On the significance of nitrification within the euphotic zone of the subpolar North Atlantic (Iceland basin) during summer 2007

The oxidation of ammonium to nitrite was investigated within surface waters (< 125 m) of the sub-polar North Atlantic (~ 60°N, 20°W) during late summer 2007. Sampling occurred within a mesoscale eddy dipole system and a definite bias towards waters beneath the euphotic zone as the focus for nitrification was evident. The patchy occurrence of significant nitrification rates within the euphotic zone is interpreted as providing minimal indication for widespread nitrification within surface waters of the sub-polar gyre. However, isolated occurrences of significant nitrification rates within the euphotic zone above the cyclonic eddy were sufficient to account for observed in-situ NO3−concentrations. It is proposed that the deeper mixed layer associated with the cyclonic eddy enhanced the likelihood of nitrification within the euphotic zone through the vertical displacement of sub-euphotic zone bacterial communities.

► Nitrification typically supplies < 10% of euphotic zone NO3− concentrations.
► Nitrification rates can be highly variable.
► The loci of nitrification activity is sensitive to the depth of the euphotic zone.