Fresh Phytoplankton Bloom Growth in the Fall and Its Control on Ocean Heating in the Pacific Arctic Region

Delayed sea ice freeze‐up and decreased sea ice extent in the Pacific Arctic region have altered light availability in marine waters, extending the ice‐free season and increasing opportunities for photosynthetic activity and energy exchanges. This study investigates phytoplankton bloom progression during the fall using chlorophyll pigments and examines the impact phytoplankton have on heat distribution within the water column. Measurements of chlorophyll‐ a concentrations and its associated degradation product pheophytin were made in conjunction with optical measurements of light transmittance within the upper water column during the Synoptic Arctic Survey cruise in the Central Arctic Ocean and northeast Chukchi Sea in September‐October 2022. Pheophytin proportions relative to chlorophyll‐ a generally increased over the course of the 2‐month cruise, indicating an overall seasonal decline in biological production. However, renewed phytoplankton growth indicated by reduced pheophytin proportions was observed within the surface ∼20 m of the water column on the Chukchi Shelf in late October. These subsurface blooms absorbed incoming solar radiation and enhanced ocean heating in the surface waters associated with the bloom while shading deeper depths from heating. Log‐linear regressions of heating rates associated each 1 μg L −1 increase of chlorophyll‐ a (at the observed chlorophyll‐ a maximum) with an approximate 15% decrease in heat input within the 10 m below the chlorophyll‐ a maximum depth. The altered vertical heat distribution caused by fall blooms trapping heat at the bloom depth can influence upper ocean stratification.