Interannual variations of net community production and air—sea CO2 flux from winter to spring in the western subarctic North Pacific

Abstract

The role of spring biological production for the air—sea CO2 flux was quantified in the Western Subarctic Gyre (48°N, 165°E), where the vertical profile of temperature revealed the existence of a temperature minimum (Tmin) layer in the North Pacific. The vertical profiles of temperature, salinity, dissolved oxygen, nutrients and dissolved inorganic carbon, DIC, in the upper water column were significantly variable year by year in spring, 1996–2000. Correspondingly, surface seawater at this site in spring was supersaturated with CO2 in 1997, 1999 and 2000, but was undersaturated in 1996 and 1998. The concentrations of DIC and nutrients in the winter mixed layer were estimated from those in the Tmin layer in spring with a correction for particle decomposition based on the apparent oxygen utilization. The net community production (NCP) and air—sea CO2 flux from winter to spring were calculated from the vertically integrated deficits of DIC and nutrients in the upper water column between the two seasons. The calculation of the carbon budget indicated large interannual variations of NCP (0–13 mmol m−2 d−1) and CO2 efflux (4–16 mmol m−2 d−1) for this period. The CO2 efflux was generally low in the year when NCP was high. The close coupling between biological production and CO2 efflux suggested the important role of the changes in the mixed-layer depth, as a key process controlling both processes, especially of the timing, so that a decrease in the mixed-layer depth could result in the activation of biological production. The early biological consumption of the surface DIC concentration could shorten the period for acting as a source for atmospheric CO2 and depress the CO2 efflux in the Western Subarctic Gyre from winter to spring in 1996 and 1998. On the contrary, in 1997, persistently deep vertical mixing until late spring could suppress the biological activity and give rise to long-lasting CO2 efflux.