The fate of the missing 15N differs among marine systems

In nitrogen uptake studies with 15 N tracer techniques, a common observation is that the 15 N label added at the start of the experiment is not fully recovered at the end of the experiment in the particulate and dissolved N pools measured. We have made direct measurements of the 15 N content of the traditionally measured NH 4 + and particulate N pools, as well as the dissolved organic N (DON) and bacterial pools in studies conducted in Chesapeake Bay, the Caribbean Sea, and the Choptank River (a subestuary of Chesapeake Bay). We found that the fate of the “missing 15 N” differs from eutrophic to oligotrophic waters and is dependent on the length of incubation, the dissolved inorganic N substrate used, and the length of time the sample was contained before the 15 N uptake experiment. In general, it appears that it is more important to include the 15 N that enters the DON and bacterial pools to achieve a 15 N mass balance in more oligotrophic ecosystems and when longer (>1 h) incubations are used.