Importance of N.sub.2-fixation on the productivity at the north-western azores current/front system, and the abundance of diazotrophic unicellular cyanobacteria

To understand the impact of the northwestern Azores Current Front (NW-AzC/AzF) system on HCO.sub.3 .sup.- -and N.sub.2 -fixation activities and unicellular diazotrophic cyanobacteria (UCYN) distribution, we combined geochemical and biological approaches from the oligotrophic surface to upper mesopelagic waters. N.sub.2 -fixation was observed to sustain 45-85% of the HCO.sub.3 .sup.- -fixation in the picoplanktonic fraction performing 47% of the total C-fixation at the deep chlorophyll maximum north and south of the AzF. N.sub.2 -fixation rates as high as 10.9 [mu]mol N m.sup.-3 d.sup.-1 and surface nitrate [delta].sup.15 N as low as 2.7â° were found in the warm (18-24°C), most saline (36.5-37.0) and least productive waters south of the AzF, where UCYN were the least abundant. However, picoplanktonic UCYN abundances up to 55 cells mL.sup.-1 were found at 45-200m depths in the coolest nutrient-rich waters north of the AzF. In this area, N.sub.2 -fixation rates up to 4.5 [mu]mol N m.sup.-3 d.sup.-1 were detected, associated with depth-integrated H.sup.13 CO.sub.3 .sup.- -fixation rates at least 50% higher than observed south of the AzF. The numerous eddies generated at the NW-AzC/AzF seem to enhance exchanges of plankton between water masses, as well as vertical and horizontal diapycnal diffusion of nutrients, whose increase probably enhances the growth of diazotrophs and the productivity of C-fixers.