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

Abstract

The balance between N-2 fixation and diffusive NO3- 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 NO3- supply. We find the average rate of N-2 fixation is equivalent to 62% of the diffusive NO3- 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 NO3- 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 NO3- supply in this region. The diffusive NO3- supply provides <20% of measurable NO3- 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 NO3- consumed on a daily basis by the phytoplankton community.