Estimation of seasonal net community production and air–sea CO2 flux based on the carbon budget above the temperature minimum layer in the western subarctic North Pacific

The distributions of the partial pressure of CO2, pCO2sea, total concentration of dissolved CO2 species, DIC, and nutrients in the upper 300 m in water column along 165°E in the western subarctic North Pacific were investigated in June and September, 1997. The high levels of pCO2sea observed north of 40.5°N in June decreased in September. The region north of 47°N changed from being a source for atmospheric CO2 in June to a sink in September. The surface mixed-layer depth decreased from June to September and the concentrations of DIC and nutrients in the mixed layer greatly decreased during that period. Vertical observations at 48°N, 165°E in the Western Subarctic Gyre showed the occurrence of a dichothermal structure, characterized by a temperature minimum (Tmin) layer, in both June and September. The concentrations of DIC and nutrients in the wintertime mixed layer were estimated from those in the Tmin layer in June with a correction for particle decomposition based on the apparent oxygen utilization. The vertically integrated seasonal deficits of DIC and nutrients from winter to June and June to September were calculated from the differences in the concentrations between the respective times. The calculation of the carbon budget indicated that about a half of the DIC deficit (3.9±0.4 mol C m−2) estimated for the period from winter to September would take place during each period. Seasonal DIC deficits were primarily due to a large CO2 efflux (1.4±0.4 mol C m−2) from sea to air during the earlier period and biological consumption (2.4±0.2 mol C m−2) during the later period. A small influx (−0.4±0.2 mol C m−2) of CO2 was estimated for the period from June to September, consistent with the seasonal changes in pCO2sea in this region. The net CO2 flux from winter to September was estimated to be 1.0±0.4 mol C m−2. The seasonally integrated net community production (NCP) was estimated to be 2.9±0.2 mol C m−2 from winter to September, and 83% of NCP occurred between June and September. A supply of DIC due to the continual deep vertical mixing and a low biological production during winter through June resulted in the high DIC concentration in surface waters and the high CO2 efflux during this period.