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

To better constrain the dynamics of the dissolved organic nitrogen (DON) pool and the role of N 2 fixation in the nitrogen cycle at the Bermuda Atlantic Time‐series Study (BATS) site, we measured the 15 N/ 14 N 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 δ 15 N 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 δ 15 N, which suggests that most of the DON pool is recalcitrant on this timescale. The TN data require a δ 15 N for the sinking flux that is similar to previous measurements, suggesting that N 2 fixation is a minor component of new nitrogen at BATS. Small but measurable differences in the concentration and 15 N/ 14 N 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 15 N/ 14 N of nitrate in the thermocline at BATS.