New Insights Into the Ocean as a Source of Atmospheric N in the North Atlantic Region

Abstract:

The impact of increased reactive nitrogen (N) deposition to terrestrial and coastal systems is well documented, but the implications of N deposition from the atmosphere to the open ocean remain uncertain. Current estimates suggest that anthropogenic emissions contribute up to 80% of N deposition to the oceans (e.g. nitrate, ammonium, and water-soluble organic nitrogen (WSON)), and this contribution represents new N to the ocean, stimulating production and affecting carbon cycling [e.g., Duce et al., 2008]. Further, these estimates suggest that N deposition may rival oceanic N-fixation (38-96 Tg N yr^-1 vs. 60-150 Tg N yr^-1, respectively). Clearly, it is critical to understand the impact this N has on the ocean and the marine atmosphere.

An important underlying assumption to the global input of N to the ocean is that the ocean is a passive receptor of N from the atmosphere, adding to the ocean’s nutrient budget. On the island of Bermuda, rainwater and aerosols were collected over 18-months and analyzed for concentrations, isotopes, and chemical composition of atmospheric N. For wet deposition at Bermuda, ammonium represents 45%, nitrate 41%, and WSON 14%. For dry deposition, these numbers change to 15% ammonium, 76% nitrate, and 9% WSON. Along with air mass history and a box model of ammonia/ammonium air-sea exchange, the results suggest that: the isotopes of nitrate depend on source region (i.e. continental vs. marine), are influenced by pollution and marine boundary layer chemistry, and atmospheric nitrate contributes low-δ¹⁵N to the N. Atlantic on an annual basis; rainwater ammonium isotopes and concentration are not as dependent upon source region, ammonium is primarily sourced from the ocean year-round; and the composition of WSON is different between aerosols and rainwater, with aerosol ON primarily derived from the ocean. Based on this interpretation (and assuming the results are broadly applicable), the input of anthropogenic N to the global oceans would be revised down from ~80% to 32%. In this case, a significant percentage of deposition is not a new input of N; rather, the ocean plays an important role in determining the N content of the marine atmosphere.