Changes in Natural Isotopic Composition of Nitrate during the Development of Hypoxia in Ise Bay,Japan

摘要

Temporal changes in nitrogen isotopic composition(delta ~(15)N)of NO3~-pooled within the lower layers in Ise Bay,Japan were investigated to clarify the influence of microbial processes such as nitrification and denitrification in sediments on SI5N of NO3~-in the overlying water column.delta ~(15)N of NO3~-in the lower layers varied from-10per thousand in May to+9per thousand in July with development of seasonal hypoxia and associated changes in the concentrations of NO3~-andNH4~+.The significantly ~(15)N-depleted NO3~-in May most likely originated in nitrification.The apparent isotopic discrimination(s)for nitrification was estimated 23.4per thousand,which is within the range of the previous estimates by laboratory experiments.On the other hand,an NO3~-anomaly relative to PO4~(3-)(N*)showed the occurrence of NO3~-reduction in the bottom water in July.However,delta ~(15)N of NO3~-in the bottom water was similar to that within the upper hypoxia.This indicates denitrification occurred in sediments in the hypoxia with an apparent s of 1.5per thousand.delta ~(15)N of NO3~-in the hypoxia did not reflect the isotopic effect of denitrification but nitrification.