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 this week 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 Hawai'i, 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.
Their analysis, which could discern human-derived nitrogen from
natural nitrogen fixation, 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.
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 photosysnthesis 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.
In particular, the shift in nutrient availability could favor marine
organisms that thrive under the high nitrate and low phosphorus
conditions. If similar trends are confirmed in the Atlantic and Indian
Oceans, it would constitute another example of a global-scale alteration
of Earth system. Further, the findings of this study of the North
Pacific highlight the need for greater controls on the emission of
nitrogen compounds during combustion and agricultural processes.
