Planktonic associations in the Northern Bering and Chukchi seas during the 2017–2019 warm period

Pacific Arctic ecosystems are changing due to ocean warming and sea ice loss. Increases in primary production and shifts towards smaller phytoplankton and zooplankton have been recently documented, yet understanding interactions among plankton components and their responses to changing oceanographic conditions are still needed. Herein, we assess plankton responses to unprecedented warm water temperatures and low sea ice conditions during springs and summers of 2017–2019 in the Northern Bering and Chukchi seas. Record low sea ice in winter 2017–2018 was followed by high biomass of large phytoplankton (>5 μm) but low abundances of large mesozooplankton (>500 μm) species in spring of 2018, potentially due to a temporal mismatch between zooplankton and phytoplankton. The widespread distribution of warm Coastal Water in the Chukchi Sea during summer of 2019 resulted in increased biomass of small-sized phytoplankton and a mesozooplankton community characterized by small copepod species and neritic copepods. Planktonic food webs changed seasonally, with phytoplankton and mesozooplankton directly linked in spring but mediated by microzooplankton in summer. Shifts towards smaller plankton with warming will increase the number of trophic levels and reduce trophic transfer efficiencies with potential impacts on fish and shellfish resources and benthic-pelagic coupling in these ecosystems.