An ecosystem model including nitrogen isotopes: Perspectives on a study of the marine nitrogen cycle

We have developed an ecosystem model including two nitrogen isotopes (N-14 and N-15), and validated this model using an actual data set. A study of nitrogen isotopic ratios (delta N-15) using a marine ecosystem model is thought to be most helpful in quantitatively understanding the marine nitrogen cycle. Moreover, the model study may indicate a new potential of delta N-15 as a tracer. This model has six compartments: phytoplankton, zooplankton, particulate organic nitrogen, dissolved organic nitrogen, nitrate and ammonium in a two-box model, and has biological processes with/without isotopic fractionation. We have applied this model to the Sea of Okhotsk and successfully reproduced the delta N-15 of nitrate measured in seawater and the seasonal variations in delta N-15 of sinking particles obtained from sediment trap experiments. Simulated delta N-15 of phytoplankton are determined by delta N-15 of nitrate and ammonium, and the nitrogen f-ratio, defined as the ratio of nitrate assimilation by phytoplankton to total nitrogenous nutrient assimilation. Detailed considerations of biological processes in the spring and autumn blooms have demonstrated that there is a significant difference between simulated delta N-15 values of phytoplankton, which assimilates only nitrate, and only ammonium, respectively. We suggest that observations of delta N-15 values of phytoplankton, nitrate and ammonium in the spring and autumn blooms may indicate the ratios of nutrient selectivity by phytoplankton. In winter, most of the simulated biogeochemical fluxes decrease rapidly, but nitrification flux decreases much more slowly than the other biogeochemical fluxes. Therefore, simulated delta N-15 values and concentrations of ammonium reflect almost only nitrification. We suggest that the nitrification rate can be parameterized with observations of delta N-15 of ammonium in winter and a sensitive study varying the parameter of nitrification rate.