Increased intrusion of warming Atlantic water leads to rapid expansion of temperate phytoplankton in the Arctic.

The Arctic Ocean and its surrounding shelf seas are warming much faster than the global average, which potentially opens up new distribution areas for temperate-origin marine phytoplankton. Using over three decades of continuous satellite observations, we show that increased inflow and temperature of Atlantic waters in the Barents Sea resulted in a striking poleward shift in the distribution of blooms of Emiliania huxleyi, a marine calcifying phytoplankton species. This species' blooms are typically associated with temperate waters and have expanded north to 76°N, five degrees further north of its first bloom occurrence in 1989. E. huxleyi's blooms keep pace with the changing climate of the Barents Sea, namely ocean warming and shifts in the position of the Polar Front, resulting in an exceptionally rapid range shift compared to what is generally detected in the marine realm. We propose that as the Eurasian Basin of the Arctic Ocean further atlantifies and ocean temperatures continue to rise, E. huxleyi and other temperate-origin phytoplankton could well become resident bloom formers in the Arctic Ocean.

Photosynthetic light requirement near the theoretical minimum detected in Arctic microalgae.

Photosynthesis is one of the most important biological processes on Earth, providing the main source of bioavailable energy, carbon, and oxygen via the use of sunlight. Despite this importance, the minimum light level sustaining photosynthesis and net growth of primary producers in the global ocean is still unknown. Here, we present measurements from the MOSAiC field campaign in the central Arctic Ocean that reveal the resumption of photosynthetic growth and algal biomass buildup under the ice pack at a daily average irradiance of not more than 0.04 ± 0.02 µmol photons m-2 s-1 in late March. This is at least one order of magnitude lower than previous estimates (0.3-5 µmol photons m-2 s-1) and near the theoretical minimum light requirement of photosynthesis (0.01 µmol photons m-2 s-1). Our findings are based on measurements of the temporal development of the under-ice light field and concurrent measurements of both chlorophyll a concentrations and potential net primary production underneath the sea ice at 86 °N. Such low light requirements suggest that euphotic zones where photosynthesis can occur in the world's oceans may extend further in depth and time, with major implications for global productivity estimates.