Seasonal variability of primary production and phytoplankton biomass in the western Pacific subarctic gyre: Control by light availability within the mixed layer

A distinct seasonal variation of primary production was revealed from shipboard observations conducted from 2005 to 2013 at time series station K2 in the western Pacific subarctic gyre (WSG). The mean depth-integrated primary production was highest (569162 mg C m(-2) d(-1)) in summer and lowest (10116 mg C m(-2) d(-1)) in winter. Strong winter mixing enriched the mixed layer (ML) with nutrients that were not fully consumed during the remainder of the year, the result being that the WSG was a high-nutrient, low-chlorophyll (HNLC) region. The deep ML reduced primary production by reducing light availability in winter, whereas primary production was enhanced by strong light availability in the shallower ML as summer progressed. However, primary production was often attenuated by a reduction of light availability attributable to dense sea fog in summer. We found a significant relationship between primary production and light availability in this HNLC region. However, chlorophyll a was less variable seasonally than primary production. The highest depth-integrated chlorophyll a was observed in summer (54.613.4 mg m(-2)), but chlorophyll a remained high in winter (45.37.7 mg m(-2)). Reduced light availability depressed primary production, but a reduction of the chlorophyll a concentration was prevented by a relaxation of grazing in the deep ML during winter. We found that light availability exerted an important control on the seasonal variability of primary production and phytoplankton biomass in the WSG.