Biophysical Interactions Stimulate the Spring Phytoplankton Bloom in the North Pacific Subtropical Recirculation Gyre

Abstract

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.