Thermohaline structure, ageostrophic vertical velocity fields and phytoplankton distribution and production in the northeast Atlantic subtropical front

Abstract

Two oceanographic cruises were carried out in the Azores Current/Subtropical Front region in July 1997 and April 1999 to study the seasonal and mesoscale spatial variability in the relationship between the hydrodynamic characteristics of the water column and the distribution of nutrients, phytoplankton biomass, and production. Both the magnitude of chlorophyll‐a values and primary production rates as well as their vertical distribution varied sharply associated with subtle changes in seasonal thermal stratification. Successive upwelling and downwelling regions were detected along the track of a cyclonic meander, with maximum ageostrophic vertical velocities of 3 m d−1 and −4 m d−1, respectively, at 375 dbar. Relatively high nitrate concentrations (>3.5 μM) were found associated with denser waters at the center of the meander, where the deep chlorophyll maximum (DCM) uplifted ∼20 dbar. Chlorophyll‐a concentrations ranged from 0.1 to 0.4 mg m−3, with slightly higher values (>0.35 mg m−3) at the northern side of the current. A significant relationship was not detected between the vertical ageostrophic velocity field and nitrate and chlorophyll‐a distributions, which were significantly correlated with the depth of the 16°C isotherm. Vertical advective fluxes of nitrate across the base of the DCM were about 2 orders of magnitude higher than vertical diffusive nitrate transport (−6.6 to 3.7 mmol m−2 d−1 versus 0.01 to 0.07 mmol m−2 d−1). Diffusive nitrate fluxes only account for <10% of the total primary production rates measured in the region.