Nitrogen biogeochemistry of adjacent mesoscale eddies in the North
Pacific Subtropical Gyre
Abstract
We examined the nitrogen (N) biogeochemistry of adjacent cyclonic and
anticyclonic eddies near Hawai’i in the North Pacific Subtropical Gyre
(NPSG) and explored mechanisms that may sustain productivity in the
cyclone after the initial intensification stage. The top of the
nutricline was uplifted into the euphotic zone in the cyclone and
depressed in the anticyclone. Subsurface nutrient concentrations and
apparent oxygen utilization at the cyclone’s inner periphery were higher
than expected from isopycnal displacement, suggesting that shallow
remineralization of organic material generated excess nutrients in the
subsurface. The excess nutrients may provide a supply of subsurface
nutrients to sustain productivity in maturing eddies. The shallow
remineralization also raises questions regarding the extent to which
cyclonic eddies promote deep carbon sequestration in subtropical gyres
such as the NPSG. An upward increase in nitrate 15N/14N isotope ratios
below the euphotic zone, indicative of partial nitrate assimilation,
coincided with negative preformed nutrients – potentially signaling
heterotrophic bacterial consumption of carbon-rich (nitrogen-poor)
organic material. The 15N/14N of material collected in shallow sediment
traps was significantly higher in the cyclone than the anticyclone and
showed correspondence to the 15N/14N ratio of the nitrate supply, which
is acutely sensitive to sea level anomaly in the region. A number of
approaches were applied to estimate the contribution of N2 fixation to
export production; results among approaches were inconsistent, which we
attribute to non-steady state conditions during our observation period.