Temporal variability of excess nitrate in the subtropical mode water of the North Atlantic Ocean

A dichotomy exists between rates of nitrogen fixation directly measured by biological techniques, and rates inferred from the geochemical distributions of excess nitrate within the thermocline of the North Atlantic Ocean. Part of the dichotomy relates to the temporal and spatial uncoupling between the event (i.e., nitrogen fixation by diazotrophs) and signal (i.e., excess nitrate (DINxs) in the thermocline), as well as the interannual variability of both. Here, temporal variability of excess nitrate in the subtropical mode water (STMW) of the North Atlantic Ocean is evaluated for the 1988–2001 period. The excess nitrate signal has a maximum in this water mass, and it is by far the largest volumetric component of thermocline waters in the subtropical gyre. DINxs variability and excess nitrate production rates in the STMW layer were well correlated with the North Atlantic Oscillation (NAO). For example, DINxs values ( 1.5–2.8 μmol kg−1) and excess nitrate production rates ( 3.7 Tg N year−1) were generally high during positive phases of the NAO (e.g., 1989–1994; 1997–2000) and coincident with periods of higher atmospheric mineral dust input to the ocean. When the NAO was in its negative phase and dust inputs lower (e.g., 1995–1996; 2001), DINxs values ( 0–1.0 μmol kg−1) and excess nitrate production rates were generally low (up to 0.6 Tg N year−1). The NAO potentially influences DINxs variability by modulating the extent and magnitude of STMW formation, thereby changing the fate of accumulated DINxs during circulation of STMW in the subtropical gyre, and the variability of nitrogen fixers through changes in dust inputs to the subtropical gyre.