Time series of seasonal variation of primary productivity at station KNOT (44°N, 155°E) in the sub-arctic western North Pacific

Phytoplankton biomass and primary productivity were measured as a part of the station KNOT (44°N, 155°E) time-series observation program in the western North Pacific, which began in June 1998 and continued on a nearly monthly basis until February 2000. Primary productivity was measured by the 13C uptake method using a clean sampling technique. The surface temperature changed from 2°C to 15°C, and the minimum and maximum temperatures were in February and August, respectively. The minimum and maximum concentrations of surface-layer nutrients were 2.2 and 19.6 μM for nitrate+nitrite, 0.32 and 1.64 μM for phosphate, and 3.3 and 33.3 μM for silicate. The averaged depth of euphotic zone was about 50 m, which did not show clear seasonality. Integrated chlorophyll a in the euphotic zone ranged between 20 and 40 mg Chl a m−2, except for the high values (51–64 mg Chl a m−2) in early June and middle October 1998 and May 1999, and the low value (17 mg Chl a m−2) in late June 1998. The integrated primary productivities in the upper part of the euphotic zone for spring (May), summer (June–August), autumn (October–November), and winter (December–February) were 517–535 (av. 526), 153–304 (av. 227), 101–307 (av. 173), and 25–121 (av. 48) mg C m−2 d−1, respectively. The annual productivity was estimated to be 90 g C m−2 yr−1. The results showed that primary productivity had a distinct seasonal variation, with a 10-fold range during winter–spring, which is far larger than that at Ocean Station Papa (OSP) in the eastern North Pacific. However, the annual productivity at station KNOT was lower than the at OSP. It was found that the light utilization index was constant during all seasons, and the annual average (±1 SD) was 0.30±0.10 g C (g Chl a)−1 m2 (mol quanta)−1.