Seasonal and Interannual Variations in Nitrogen Availability and Particle Export in the Northwestern North Pacific Subtropical Gyre. Issue 5 (18th May 2020)

• Record Type: Journal Article
• Authors: Mino, Y.; Sukigara, C.; Honda, M. C.; Kawakami, H.; Wakita, M.; Sasaoka, K.; Yoshikawa, C.; Abe, O.; Kaiser, J.; Kimoto, K.; Kitamura, M.; Fujiki, T.; Matsumoto, K.; Saino, T.
• Is Part Of: Journal of geophysical research. Volume 125:Issue 5(2020)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2020-05-18
• Subjects: delta N‐15 of particles -- time‐series sediment trap -- the western north pacific subtropical gyre -- nitrate supply -- coccolithophore -- POC:PIC export ratio; Oceanography -- Periodicals; 551.4605
• DOI: 10.1029/2019JC015600
• Languages: English
• ISSNs: 2169-9275

Abstract

Abstract: Time series of particulate nitrogen isotope delta (PN‐ δ 15 N) 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 15 N (<4‰) seen every winter corresponded to convective nitrate supply triggering phytoplankton blooms. Greater particle fluxes were associated with higher CaCO3 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 15 N 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 15 N decrease appeared earlier in the 2011–2012 blooms than those in 2013–2014, 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 2011–2012, 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 North Pacific subtropical gyre where anthropogenic CO2 accumulates in subtropical mode waters. Plain Language Summary: Nutrient availability limits primary productivity in the low‐ and middle‐latitude oceans. This constrains the export of sinking biogenic particles, which removes carbon from the atmosphere, the so‐called biological pump. We examined temporal variations in nitrogen availability and particle export in the northwestern North Pacific subtropical gyre, using sediment trap time series observations at 200 m depth between 2010 and 2014. We found that the nitrogen isotope ratio ( 15 N/ 14 N) of trapped particles varied according to the external input of "new" nitrogen. 86–93% of "new nitrogen" was nitrate supplied from subsurface waters. Reduced 15 N/ 14 N ratios of high particle fluxes every winter indicated that nitrate was mainly supplied via convective mixing, triggering phytoplankton blooms. Interestingly, the magnitude of convective nitrate supply varied from year to year, depending on winter monsoon intensity. This not only controls particle fluxes but also their composition. Stronger mixing in 2011–2012 than in 2013–2014 enhanced CaCO3 export and hence reduced the organic carbon to inorganic carbon export ratio, R (POC:PIC). Such interannual changes of winter R (POC:PIC) can affect air‐sea CO2 fluxes through modulating CO2 partial pressure in the northwestern North Pacific subtropical gyre. Key Points: Changes in nitrogen availability in the western NPSG were revealed by delta nitrogen‐15 time series of settling particles at 200 m during 2010–2014 Reduced delta nitrogen‐15 every winter corresponded to convective nitrate supply that triggered algal blooms and enhanced downward CaCO3 transport Stronger convective mixing in 2011–2012 than 2013–2014 enhanced particle flux with a reduced organic carbon to inorganic carbon export ratio