The Formation of Subtropical Phytoplankton Blooms Is Dictated by Water Column Stability During Winter and Spring in the Oligotrophic Northwestern North Pacific

Subtropical phytoplankton blooms were observed in winter and late spring (rather than in early spring, as is typical) via shipboard observations in an area south of the Kuroshio Extension in the northwestern North Pacific subtropical gyre. Satellite-based observations revealed that these submesoscale blooms occurred in warmer water masses in winter and in cooler water masses in late spring. The fact that winter blooms occurred in warmer areas suggests that they depend on water column stratification caused by solar heating. In contrast, the fact that the late-spring blooms occurred in cooler areas suggests a breakdown of stratification due to a recurrence of convective mixing. Mesoscale blooms occurred at intermediate water temperatures in early spring, suggesting a repeating sequence of stratification and mixing during this period. Wintertime deep convective mixing in the northwestern North Pacific subtropical gyre creates Subtropical Mode Water. The behavior of the wintertime mixed-layer depth, which determines the thickness of Subtropical Mode Water, also dictates the characteristics of subtropical blooms such as their magnitude and the timing of onset and demise. Based on in situ observations and a model analysis, we conclude that deeper winter mixing, which increases nutrient concentrations, will intensify early-spring blooms and facilitate the formation of episodic blooms in winter and late spring. On the other hand, shallower winter mixing should increase stratification and thus facilitate the formation of smaller blooms, even in winter. Plain Language Summary In the subtropical northwestern North Pacific, nutrients brought into shallow water by winter mixing were observed to fuel local increases in phytoplankton from winter to late spring. Surprisingly, these increases in phytoplankton occurred in warm waters in winter and in cold waters in late spring. Large increases in phytoplankton require ample supplies of both light and nutrients. Our data suggest that phytoplankton increased in winter in waters that were warmed by solar heating after the cessation of vertical mixing and in late spring in waters that were cooled by the recurrence of vertical mixing. Wintertime mixed-layer depth fluctuates from year to year. Based on field observations and a model analysis, we suggest that the timing and magnitude of phytoplankton increases vary with the depth of the wintertime mixed layer, which in turn affects year-to-year variability of carbon export in this region.