Importance of N-2-fixation on the productivity at the North-Western Azores current/front system, and the abundance of diazotrophic unicellular cyanobacteria

2016

Article référencé dans le Web of Science WOS:000371992300077

Riou V., Fonseca-Batista D., Roukaerts A., Biegala Isabelle, Prakya S. R., Loureiro C. M., Santos M., Muniz-Piniella A. E., Schmiing M., Elskens M., Brion N., Martins M. A., Dehairs F.

Plos One, 2016, 11 (3), p. e0150827 [22 p.] ISSN 1932-6203

To understand the impact of the northwestern Azores Current Front (NW-AzC/AzF) system on HCO3-- and N-2-fixation activities and unicellular diazotrophic cyanobacteria (UCYN) distribution, we combined geochemical and biological approaches from the oligotrophic surface to upper mesopelagic waters. N-2-fixation was observed to sustain 45-85% of the HCO3--fixation in the picoplanktonic fraction performing 47% of the total C-fixation at the deep chlorophyll maximum north and south of the AzF. N2-fixation rates as high as 10.9 mu mol N m(-3) d(-1) and surface nitrate delta N-15 as low as 2.7 parts per thousand were found in the warm (18-24 degrees 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(-1) were found at 45-200m depths in the coolest nutrient-rich waters north of the AzF. In this area, N-2-fixation rates up to 4.5 mu mol N m(-3) d(-1) were detected, associated with depth-integrated (HCO3-)-C-13-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.

Limnologie physique / Océanographie physique [032] ; Ecologie, systèmes aquatiques [036]

ATLANTIQUE ; PORTUGAL ; ACORES

Fonds IRD [F B010066683]

fdi:010066683