Nitrate fluxes induced by turbulent mixing in dipole eddies in an oligotrophic ocean

The vertical turbulent fluxes of nitrate to the base of the euphotic zone were quantified to assess the role of turbulent mixing in modulating new production inside cyclonic and anticyclonic eddies in an oligotrophic ocean. Although the turbulent nitrate flux (0.35 +/- 0.11 mmol N m(-2) d(-1)) at the base of the euphotic zone inside the cyclonic eddy was higher than that (0.07 +/- 0.03 mmol N m(-2) d(-1)) inside the anticyclonic eddy, this flux contributed to 19.7-76.3% of new production in the anticyclonic eddy and less than 4.94% in the cyclonic eddy, wherein new production was primarily driven by upwelling. Moreover, the turbulent nitrate flux in the cyclonic eddy center was higher than that at its edge because of differences in turbulent diffusivity. However, the opposite pattern appeared in the anticyclonic eddy, mainly due to discrepancies in the nitrate gradients and turbulent diffusivity. Elevated turbulent diffusivity at the edge of the anticyclonic eddy is the most likely attributed to submesoscale processes. These findings provide observatory evidence, confirming that a higher phytoplankton biomass exists in the anticyclonic eddy edge than in its center.

Automated summary

This study quantified the vertical turbulent fluxes of nitrate and assessed its role in modulating new production inside cyclonic and anticyclonic eddies. The results showed that although the turbulent nitrate flux was higher in cyclonic eddies, it contributed less to new production compared to anticyclonic eddies. Differences in turbulent diffusivity and nitrate gradients influenced the distribution pattern of fluxes in the eddies.