Under-ice phytoplankton blooms in the central Arctic Ocean: Insights from the first biogeochemical IAOOS platform drift in 2017

In April 2017, the first Ice Atmosphere Arctic Ocean Observing System (IAOOS) platform equipped with biogeochemical sensors was deployed near the North Pole. Over the next 8 months it meandered southwards with the ice, collecting measurements in the upper 250 m of Amundsen Basin, Gakkel Ridge, Nansen Basin and Fram Strait. Two distinct periods of primary production were encountered. The first bloom developed in the mixed layer in Amundsen Basin in May despite very limited under-ice light, reaching maximum chlorophyll-a concentrations on the order of ~ 0.4 mg m^-3 by the end of June. This bloom occurred earlier in the year than any other recorded bloom in Amundsen Basin, despite very limited under-ice light due to a thick layer of snow. The second bloom, encountered in Nansen Basin in August, was significantly larger (maximum chlorophyll-a concentration = 2.8 mg m^-3). Examinations of the optical community index and CDOM concentrations during both blooms suggest that the spring bloom consisted of small phytoplankton which may have been mixotrophic, while the summer bloom contained a greater diversity of planktonic size classes. The dataset demonstrates the heterogeneity of Arctic under-ice primary production, and is a valuable resource for validating coupled Earth System models.