Seasonal and interannual variations in nitrogen availability and particle export in the northwestern North Pacific subtropical gyre

Y. Mino, C. Sukigara, M. C. Honda, H. Kawakami, M. Wakita, K. Sasaoka, C. Yoshikawa, O. Abe, J. Kaiser, K. Kimoto, M. Kitamura, T. Fujiki, K. Matsumoto, T. Saino

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Abstract

Time series of particulate nitrogen isotope delta (PN-δ 15N) and flux of trapped particles at 200 m were determined in the northwestern North Pacific subtropical gyre between 2010 and 2014 in order to examine temporal variations in mixed layer nitrate availability and downward particle export. Lower PN-δ 15N (<4‰) seen every winter corresponded to convective nitrate supply triggering phytoplankton blooms. Greater particle fluxes were associated with higher CaCO 3 concentrations (mass fraction >50%), implying that new nitrogen supply stimulated predominantly coccolithophore growth and downward transport of coccoliths. Even in the stratified summer of 2011, a δ 15N drop associated with a spike in particulate flux was found and was attributed to the uplift of nitrate-rich deep waters due to a mesoscale cyclonic eddy passing the mooring station. Total nitrate input sustains 86–93% of annual new production in this region while nitrogen fixation accounts for the rest. Trapped particles also showed that the winter δ 15N decrease appeared earlier in the 2011–2012 blooms than those in 2013–2014, coinciding with 8% higher CaCO 3 concentrations and a 40% lower particulate organic carbon to inorganic carbon export ratio, R(POC:PIC). This reflected stronger convective mixing and hence larger nutrient supply in 2011–2012, caused by larger ocean heat loss related to winter monsoon intensity. Such interannual change of winter R(POC:PIC) can affect CO 2 uptake rate in the northwestern North Pacific subtropical gyre where anthropogenic CO 2 accumulates in subtropical mode waters.

Original languageEnglish
Article numbere2019JC015600
Number of pages18
JournalJournal of Geophysical Research: Oceans
Volume125
Issue number5
Early online date16 Apr 2020
DOIs
Publication statusPublished - May 2020

Keywords

  • POC:PIC export ratio
  • coccolithophore
  • delta N-15 of particles
  • nitrate supply
  • the western north pacific subtropical gyre
  • time-series sediment trap

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