FY 2025

Biophysical interactions stimulate the spring phytoplankton bloom in the North Pacific subtropical recirculation gyre

Sukigara, C., R. Inoue, K. Sato, Y. Mino, T. Nagai, A.J. Fassbender, Y. Takeshita, and E. Oka

J. Geophys. Res., 130(7), e2025JC022718, doi: 10.1029/2025JC022718, View open access article at AGU/Wiley (external link) (2025)

Although previous studies have found that restratification caused by mixed layer eddies induces early spring blooms near fronts under the nutrient-replete conditions of the subarctic ocean, the genesis of the spring bloom in nutrient-depleted subtropical regions has been unclear. To investigate the biogeochemical responses to changes of ocean dynamics during the spring transition season in the oligotrophic subtropical gyre of the western North Pacific from January to April of 2018, we used two Biogeochemical Argo floats equipped with oxygen, fluorescence (to estimate chlorophyll concentrations), backscatter (to estimate the concentration of particulate organic carbon [POC]), and nitrate and nitrite sensors to collect daily vertical profiles of the water column from a depth of 2,000 m to the sea surface. During February and March, as the mixed layer gradually deepened, there were slight increases of the concentrations of chlorophyll and POC after temporary restratification. At the end of March, the depth of the mixed layer reached a maximum, and the water column rapidly stratified. Chlorophyll concentrations increased significantly in the surface water, and low-salinity water with high concentrations of nutrients and low concentrations of oxygen and POC apparently entered the subsurface layer from greater depths. We hypothesize that this vertical structure was created by ageostrophic secondary circulation in frontal areas that enhanced the upward transport of nutrients into the euphotic zone and resulted in rapid phytoplankton growth in the surface layer as the light environment improved.

Plain Language Summary. In subtropical areas where nutrient concentrations are low in surface waters, the supply of nutrients from deeper water by deep winter mixing has been thought to cause spring phytoplankton blooms after warming causes the water column to restratify. However, the relationship between restratification and phytoplankton blooms is still under debate because the relevant phenomena have been difficult to observe. In this study, two autonomous profiling floats with temperature, salinity, and pressure sensors as well as dissolved oxygen, nitrate and nitrite, chlorophyll, and backscatter sensors were deployed in the subtropical region of the western North Pacific from January to April 2018. The results revealed restratification and high chlorophyll concentrations in the surface layer (0–20 m) within a few days after the depth of the mixed layer reached a maximum in late March. Water with high nutrient concentrations and low oxygen concentrations was transported into the subsurface layer (20–100 m) from greater depths. These phenomena were hypothesized to have been caused by a mechanism called secondary circulation and resulted in higher primary production than would be associated with the traditional formation of a deep mixed layer in the winter followed by restratification during the spring.