The role of net community production in air-sea carbon fluxes at the North Pacific subarctic-subtropical boundary region

The North Pacific subarctic-subtropical boundary region is an important sink for atmospheric CO2, with high annual influx relative to surrounding waters and large seasonal variability. The location of the subarctic-subtropical gyre boundary is relatively stable, but there is a large seasonal migration of the North Pacific Chlorophyll Front, between, similar to 30 degrees N and similar to 45 degrees N. To determine the role of biological productivity on the seasonal CO2 flux, we measured sections of oxygen-argon gas ratios on cruises across the transition zone in November 1997 (autumn) and September 2008 (summer). A simple upper ocean model was used with the O-2 : Ar data to estimate the net community production (NCP) in the euphotic zone. On both cruises the NCP was highest at the chlorophyll front with average values from 30 degrees N to 45 degrees N of 3.4 +/- 2.0 and 8.1 +/- 2.7 mmol C m(-2) d(-1) in autumn and summer, respectively. These values are sufficient to make biologically induced carbon export an important component of the CO2 drawdown in this region. Processes that control the relatively high NCP at this boundary are not certain, but it has been demonstrated that horizontal convergence of nitrate plays some role.