Abstract
To understand the impact of the northwestern Azores Current Front (NW-AzC/AzF) system on HCO3−-and N2-fixation activities and unicellular diazotrophic cyanobacteria (UCYN) distribution, we combined geochemical and biological approaches from the oligotrophic surface to upper mesopelagic waters. N2-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 μmol N m-3 d-1 and surface nitrate δ15N as low as 2.7‰ 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-1 were found at 45–200m depths in the coolest nutrient-rich waters north of the AzF. In this area, N2-fixation rates up to 4.5 μmol N m-3 d-1 were detected, associated with depth-integrated H13CO3−-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.
Highlights
The increase in atmospheric CO2 concentration has stressed the need to quantify the transfer of CO2 by the marine biological carbon pump to the deep sea, where it can be trapped for centuries [1]
The Azores Front (AzF) was located between stations C and D, as derived from the 16°C isotherm at 200m depth [11] (Fig 3A)
South of the AzF, the surface T-S diagram patterns were typical of the 18°C Mode Water of subtropical origin (18MW, σΘ = 26.5, Fig 3B)
Summary
The increase in atmospheric CO2 concentration has stressed the need to quantify the transfer of CO2 by the marine biological carbon pump to the deep sea, where it can be trapped for centuries [1]. Once all the nitrate and nitrite have been used by phytoplankton in the euphotic zone, new primary production at the surface is only possible if N2-fixation occurs or if new nitrate sources appear. While upwelling provides new nitrate for phytoplankton growth, it delivers deep ocean CO2, leading to less atmospheric CO2 sequestration by the biological pump. In the N depleted North Atlantic (Sub)Tropical gyre (NAST), Fe-enhanced surface N2-fixation, which is limited by P availability, was estimated to add the equivalent of 50–180% of the deep ocean nitrate flux into the euphotic zone [4,5]. The geochemical, biological, and numerical modeling estimates vary widely [6] and there is an urgent need to evaluate the impact of N2-fixation on sea surface productivity with more accuracy to be able to predict the future efficiency of the biological carbon pump
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