A d15N-NO3 AND N* COMBINED SCALE TO IDENTIFY ANTHROPOGENIC NITROGEN SUPPLY INTO SURFACE WATER OFFSHORE JAPAN

Industrial and agricultural activities have greatly modified the global biogeochemical cycles of nitrogen. Anthropogenic nitrogen fixation has been estimated to double the natural rate and atmospheric N deposition have been increased more than ten times, compared to pre-industrial times. Such abrupt increase of N supply has also changed the N balance in marine environments. To study how the anthropogenic N has been involved into the nitrogen cycles in the ocean, nitrogen isotope analysis can be one of the most promising approaches, even if it is well known that d ¹⁵N of nitrate and PON are directly controlled by isotope effect on various biological processes in seawater, such as N uptake, N₂fixation and denitrification. Hence the N isotopes in surface water seems not be a simple tracer indicating anthropogenic sources. On the other hand, N* was developed to characterize a water mass which has different nutrient history, and was defined by deviation of N/P ratio in a water from a mean planktonic N/P ratio (Gruber and Salmiento, 1997). Because N* is sensitive to N₂ fixation or denitrification, both N* and d ¹⁵N can be a complementary scale to see the history of nutrient transform. We proposed a hypothetical nitrogen balance model based on N₂ fixation and denitrification in seawater and applied it to distinguish nitrogen dynamics in the North Pacific (Minagawa et al., 2001). Here we apply the model to interpret d ¹⁵N-NO ₃- and N* observation from some offshore sites near Japan. The result showed significant increase of N* and lowering of d ¹⁵N in the surface water. The analysis of d ¹⁵N of rain water at Sapporo, Japan gave between -4 to 3.7 %o and -22.9 to -8.4 %o for NO₃ and NH4 respectively, indicating atmospheric deposition in Japan sea can lower d ¹⁵N and rise N* in seawater. In conclusion, nitrogen deposition or riverine transportation would be already evident in the surface water at the offshore Japan both the Japan sea and the Pacific sides.