Evolution of an Extensive Under‐Ice Phytoplankton Bloom in the Chukchi Sea

Abstract

In recent decades, evidence has emerged of phytoplankton growing beneath Arctic sea ice, in some cases forming massive blooms. Observed and predicted changes in the cryosphere highlight the need to better understand the development and progression of under‐ice blooms. During June and July 2023, as a part of the Tale of Three Systems (ToTS) field program, we characterized the environmental conditions that led to the initiation, peak, and decline of a substantial under‐ice phytoplankton bloom in the Chukchi Sea. By sampling two transects 7–8 times over a period of approximately 6 weeks, we are able to describe spatial and temporal patterns in hydrography, nutrient distributions, and phytoplankton biomass, allowing for a thorough analysis of a generally understudied phenomenon. Along both transects, phytoplankton biomass was five times higher under consolidated sea ice (maximum 661 mg Chl a m ⁻² ) than they were in open water (114 ± 135 mg Chl a m ⁻² ). Nitrate (NO ₃ ⁻ ) was the limiting nutrient and controlling factor of the magnitude of the bloom. The under‐ice environment also exhibited much greater surface water NO ₃ ⁻ depletion and dissolved oxygen (O ₂ ) supersaturation than in open water. The bloom was dominated by Thalassiosira spp., a large chain‐forming diatom, and much of the organic matter produced at the surface was exported below the mixed layer, suggesting tight benthic‐pelagic coupling. In the absence of widespread melt ponds, leads appeared to be the most important factor in alleviating light limitation of phytoplankton growth. An increasingly dynamic ice pack could therefore have important impacts on phytoplankton and the broader ecosystem.