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 , 30uN and , 45uN. 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 O2: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 30u Nt o 45uN of 3.4 6 2.0 and 8.1 6 2.7 mmol C m22 d21 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. The oceanic response to anthropogenic carbon dioxide input to the atmosphere is an important scientific and social focus due to the potentially significant effects of increasing atmospheric partial pressure of CO2 (pCO2 )o n ecology and economies globally. The sum of biologically mediated processes affecting the fixation of CO2 and export of carbon from the euphotic zone to the deep ocean is a major control on atmospheric pCO2. This mechanism of carbon export from the surface ocean is known as the biological carbon pump (Volk and Hoffert 1985). Our focus in this paper is the North Pacific Ocean where the pCO2 drawdown at the subarctic–subtropical boundary causes an annual influx of carbon to the ocean (1.8–2.9 mol Cm 22 yr21) that is roughly equal to the carbon efflux to the atmosphere in the equatorial Pacific (Zeng et al. 2002; Takahashi et al. 2009). The carbon drawdown has a dramatic seasonal variation, and is strongest in autumn and weakest in summer (Zeng et al. 2002). In spite of the global importance of this CO2 sink, there is uncertainty about the importance of biological and physical mechanisms that affect carbon exchange in this region of the ocean. The present balance between these forcings must be better understood to evaluate how changing climate might affect carbon fluxes in this region. We report measurements from two cruises crossing the subarctic–subtropical boundary region with the express purpose of determining the importance of biological processes in creating an ocean sink for atmospheric CO2. The cruises were conducted , 10 yr apart aboard the R/V Thomas G. Thompson, in November of 1997 (TTN072) and September 2008 (TTN224). We refer to these two cruises throughout the paper as UWStu97 and UWStu08, respectively. This paper uses background hydrography, determinations of upper-ocean oxygen and argon concentrations, and a simple mass balance model to yield estimates of biologically produced oxygen from which we infer the seasonal pattern of net community production (NCP) across the subtropical–subarctic boundary. NCP is the difference between net photosynthesis and respiration in the biological community of a water parcel. At steady state (no increase in organic matter in the water parcel), NCP should equal the flux of organic carbon from the euphotic zone to the deep ocean, or carbon export (Emerson and Hedges 2008).