N isotopic composition of dissolved organic nitrogen and nitrate at the Bermuda Atlantic Time‐series Study site

Abstract

To better constrain the dynamics of the dissolved organic nitrogen (DON) pool and the role of N2 fixation in the nitrogen cycle at the Bermuda Atlantic Time‐series Study (BATS) site, we measured the 15N/14N ratio of total nongaseous nitrogen (TN) in the upper 250 m and of nitrate in the upper 1000 m of monthly water column profiles from June 2000 through May 2001. The annually averaged TN δ15N in the upper 100 m is 3.9‰, which is greater than thermocline nitrate (2–3‰ at 250 m) and similar to literature values for shallow sinking nitrogen at BATS (3.7‰). We discern no seasonal variation in TN δ15N, which suggests that most of the DON pool is recalcitrant on this timescale. The TN data require a δ15N for the sinking flux that is similar to previous measurements, suggesting that N2 fixation is a minor component of new nitrogen at BATS. Small but measurable differences in the concentration and 15N/14N of total organic nitrogen (TON) between the surface and subsurface (∼250 m) suggest that subsurface remineralization of ∼0.25 μM of the surface TON acts to lower the 15N/14N of nitrate in the thermocline at BATS.