Seasonal variations of carbon dioxide system and nutrients in the surface mixed layer at station KNOT (44°N, 155°E) in the subarctic western North Pacific

Dissolved inorganic carbon (DIC) and related chemical species have been measured since June 1998 at the western subarctic time-series station KNOT (44°N, 155°E). Seasonal changes in hydrography, concentrations of nutrients and DIC, and total alkalinity (TAlk) were observed above the pycnocline, which exists at 150–300 m. DIC in the surface mixed layer showed a maximum in February 2000 (2093 μmol kg–1) and minima in August 1998 and 1999 (1980 and 1987 μmol kg–1, respectively). The seasonal amplitude of DIC was 107 μmol kg–1 in the 1999–2000 time series, which is larger than those at other pelagic ocean time-series sites. This large variation is due mainly to biological production in spring to fall and strong vertical mixing in winter. Surface fugacity of CO2 (fCO2) ranged from 300 to 390 μatm in the 1999–2000 time series, generally decreasing in spring and summer and increasing in fall and winter. The seasonal change in fCO2 was controlled mainly by the large variation in surface DIC. The invasion of atmospheric CO2 into the surface seawater continues from June to January, showing a maximum in October. The decrease in carbon in summer was explained by biological uptake on the basis of Redfield stoichiometry and air–sea CO2 exchange. Net community production in the surface mixed layer estimated from the change in nutrients ranged from 250 to 600 mg C m–2 day–1 and was higher in spring.