Evaluating the balance between vertical diffusive nitrate supply and nitrogen fixation with reference to nitrate uptake in the eastern subtropical North Atlantic Ocean

The balance between N 2 fixation and diffusive NO 3 − supply is a key determinant for assessing the importance of both processes for new production in subtropical waters. Here we report observations of integrated N 2 fixation rates from the eastern subtropical North Atlantic Ocean with coincident estimates of diffusive NO 3 − supply. We find the average rate of N 2 fixation is equivalent to 62% of the diffusive NO 3 − supply, though N 2 fixation could exceed the diffusive flux at individual stations. Turbulent diffusivity measurements across the nitracline indicate a mean diffusivity of 0.077 cm 2 s −1 . If approximations for methodological underestimates in the dominant N 2 fixation technique are considered, the magnitude of N 2 fixation is shown to represent 100% of the NO 3 − flux on average, and can be almost threefold higher at individual stations. As the study site is characterized by low rates of N 2 fixation compared to other sectors of the North Atlantic this confirms N 2 fixation as a major source term across the subtropical North Atlantic. The seasonal context of our observations suggests environmental factors underlie the in situ variability in observed N 2 fixation rates, and may well explain lower previous assessments of the importance of N 2 fixation relative to diffusive NO 3 − supply in this region. The diffusive NO 3 − supply provides <20% of measurable NO 3 − uptake with the remainder supplied via other mechanisms, most notably nitrification. The mean integrated rate of N 2 fixation equates to just 8% of the NO 3 − consumed on a daily basis by the phytoplankton community.