Atmospheric deposition of nutrients and excess N formation in the North Atlantic

• Published in: Biogeosciences
• Main Authors: Zamora, Lauren, Landolfi, Angela, Oschlies, Andreas, Hansell, D., Dietze, Heiner, Dentener, F.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications (EGU) 2010
• Subjects: North Atlantic
• Online Access: https://oceanrep.geomar.de/id/eprint/7341/; https://oceanrep.geomar.de/id/eprint/7341/1/1034_Zamora_2010_AtmosphericDepositionOfNutrientsAnd_Artzeit_pubid13301.pdf; http://www.biogeosciences.net/7/777/2010/; https://doi.org/10.5194/bg-7-777-2010

Anthropogenic emissions of nitrogen (N) to the atmosphere have been strongly increasing during the last century, leading to greater atmospheric N deposition to the oceans. The North Atlantic subtropical gyre (NASTG) is particularly impacted. Here, upwind sources of anthropogenic N from North American and European sources have raised atmospheric N deposition to rates comparable with N2 fixation in the gyre. However, the biogeochemical fate of the deposited N is unclear because there is no detectable accumulation in the surface waters. Most likely, deposited N accumulates in the main thermocline instead, where there is a globally unique pool of N in excess of the canonical Redfield ratio of 16 N:1 phosphorus (P). To investigate this depth zone as a sink for atmospheric N, we used a biogeochemical ocean transport model and year 2000 nutrient deposition data. We examined the maximum effects of three mechanisms that may transport excess N from the ocean surface to the main thermocline: physical transport, preferential P remineralization of sinking particles, and nutrient uptake and export by phytoplankton at higher than Redfield N:P ratios. Our results indicate that atmospheric deposition may contribute 13-19% of the annual excess N input to the main thermocline. Modeled nutrient distributions in the NASTG were comparable to observations only when non-Redfield dynamics were invoked. Preferential P remineralization could not produce realistic results on its own; if it is an important contributor to ocean biogeochemistry, it must co-occur with N2 fixation. The results suggest that: 1) the main thermocline is an important sink for anthropogenic N deposition, 2) non-Redfield surface dynamics determine the biogeochemical fate of atmospherically deposited nutrients, and 3) atmospheric N accumulation in the main thermocline has long term impacts on surface ocean biology.