Interannual Variability of Nitrogen Cycle

Interannual Variability of the Oceanic Nitrogen Cycle in the North Atlantic Ocean
Evaluating The Impact of Nitrogen Fixation in 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 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 Figure 1). DINxs variability and excess nitrate production rates in the STMW layer were well correlated with the North Atlantic Oscillation (NAO) (Bates and Hansell, 2004; Figure 2).
		Figure 2. The NAO, a component of the Arctic Oscillation (AO), is the sea level pressure (SLP) difference between Ponta Delgado, Açores, and Stykkisholmur, Icealand (Hurrell, 1985). Figure from Stephenson. The Arctic Oscillation is a pattern in which atmospheric pressure at polar and middle latitudes fluctuates between negative and positive phases. The negative phase brings higher-than-normal pressure over the polar region and lower-than-normal pressure at about 45 degrees north latitude. The negative phase allows cold air to plunge into the Midwestern United States and western Europe, and storms bring rain to the Mediterranean. The positive phase brings the opposite conditions, steering ocean storms farther north and bringing wetter weather to Alaska, Scotland and Scandinavia and drier conditions to areas such as California, Spain and the Middle East. In recent years research has shown, the Arctic Oscillation has been mostly in its positive phase. Some researchers argue that the North Atlantic Oscillation is in fact part of the AO.
For example, DINxs values (~1.5-2.8 µmoles kg^-1) and excess nitrate production rates (~3.7 Tg N yr^-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 µmoles kg^-1) and excess nitrate production rates were generally low (up to ~0.5 Tg N yr^-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.

Selected Publications:
Bates, N.R., and Hansell, D.A., 2004. Temporal variability of excess nitrate in the Subtropical Mode Water of the North Atlantic Ocean. Marine Chemistry, 84, 225-241. Hansell, D.A., Bates, N.R., and Olson, D.B., 2004. Excess nitrate and nitrogen fixation in the Subtropical North Atlantic. Marine Chemistry 84, 243-265. Lipshultz, F.J., Bates, N.R., Carlson, C.A., and Hansell, D.A., 2002. New production in the Sargasso Sea: history and current status. Global Biogeochemical Cycles 15(1-16) Hood, R.R., Bates, N.R., Capone, D.G., and Olson, D.B., 2001. Modeling seasonal and interannual biogeochemical and N2-fixation cycles at BATS. Deep-Sea Research II, 48(8-9), 1609-1648. abstract pdf
Past Funding: NSF *Elevated N in the Western Sargasso Sea: Local or Non-local Origin in the Western Sargasso Sea. P.I.'s and Collaborators:** D.A. Hansell (RSMAS); N.R. Bates (BIOS)