Sea ice breakup and nutrient supply regulate the timing and magnitude of algal export over the slopes of the Pacific Arctic region

Few in situ measurements are conducted early in the productive season in the Arctic Ocean, making it difficult to accurately assess the impact of the recent decline in sea ice cover on algal fluxes. Since 2017, time‐series sediment traps have been deployed to collect sinking particles at two sites in the Pacific Arctic region. Here, we present algal fluxes obtained in the East Siberian Sea (ESS) and in the northern Chukchi Sea (NCS) between August 2017 and November 2022. Despite interruptions in sampling, algal fluxes reflected large spatial and temporal variations in the magnitude and timing of algal blooms and export in the region. In the ESS, peak diatom fluxes > 600 million cells m⁻² d⁻¹ in early August 2018 reflected the occurrence of an unusually large and long‐lasting diatom bloom sustained by an episodic inflow of nutrient‐rich waters. Diatom fluxes were usually lower in the NCS than in the ESS, reaching a peak flux of > 170 million cells m⁻² d⁻¹ in late August 2017. As diatoms consistently contributed to the majority of algal fluxes, their rapid decrease and near absence frequently observed during July or August at both sites reflected low nutrient concentrations in the upper water column leading to a dominance of small algal cells typically retained in upper waters. Overall, whereas nutrient supply influenced the magnitude and duration of the diatom fluxes, sea ice breakup determined the timing of peak diatom export in the Pacific Arctic slope region.