An Ecosystem Model Including Nitrogen Isotopes: Perspectives on a Study of the Marine Nitrogen Cycle |
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| Authors: | Email author" target="_blank">Chisato?YoshikawaEmail author Yasuhiro?Yamanaka Takeshi?Nakatsuka |
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| Institution: | (1) Graduate School of Environmental Earth Science, Hokkaido University, Kita-ku, Sapporo 060-0810, Japan;(2) Present address: Frontier Research Center for Global Change, Japan Agency for Marine-Earth Science and Technology, Showamachi, Kanazawa-ku, Yokohama 236-0001, Japan;(3) Institute of Low Temperature Science, Hokkaido University, Kita-ku, Sapporo 060-0819, Japan |
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| Abstract: | We have developed an ecosystem model including two nitrogen isotopes (14N and 15N), and validated this model using an actual data set. A study of nitrogen isotopic ratios (δ15N) 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 δ15N 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 δ15N of nitrate measured in seawater and the seasonal variations in δ15N of sinking particles obtained from sediment trap experiments. Simulated δ15N of phytoplankton are determined by δ 15N 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 δ15N values of phytoplankton, which assimilates only nitrate, and only ammonium, respectively. We suggest that observations of
δ 15N 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 δ15N values and concentrations of ammonium reflect almost only nitrification. We suggest that the nitrification rate can be parameterized
with observations of δ15N of ammonium in winter and a sensitive study varying the parameter of nitrification rate. |
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| Keywords: | Nitrogen isotopes ecosystem model sediment traps seasonal variation Sea of Okhotsk |
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