Nitrogen Isotopes of Sinking Particles Reveal the Seasonal Transition of the Nitrogen Source for Phytoplankton

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...

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Bibliographic Details
Published in:Geophysical research letters 2022-09, Vol.49 (17), p.n/a
Main Authors: Yoshikawa, Chisato, Ogawa, Nanako O., Chikaraishi, Yoshito, Makabe, Akiko, Matsui, Yohei, Sasai, Yoshikazu, Wakita, Masahide, Honda, Makio C., Mino, Yoshihisa, Aita, Maki N., Fujiki, Tetsuichi, Nunoura, Takuro, Harada, Naomi, Ohkouchi, Naohiko
Format: Article
Language:English
Subjects:
Amino acids
Ammonium
Ammonium compounds
Analysis
Autumn
Availability
Deamination
Isotope composition
Isotope ratios
Isotopes
marine nitrogen cycle
Nitrates
Nitrogen
nitrogen isotope
Nitrogen isotopes
Nitrogen sources
Nutrient availability
Nutrients
Oceans
Organic matter
Particulate flux
Phytoplankton
Productivity
Seasonal variation
Seasonal variations
Sinking
sinking particles
Surface water
Switches
Zooplankton
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Summary: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 (δ15N) 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 δ15N of sinking particles, we used compound‐specific δ15N analysis of amino acids (δ15NAA) in sinking particles. The seasonal change in δ15NAA revealed that most of the seasonal variation in δ15N of sinking particles arose from changes in the availability of nitrogenous nutrients in the euphotic layer. The δ15N of sinking particles suggested that the dominant nitrogen source for phytoplankton switches from nitrate to ammonium in autumn. Plain Language Summary Exploring the year‐round utilization of nitrate and ammonium by phytoplankton is essential for understanding marine productivity. The nitrogen isotope ratio of sinking particles has long been used for reconstructing the availability of nitrogenous nutrients in the overlying surface water. However, the nitrogen isotope signal of sinking particles represents the combined effects of nitrogen sources for phytoplankton and subsequent trophic transfers, such as zooplankton grazing on phytoplankton and bacterial deamination of organic matter. This study shows the seasonal transition of the dominant nitrogen source for phytoplankton in the northwestern North Pacific by using compound‐specific nitrogen isotope analysis of amino acids in sinking particles that can deconvolute the effect of trophic alterations on bulk nitrogen isotope signals. The compound‐specific nitrogen isotope analysis of amino acids revealed that most of the seasonal variation in the bulk nitrogen isotope signal of sinking particles arises from changes in the availability of nitrogenous nutrients. The seasonal patterns of the bulk nitrogen isotope ratio demonstrated that the dominant nitrogen source for phytoplankton switches from nitrate to ammonium in autumn. Our findings highlight the importance of ammonium as a major nitrogen source for phytoplankton in the high‐productivity subarctic ocean. Key Points Nitrogen isotope analysis of amino acids in sinking particles
ISSN:0094-8276
1944-8007
DOI:10.1029/2022GL098670