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.