Pan-Arctic analysis of the frequency of under-ice and marginal ice zone phytoplankton blooms, 2003-2021

Under-ice (UI) phytoplankton blooms have been observed in most of the marginal seas of the Arctic Ocean and are often found to contribute substantially to total primary production. However, because remote sensing studies cannot directly measure UI blooms and limited in situ observations prevent analysis of their frequency across the region as a whole, their distribution has not been characterized across the Arctic Ocean. Here, we use remote sensing data to discern which parts of the seasonally ice-free Arctic Ocean demonstrate evidence of UI blooms and whether UI bloom frequency changed between 2003 and 2021. Results suggest that UI blooms were generated frequently, with evidence of UI blooms over nearly 40% of the observable seasonally ice-free Arctic Ocean, while marginal ice zone blooms covered 60% in any given year. However, while there was no significant change in the UI bloom area over the study period, there was a 7% decline in the proportion of UI area in the seasonal sea ice zone. This decline was driven largely by declines at lower latitudes, where sea ice retreats earlier in the year, and in the Chukchi Sea, where UI blooms were also most prevalent. Our analysis demonstrates the need for increased observational studies of UI blooms and their ecological and biogeochemical consequences throughout the Arctic Ocean.