Seasonal dynamics of sinking organic matter in the Pacific Arctic Ocean revealed by nitrogen isotope ratios of amino acids

The Pacific Arctic Ocean has experienced a rapidly changing climate, sea-ice retreat, and enhanced primary production over the past few decades. The export production generated by photoautotrophs and heterotrophs has been characterized in the Arctic Ocean, but their seasonal variations in relative proportion are largely unknown due to the limited access in the ice-covered season. We measured the concentration and nitrogen isotope ratio of individual amino acids from sinking particles in the northern east Siberian Sea (KAMS1), northern Chukchi Sea (KAMS2), and Northwind Ridge (KAMS4) from August 2017 to July 2019. The average trophic position, based on differences in the nitrogen isotope ratios of glutamic acid and phenylalanine, can indicate the relative proportions of biogenic organic matters derived from photoautotrophs and heterotrophs in sinking particles. Decreasing values (close to 1.0) in summer at KAMS2 in 2018 suggest that primary producers are responsible for most of the downward flux of sinking particles. However, the average trophic position at KAMS1 in 2017 increased to > 1.5 in autumn and was maintained at approximately 1.7 during ice-covered winter periods, likely due to greater contributions from heterotrophic organisms. Exceptionally high average trophic positions (close to 2.0) of sinking particles in summer at KAMS1 in 2017 and KAMS4 in 2018 were likely due to small export of photoautotrophs due to the surface seawater stratification and limited pelagic production. The average trophic position in sinking particles should reflect the spatiotemporal variation in export particle composition in the Pacific Arctic Ocean.