also from our own excrement.”
abc.net.au, 11 November 2021