Physical and biological controls of nitrate concentrations in the upper subtropical North Pacific Ocean

Vertical profiles of nitrate down to 1000 m depth were obtained about every 5 days and over several years by four profiling floats deployed near Station ALOHA in the North Pacific subtropical gyre. As a first step, we study the episodic and rapid (10–30 days) changes in the depth of constant-nitrate surfaces observed in the float records. These changes are in general correlated with similar changes in the depth of isopycnal surfaces and have a small horizontal scale (horizontal wavelength less than 2°). They are furthermore observed over the whole water column sampled by the floats and throughout the year, with no apparent seasonal cycle. Using these characteristics as well as a 7-year high-resolution time series of potential density at Station ALOHA and a high-resolution numerical simulation of the circulation around the station, we conclude that these episodic changes correspond to the depth anomalies associated with the rapid changes and/or small-scale features of the eddy field. Large vertical velocities associated with submesoscale frontal processes are confined to the surface mixed layer (SML) and play no role in the episodic nitrate events except, perhaps, in late winter to early spring when the SML reaches the top of the nutricline. As a second step, we study the variations of nitrate concentration along isopycnal surfaces, which enables us to isolate the effects of biological processes. In the lower euphotic zone (125–200 m), the nitrate variations reflect the response of the ecosystem to the eddy variability: the shallower the isopycnal surface, the lower the nitrate concentration, and conversely, with nitrate varying in Redfield stoichiometric proportions with oxygen anomaly (defined here as dissolved oxygen minus oxygen saturation). In the upper euphotic zone (0–125 m), in contrast, eddy-induced variations in along-isopycnal oxygen anomaly do not conform with current knowledge about supply and variability of nitrate, ammonium and dissolved organic matter; some other nitrogen source that remains to be identified is required for mass balance.