Changes in the lower-trophic marine ecosystem and biogeochemical cycles were considered with respect to the warming Arctic Ocean. The time series monitoring of the hydrography and marine settling particles supplies a large amount of information regarding the marine lower-trophic ecosystem in with respect to the changing Arctic Ocean. To investigate the interannual relationship between the settling particles and hydrographic conditions of the western Arctic Ocean, bottom-tethered sediment trap moorings were deployed at the Northwind Abyssal Plain (NAP) and at the Chukchi Abyssal Plain from October 2010 to September 2014 to the east and west of the Chukchi Borderland. The settling particles at both stations contained a significant amount of lithogenic matter, with biogenic particles being a lateral advection of shelf materials from shelf to basin. The many peaks of settling particle fluxes at Station NAP in 2013–2014 corresponded to the hydrographic events of passing oceanic eddies over the station, in addition to seasonal biological production. Additionally, abundant gelatinous matter was found in some summer samples. The settling flux data of particulate organic matter and the mole ratio and stable isotope ratios of particulate organic carbon and nitrogen in the settling particles reflected the changes in the composition of trapped plankton assemblages under various hydrographic conditions over the course of the four years of the study.

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.