Analysis of the deep chlorophyll maximum across the Azores Front

Physical, chemical and biological observations made in late July and August 1997 across the Azores Front (37 degrees N, 32 degreesW to 32 degrees N, 2 9 degreesW) are presented. The objectives of the study were: (1) to analyse horizontal and vertical profiles of temperature, salinity, density, nutrie nts and chlorophyll-a (Chl a) of the top 350 m; (2) to identify the main di fferences in the deep Chl a Maximum (DCM) and hydrographic structure betwee n the water masses that pass north and south of the Azores Front; and (3) t o estimate phytoplankton primary production in these water masses. Horizont al and vertical profiles of salinity, temperature, density, nutrients and p hytoplankton pigments in the top 350 m were analysed. The Front separates t wo distinct water types: the 18 degreesC Mode Water (18 MW) of sub-tropical origin, and the 15 degreesC Mode Water (15 MW) of sub-polar origin. Differ ences in the DCM and hydrographic structure between 18 MW and 15 MW were ob served in the contour plots of each section. The average Chl a concentratio n between 5 and 200 m depth decreased significantly from 15 MW to 18 MW. Th e same pattern was observed for the Chl a concentration at the DCM depth. A vertical one-dimensional model was used to estimate the phytoplankton prim ary production in the 15 MW and 18 MW and led to an estimated water column average gross primary productivity (GPP) between 1.08 and 2.71 mg C m(-)3 d (-)1 for the 15 MW and about half of these values for the 18 MW. These resu lts indicate that the typical south-north positive slope on DCM depth paral lels a latitudinal increase on GPP, suggesting that the location of the Azo res Front may have a significant regional impact on GPP.