Comparison of seasonal characteristics in biogeochemistry among the subarctic North Pacific stations described with a NEMURO-based marine ecosystem model

A NEMURO-based 16-compartment marine ecosystem model is applied to Stations A7 (41.5°N, 145.5°E) and KNOT (44°N, 155°E) in the subarctic western North Pacific and Station PAPA (50°N, 145°W) in the subarctic eastern North Pacific. Model results show significant west-east differences in seasonal characteristics of physical environmental conditions and biogeochemistry, such as larger seasonal amplitudes in sea surface temperature, mixed layer depth, surface nutrients, chlorophyll and partial pressure of CO2 at the sea surface (pCO2)sea, and higher primary productivity, at Stations A7 and KNOT than at Station PAPA. The modeled annual-mean e-ratios are higher at Stations A7 (0.32) and KNOT (0.33) than at Station PAPA (0.27) due to higher plankton biomass and mortality in the western North Pacific. Modeled annual-mean f-ratios are systematically higher than e-ratios under the influence of nitrification. The f-ratios are lower at Stations A7 (0.57) and KNOT (0.58) than at Station PAPA (0.64) because of higher ammonium concentrations in the western North Pacific. The e-ratio increases and f-ratio decreases with primary productivity, and the relationships can be described by exponential functions at any of the sites. The sea-to-air CO2 flux increases at Stations A7 and KNOT when calculated using daily wind data, instead of climatological wind data, which have been used in most of the previous studies. The increase is attributed to the strong winds in late winter in the daily wind data, suggesting that the sea-to-air CO2 flux was probably underestimated in previous studies and that frequent monitoring of winds and (pCO2)sea is necessary to reduce uncertainties in estimating air–sea CO2 flux. Phytoplankton growth is severely limited by light at any of the stations throughout the year. Diatom growth is regulated by silicate rather than nitrate and ammonium at each site, particularly in late summer and early autumn at Stations A7 and KNOT. We conclude that the west-east differences in the biogeochemistry are primarily caused by differences in the physical environmental conditions. The biogeochemical differences are also suggested to be caused by differences in the ecosystem dynamics resulting from differences in the iron bioavailability among the stations.