Influence of sea ice concentration on phytoplankton community structure in the Chukchi and East Siberian Seas, Pacific Arctic Ocean

To clarify the influence of sea ice concentration on phytoplankton community distribution in the rapidly warming Arctic Ocean, we conducted field surveys in August 2012 and August 2015 in the Chukchi and East Siberian Seas. The phytoplankton community structures (derived from pigment analysis) were compared according to environmental variables between the two periods. August 2012 had the lowest average sea ice concentration since sea ice observations began in 1979, along with higher sea surface temperatures. A deeper mixed layer depth and higher surface nitrate concentrations were observed in 2012, caused by exposure of the sea surface to wind. The euphotic depth and subsurface chlorophyll maximum layer were deeper in 2015 than in 2012, and whereas the low surface phytoplankton biomass in both years indicated post-bloom periods. Similar phytoplankton biomass and different phytoplankton community structure were found between ice and melt years. Small phytoflagellates (e.g., prymnesiophytes, prasinophytes, dinoflagellates, and cryptophytes) dominated the phytoplankton biomass with different community structures in the eastern and western areas in 2012, while diatoms dominated over the whole study area in 2015. Several environmental factors affect the variation in phytoplankton community structure in the Chukchi and East Siberian Seas; however, the phytoplankton, especially diatoms, appeared to be controlled mainly by seeding from sea ice in the surface layer. Furthermore, euphotic depth and water mass structure are likely the main factors influencing the spatial distribution of phytoplankton in the subsurface layer of this area, indicating that the phytoplankton community structure might be influenced by the water mass distribution under future ice-free conditions.