Marine ammonium deposition: anthropogenic input or ocean emission?

The industrialization processes have seen a great increase in the anthropogenic emission of nitrogen (N) to the atmosphere and its subsequent deposition to terrestrial and coastal ecosystems. A clear increase in nitrate deposition to the open ocean has been detected due to NOx emissions, but little has been understood about marine ammonia/um (NH₃/NH₄⁺), especially with regard to its source of deposition.

A previous estimation has associated ~90% of the ammonium deposition to the ocean with anthropogenic sources. Opposed to that, findings of a recent study, published in Global Biogeochemical Cycles and cited by a Science Daily article, suggest that almost the entire deposition should come from natural marine source.

The study obtained its data from event-based rainwater sample collection in Bermuda over two years, with each sample classified as continental (North America) or marine originated according to its air mass location 36 hours prior to the collection of a rain event. Each sample was analyzed for its NH₄⁺ concentration and the ratio of ¹⁵N/¹⁴N in NH₄⁺, since the difference in isotopic composition may be used to distinguish the source of emission. The results showed that the NH₄⁺ concentration did not vary significantly among the samples, and that surprisingly, there lacked a correlation between NH₄⁺ isotopic ratio and air mass origin - no evidence pointed to a clear anthropogenic source. On the other hand, using a box model, the researchers found out that sources from the ocean itself could explain the concentration and isotopic composition of NH₄⁺ in marine rainwater, which also implied that the deposition of anthropogenic NH₃ would approach equilibrium near the coast before reaching the open ocean.

Geographic distribution of the air mass origin of samples
(isotopic composition represented by colored symbols)

Reading the Science Daily article gives a good grasp of the main findings of the study. It described, supplemental to the publication, why Bermuda was considered an ideal testing site. For different times of the year, the island receives air masses either blowing in northward from the Atlantic Ocean or coming off the continental US carrying anthropogenic pollutants, which makes it possible to separate the anthropogenic and natural marine sources. While the publication mainly aimed to communicate scientific discoveries, the news article was obviously more interested in the political implications with regard to limiting the emissions of N-containing pollutants. Furthermore, a co-author of the journal article and major member on the research team was quoted several times, making the news article more convincing in its tone and more appealing in the content. However, the article was not free from flaws when it tried to retell the story with plain words. An example would be the confusing interpretation of ‘isotopic ratio variation not correlated with air mass origin’ as ‘generally the same nitrogen isotopes regardless of air mass‘. Overall, I would give the news article a score of 7/10. 


Links:

Global Biogeochemical Cycles article
http://onlinelibrary.wiley.com/doi/10.1002/2014GB004809/abstract;jsessionid=F982F2481807844E08BB65F784E08D2A.f01t01

Science Daily article
http://www.sciencedaily.com/releases/2014/10/141029145642.htm