Smoke stacks like these emit nitrogen oxides, which are converted to nitrate in the atmosphere. A recent series of studies found nitrate to be one of the primary anthropogenic sources of nitrogen in the ocean. Credit: Tony Webster, CC BY-SA 2.0

Since the Industrial Revolution, humans have been releasing nitrogen into the atmosphere and, from there, into the ocean, where it acts as a nutrient but also poses dangers to aquatic ecosystems in high quantities. Now, new research finds that far less human-generated nitrogen is reaching the open ocean than previously thought.

Although atmospheric models suggest that up to 80% of the ocean’s reactive nitrogen—that is, nitrogen that can be used by organisms—comes from anthropogenic sources, such as livestock production, agriculture, and coal-fired power plants, a recent series of observational studies indicates that only 27% can be traced to human activities.

The results may lead to future research that could alter how scientists view fundamental interactions between the ocean and the air. “We’re lacking important understandings of the nitrogen cycle because the traditional idea is that nitrogen coming in via rain and aerosol deposition is really anthropogenic in origin,” said Meredith Hastings, an associate professor of Earth, environmental, and planetary sciences at Brown University. “If it’s not, and the ocean is playing a significant role in recycling, then that’s something that’s been totally missing in our thinking.”

A Rigorous Analysis

Hastings participated in the new research, which was led by Katye Altieri, a senior researcher at the University of Cape Town in South Africa. At the time of the studies, Altieri worked as a National Oceanic and Atmospheric Administration postdoc jointly appointed between Brown University and Princeton University.

Scientists used this precipitation collector on the top of a research tower in Bermuda to collect rainwater samples. Along with aerosols, they analyzed these samples for nitrate, ammonium, and organic nitrogen. Credit: Katye Altieri

The researchers spent 18 months collecting aerosol and precipitation samples from a National Science Foundation atmospheric sampling tower in Bermuda that rises 65 meters above sea level. Because the island is at times “downstream” from winds that blow over the North American continent, it has proven useful for atmospheric sampling in the past.

During its warm season, Bermuda receives air masses only from farther out in the ocean, whereas its cool season brings an influx of air masses directly from North America in addition to marine ones. Using weather modeling, the team could be sure where each air mass came from.

To gather rainwater, the team used a precipitation collector—basically a glorified bucket equipped with a sensor that opened its lid to collect new samples whenever it began to drizzle. Low-lying aerosol particles that hang from the air regardless of rain were pumped through filters. The samples were later analyzed for nitrate, ammonium, and organic nitrogen in what chemical oceanographer Angela Knapp of Florida State University in Tallahassee, who was not involved in the study, called “one of the most rigorous analytical studies of atmospheric deposition that’s been done.” The researchers scaled up their results to represent the world as a whole so they could compare their findings with previous models.

Revisiting the Nitrogen Cycle

Human activities produce nitrogen mainly as ammonia, nitrogen oxides, and organic nitrogen-containing compounds.  In the atmosphere, chemical reactions convert ammonia to ammonium and nitrogen oxides to nitrate.  A lack of data about the fates of nitrogen compounds motivated the research, Hastings said, particularly with regard to ammonium and organic nitrogen deposition.

“We would have thought we would’ve seen significant agriculture or industrial activity influence, but the ammonium seems to all be coming from the ocean itself.”

She and her colleagues found that approximately 60% of nitrate in their samples hails from human sources. The ammonium came entirely from evaporated seawater, although the researchers were able to use only precipitation, not aerosol, data for that compound. Although the nitrate data aligned with results Hastings had seen before, the ammonium data were unprecedented. “We would have thought we would’ve seen significant agriculture or industrial activity influence, but the ammonium seems to all be coming from the ocean itself,” she says. “That was a particularly big surprise.” The results of their analyses for these nitrogen sources were published in the Journal of Geophysical Research: Atmospheres in 2013 and Global Biogeochemical Cycles in 2014, respectively.

In a third study using the same samples, which was published on 6 January in the Proceedings of the National Academy of Sciences of the United States of America, the team found that only about 17% of the organic nitrogen originated from anthropogenic activities.  Hastings said she would have expected a significant portion of it to be generated by humans, as with nitrate. “This is the most organic nitrogen data that really anyone has ever had for this type of data set over the ocean,” she said.

In this final study, the team also pooled the data for all three nitrogen sources extracted from the precipitation and aerosol samples. They found that only about 27% of the total nitrogen deposition was anthropogenic in origin. Although Knapp considers the results somewhat surprising, she suspects that the discrepancy between the team’s results and past findings could be due to a paucity of research.

“This is a data-limited field and there just aren’t a lot of atmospheric deposition measurements,” said Knapp. “I don’t think it’s overturning a paradigm. I think it’s more that this is a new piece in the puzzle.”

—Shannon Kelleher, Writer Intern

Citation: Kelleher, S. (2016), Human activities account for less than a third of ocean nitrogen, Eos, 97, doi:10.1029/2016EO043963. Published on 20 January 2016.

Text © 2016. The authors. CC BY-NC 3.0
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