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

Time‐series of particulate nitrogen isotope delta (PN‐δ15N) and flux of trapped particles at 200 m were determined in the northwestern North Pacific subtropical gyre (NPSG) between 2010 and 2014 in order to exa Saino mine temporal variations in mixed layer nitrate availability and downward particle export. Lower PN‐δ15N ( 50 %), implying that new nitrogen supply stimulated predominantly coccolithophore growth and downward transport of coccoliths. Even in the stratified summer of 2011, a δ15N 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 δ15N decrease appeared earlier in the 2011–12 blooms than those in 2013–14, coinciding with 8 % higher CaCO3 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 2011–12, caused by larger ocean heat loss related to winter monsoon intensity. Such interannual change of winter R (POC:PIC) can affect CO2 uptake rate in the northwestern NPSG where anthropogenic CO2 accumulates in subtropical mode waters.