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-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.