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

Abstract

Temporal variations in excess nitrate (DINxs) relative to dissolved inorganic phosphorus (DIP) 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 DINxs 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 DINxs concentrations have increased over the last 2 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), suggesting a major role for a physical mechanism in transporting the AND signals to the subsurface. Our finding provides evidence that the DINxs 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 western North Pacific. In contrast, a decreasing trend in subsurface DINxs was observed after the 2000s in the eastern NAtl, particularly in the high latitudes. This finding was not associated with the comparable decrease in AND from Europe. Other natural processes (a possible decline in tropical N2 fixation and weakening of the Atlantic meridional overturning circulation) may be responsible, but lack of time-resolved data on N2 fixation and meridional circulation is an impediment to assessment of these processes. Our results highlight the importance of both anthropogenic and natural forcing in impacting the nutrient dynamics in the upper NAtl.