Effect of a Subsurface Dipole on the Phytoplankton Biomass in the Southern Gulf of Mexico

This study examines the influence of a subsurface dipole eddy on phytoplankton biomass in the southern Gulf of Mexico using high-resolution observations of temperature, salinity, pressure, chlorophyll-a fluorescence, and nutrients collected during a June 2015 cruise. Geostrophic circulation revealed a cyclone–anticyclone pair at 200 m depth. Across the cyclone, vertical displacement of isotherms and nutrients from 1000 m to 180 m induced a deep chlorophyll-a peak below 80 m, while the anticyclone showed depressed isotherms and nutrient layers. Nitrite+nitrate concentrations at 200 m reached 5 µM within the cyclonic core, coinciding with elevated integrated chlorophyll-a (∼10 mg m−2). Strong boundary jets suggested frontal dynamics. These findings highlight the role of mesoscale eddies in nutrient injection and phytoplankton distribution, with eddy pumping as the primary mechanism for subsurface enrichment.