Human Activities Impact the Oceanic Nitrogen Cycle in Big Ways

While scientists have known for years that human activities influence the ocean and, in particular, the carbon cycle, they didn't realize that it also influenced another oceanic cycle. Researchers have discovered that industrial and agricultural processes have had significant impacts on the upper ocean nitrogen cycle.

The rate of deposition of reactive nitrogen from the atmosphere to the open ocean has more than doubled globally over the last 100 years. This human-caused addition of nitrogen has reached a magnitude that's comparable to about half of global ocean nitrogen fixation, which is the natural process by which atmospheric nitrogen gas becomes a useful nutrient for organisms.

In order to assess changes in nitrate concentration, the researchers used ocean data in conjunction with the Earth System Model. This allowed them to reconstruct the history of the oceanic nitrate concentration and make predictions about the future state of the North Pacific Ocean. This revealed a consistent picture of increasing nitrate concentrations, the magnitude and the pattern of which can only be explained by the observed increase in atmospheric nitrogen deposition.

So what does this mean for ecosystems? Biological activity is limited by nitrate availability in the North Pacific Ocean. This means that the input of new nitrogen from the atmosphere may increase photosynthesis in the sunlit layers and export 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 David Karl, one of the researchers, in a news release.

The findings reveal that the nitrogen cycle is also being impacted by human activities. This, in turn, could have implications for biological systems in the future.

The findings are published in the journal Science.