Human activity, particularly in industrial and agricultural processes, has had significant impacts on the upper ocean nitrogen cycle, a new study has warned. The rate of deposition of reactive nitrogen (nitrogen oxides from fossil fuel burning and ammonia compounds from fertiliser use) from the atmosphere to the open ocean has more than doubled globally over the last 100 years, researchers said. 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 from the University of Hawaii teamed up with researchers from Korea, Switzerland and the US National Oceanic and Atmospheric Administration to assess changes in nitrate concentration between the 1960s and 2000s across the open North Pacific Ocean. The analysis, which could discern human-derived nitrogen from natural nitrogen fixation, showed 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.
The North Pacific is so vast it is hard to imagine that humans could impact the natural nitrogen cycle," said Karl. 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 showed 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. Enhanced nitrogen deposition has several 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. Given the likelihood that the magnitude of atmospheric nitrogen deposition will continue to increase in the future, the North Pacific Ocean could rapidly switch to having surplus nitrate. Thus, past and future increases in atmospheric nitrogen deposition have the potential to alter the base of the marine food web; and, in the long term, the structure of the ecosystem. The study was published in the journal Science.