Primary productivity at the time-series stations in the northwestern Pacific Ocean: is the subtropical station unproductive?

Abstract

A comparative study of primary productivity in the northwestern Pacific Ocean was conducted at time-series stations K2 and S1 in the nutrient-rich subarctic gyre and oligotrophic subtropical gyre, respectively. The estimated annual means of net primary production (NPP) at the two stations were virtually identical: 292 mg C m−2 day−1 at K2 and 303 mg C m−2 day−1 at S1, whereas the annual mean of gross primary production (GPP) at S1 was 1.5 times that at K2. NPP was very much limited by the supply of nutrients, typified by nitrate at S1, although it was enhanced during winter due to mitigation of nutrient limitation. The NPP/GPP ratios were remarkably lower at S1 during the spring-to-autumn time interval than in winter. The reduced NPP/GPP ratio means that photosynthetically assimilated carbon was lost at a higher rate via respiration and extracellular release of dissolved organic carbon (DOC). The carbon loss (difference between GPP and NPP) was higher at S1 than at K2, probably because of the enhanced respiration due to the relatively high temperature throughout the year, as well as the enhanced DOC release by nutrient limitation. The released DOC should be accounted for as primary production, because it contributes to oceanic biogeochemistry in a manner similar to the photosynthesized compounds. Consequently, total primary production, the sum of NPP and DOC release, was higher at S1 than at K2.