Chlorophyll a and nitrogen flux in the tropical North Atlantic Ocean

The phytoplankton chlorophyll a concentration and spatial distribution, and upward nitrogen transport toward the productive zone as controlled by the vertical turbulent diffusion and nitrate gradients, were evaluated along a 24.5° latitude section in the North Atlantic Ocean. The diffusive nitrate flux reasonably explained the expected new production in the Western Sargasso (WS), Central Sargasso (CS) and Eastern Atlantic (EA). The average upward nitrogen flux in those areas was 0.53 mmol N m−2 day−1 (±0.04 SE), representing from 0.23 to 0.34 of the total primary production. In the Canary Current (CC) zone, the estimates of upward nitrogen flux did not explain the expected new production, which appears to depend on laterally advected nutrients from coastal upwelling. In the CC, the nitracline depth was highly variable around ∼80 m depth, being more constant in the other zones around 136 m depth. The thermocline was located at depths between 28 and 44 m along the whole area except in some stations in the CC, where the upwelling system broke up the thermocline. The depth of the 26.0 isopycnal closely related to nitracline depth was always greater than that of the thermocline in the CS, EA and CC zones. In WS, the depth of the 26.0 isopycnal was highly variable responding to local subthermocline oscillations, while the nitracline remained relatively constant around 1% surface PAR depth, thus suggesting PAR as responsible instead of density for the top depth of the nitracline. Typical depth-integrated phytoplankton chlorophyll a inventories around 24 mg m−2 showed an average deep maximum of 0.27 mg m−3 at ∼119 m depth in all zones, except in the CC, where the chlorophyll a inventory was 32 mg m−2 with an average maximum concentration around 0.39 mg m−3 at ∼80 m depth. The variability of the upward diffused nitrogen was better explained by the nitrate gradient (n=86, r2=0.76; P<0.01) than by the diffusion term (n=86; r2=0.12; P<0.01). This suggests that the nitrogen fluxes are more dependent on the nitrate gradients at the nitracline than on the density gradients regulating turbulent diffusion.