Contrasting decadal trends of subsurface excess nitrate in the western and eastern North Atlantic Ocean

• Main Authors: Yang, Jin-Yu Terence, Lee, Kitack, Zhang, Jia-Zhong, Moon, Ji-Young, Lee, Joon-Soo, Han, In-Seong, Lee, Eunil
• Format: Text
• Language: English
• Published: 2020
• Subjects: Pacific; North Atlantic
• Online Access: https://doi.org/10.5194/bg-2020-87; https://www.biogeosciences-discuss.net/bg-2020-87/

Temporal variations in excess nitrate (DIN xs ) relative to phosphate were evaluated using datasets derived from repeated measurements along meridional and zonal transects in the upper (200–600 m) North Atlantic (NAtl) between the 1980s and 2010s. The analysis revealed that the DIN xs trend in the western NAtl differed from that in the eastern NAtl. In the western NAtl, which has been subject to atmospheric nitrogen deposition (AND) from the USA, the subsurface DIN xs concentrations have increased over the last two decades. This increase was associated with the increase in AND measured along the US east coast, with a mean lag period of 15 years. This time lag was approximately equivalent to the time elapsed since the subsurface waters in the western NAtl were last in contact with the atmosphere (the ventilation age). Our finding provides an evidence that the DIN xs dynamics in the western NAtl in recent years has been affected by anthropogenic nitrogen inputs, although this influence is weak relative to that in the North Pacific. In contrast, a decreasing trend in subsurface DIN xs was observed after the 2000s in the eastern NAtl, particularly in the high latitudes. This finding may be associated with a possible decline of tropical N 2 fixation and the weakening of the Atlantic meridional overturning circulation, although more time-resolved data on nutrients and meridional circulation are needed to assess this hypothesis. Our results highlight the importance of both anthropogenic and climate forcing in impacting the nutrient dynamics in the upper NAtl.