people on a ship

Decades of data were used to reconstruct historical nitrogen levels. (credit: P. Lethaby)

Human-induced changes to Earth’s carbon cycle—for example, rising atmospheric carbon dioxide and ocean acidification—have been observed for decades. However, a study published in Science showed human activities, in particular industrial and agricultural processes, have also had significant impacts on the upper ocean nitrogen cycle.

The rate of deposition of reactive nitrogen (i.e., nitrogen oxides from fossil fuel burning and ammonia compounds from fertilizer use) from the atmosphere to the open ocean has more than doubled globally over the last 100 years. This anthropogenic addition of nitrogen has reached a magnitude comparable to about half of global ocean nitrogen fixation (the natural process by which atmospheric nitrogen gas becomes a useful nutrient for organisms).

David Karl, professor of oceanography and director of the Daniel K. Inouye Center for Microbial Oceanography at the University of Hawaiʻi at Mānoa, teamed up with researchers from Korea, Switzerland and the U.S. National Oceanic and Atmospheric Administration to assess changes in nitrate concentration between the 1960s and 2000s across the open North Pacific Ocean.

Increased oceanic nitrate concentration

The analysis revealed that the oceanic nitrate concentration increased significantly over the last 30 years in surface waters of the North Pacific due largely to the enhanced deposition of nitrogen from the atmosphere.

“This is a sobering result, one that I would not have predicted,” said Karl. “The North Pacific is so vast it is hard to imagine that humans could impact the natural nitrogen cycle.”

The researchers used ocean data in conjunction with the state-of-the-art Earth system model to reconstruct the history of the oceanic nitrate concentration and make predictions about the future state of the North Pacific Ocean. Their assessment revealed a consistent picture of increasing nitrate concentrations, the magnitude and pattern of which can only be explained by the observed increase in atmospheric nitrogen deposition.

Potential ecological ramifications

Because biological activity is limited by nitrate availability in the North Pacific Ocean, the input of new nitrogen from the atmosphere may increase photosynthesis in the sunlit layers and export of carbon-rich organic material out of the surface ocean into the deep.

“The burgeoning human population needs energy and food—unfortunately, nitrogen pollution is an unintended consequence and not even the open ocean is immune from our daily industrial activities,” said Karl.

Read the UH Mānoa School of Ocean and Earth Science and Technology news release for more information.