Journal
GEOPHYSICAL RESEARCH LETTERS
Volume 49, Issue 17, Pages -Publisher
AMER GEOPHYSICAL UNION
DOI: 10.1029/2022GL098670
Keywords
nitrogen isotope; marine nitrogen cycle; sinking particles; amino acids
Categories
Funding
- Japan Society for the Promotion of Science (JSPS) KAKENHI [25281013, 15K12199, 15H05471, 15H05822, 16H02236, 19KK0293, 19H04247]
- JST CREST, Japan [JPMJCR12A3]
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This study reveals the seasonal transition of the dominant nitrogen source for phytoplankton in the high-productivity subarctic ocean. The results show that the nitrogen source switches from nitrate to ammonium in autumn.
In the high-productivity subarctic ocean, phytoplankton growth is assumed to rely mainly on nitrate, based on snapshot observations conducted in spring or summer. However, the year-round utilization of nitrate and ammonium by phytoplankton remains unexplored. Here, we show the seasonal transition of the dominant nitrogen source for phytoplankton by using the nitrogen isotopic composition (delta N-15) of nitrate, ammonium, and sinking particles in the northwestern North Pacific. To decouple the combined effects of nitrogen sources for phytoplankton and subsequent trophic transfers in delta N-15 of sinking particles, we used compound-specific delta N-15 analysis of amino acids (delta N-15(AA)) in sinking particles. The seasonal change in delta N-15(AA) revealed that most of the seasonal variation in delta N-15 of sinking particles arose from changes in the availability of nitrogenous nutrients in the euphotic layer. The delta N-15 of sinking particles suggested that the dominant nitrogen source for phytoplankton switches from nitrate to ammonium in autumn.
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