Phytoplankton chlorophyll a distribution and water column stability in the central Atlantic Ocean

The relationship between the vertical distribution of phytoplankton chlorophyll a and the physical structure of the upper central Atlantic Ocean was investigated based on results from a cruise across the tropical Atlantic (27°N to 36°S). The thermocline was very sharp and shallow in the northern part of the transect, off the coast of Mauritania and Senegal, and deepened progressively to reach > 150 m deep between 20°S and 30°S. There was a deep chl a maximum associated with the thermocline throughout the transect, reaching chl a concentrations up to 50 fold greater than those in surface waters. The chl a concentration was greatest off the NW African coast and declined towards the south, parallel to a progressive deepening of the chl a maximum closely tracking that in the thermocline. The maximal and the integrated chlorophyll a concentrations varied as the 1/2 and 1/5 power of the surface chl a. The correlation between the depth where the maximum chl a concentration was observed and the depth where the thermocline was strongest accounted for 87.2 % of the variance in the depth of the maximum chl a concentration along the transect. The maximum chl a concentration tends, on average, to be shallower than the depth where the maximal vertical stability is found when the thermocline deepens below 60 m depth, and the maximum chl a concentration tended to decline as the depth at which the Brunt-Vaisala buoyancy frequency was highest increased. Because the position of the thermocline in the tropical Atlantic Ocean may be predicted from climatology, the results reported may be useful to improve the predictions on phytoplankton biomass and production.