Distinctive spring phytoplankton bloom in the Bering Strait in 2018: A year of historically minimum sea ice extent

Abstract

Under climate change, the Bering–Chukchi continental shelves have experienced drastic sea ice loss, breaking the minimum sea ice extent record in 2018. In this paper, we report a distinctive spring phytoplankton bloom in the Bering Strait based on year-round mooring observations during 2016–2018. A near-bottom fluorescence sensor mounted on a mooring and satellite observations indicate that, despite the early sea ice retreat in spring of 2018 (mid-April), the spring phytoplankton bloom was not observed for more than 20 days after sea ice retreat. In contrast, in 2017 when sea ice retreat did not occur as early as it did in 2018 (early May), the bloom and sea ice retreat occurred in tighter synchrony. Mooring (2016–2018) and satellite observations (2003–2011, 2013–2018) suggest that an open-water bloom rather than an ice-edge bloom occurred in the Bering Strait in the spring of 2018. Early sea ice retreat apparently provided unfavorable conditions for an ice-edge bloom due to weak sunlight and winter-like weather. Mid-winter sea ice loss was promoted by unusual southerly winds, bringing in warm water from the south. For the Bering Strait, the end of April is a reference date for sea ice retreat that distinguishes between an open-water bloom and an ice-edge bloom, which is approximately one month later than that in the southeastern Bering Sea.