Phytoplankton Bloom Changes under Extreme Geophysical Conditions in the Northern Bering Sea and the Southern Chukchi Sea

Abstract

The northern Bering Sea and the southern Chukchi Sea are undergoing rapid regional biophysical changes in connection with the recent extreme climate change in the Arctic. In particular, since observations began in the 1970s, the ice concentration in 2018 was the lowest due to the unusually warm southerly wind in winter, which continued in 2019. We analyzed the characteristics of spring phytoplankton biomass distribution under such extreme environmental conditions in the two years. Our results show that higher phytoplankton biomass during late spring compared to the 18-year average was observed in the Bering Sea in both years. Their spatial distribution is closely related to the open water extent following winter-onset sea ice retreat in association with dramatic atmospheric conditions. However, despite a similar level of shortwave heat flux, the 2019 springtime biomass in the Chukchi Sea is lower than that in 2018, and instead, the bloom in 2019 was delayed to summer. We confirmed that this difference in bloom timing in the Chukchi Sea is due to the seawater property changes determined by the combination of the modulation of the northward oceanic heat flux caused by the disturbance of the enlarged sea ice in winter and the higher surface net shortwave heat flux than usual.