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

Abstract

Abstract 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 CO 2 (fCO 2 ) 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 fCO 2 was controlled mainly by the large variation in surface DIC. The invasion of atmospheric CO 2 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 CO 2 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.