Tsurushima^{1, 4}, Nobuo, Keiri Imai^{2, 4} and Yukihiro Nojiri^{3, 4}

¹National Inst. of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba, 305-8569, Japan, Tel: +81-29-861-8396, Fax: +81-29-861-8357, E-mail: tsurushima-n@aist.go.jp, ²Ocean Research Inst., University of Tokyo, 1-15-1 Minamidai, Nakano-ku, 164-8639, Japan, ³National Inst. for Environmental Science, 16-2 Onogawa, Tsukuba, 305-0053, Japan and ⁴Japan Science and Technology corporation

Seasonal carbon cycle at station KNOT in the subarctic western North Pacific

In June 1998, Japanese ocean scientists established a time-series observation station in the western North Pacific. The station, located at 44°N, 155°E, is known as KNOT, short for Kyodo North Pacific Ocean Time-series (Kyodo is a Japanese word meaning "collaborative"). The scientific focus of the program at KNOT is the seasonal carbon cycle. The KNOT program is funded by the Japan Science and Technology Corporation (JST). Large seasonal variations in surface chemical and biological parameters occur at the station KNOT. Observed seasonal variations in surface seawater nutrients and DIC at KNOT were larger than those observed at OSP in the Eastern North Pacific. The seasonal amplitude of DIC at KNOT was more than 100 µmol/kg, largely because of biological uptake in summer and strong vertical mixing in winter. As a result of these large DIC changes, surface water fCO₂ was lower than the atmospheric value in summer and autumn, and oceanic uptake of CO₂ was largest in autumn when the wind velocity starts to increase. Primary productivity was highest in May, principally because of diatom production. Although the primary productivity had a distinct seasonal variation, with a 10-fold change during the winter and spring, the light utilization index was constant in all seasons.　Although the surface nutrient and DIC variability were higher at station KNOT than those at OSP, primary productivity and estimated new production were not higher. This may be a result of shallower summer mixed layer in the western subarctic North Pacific.