Controlling factors of seasonal variation in the nitrogen isotope ratio of nitrate in a eutrophic coastal environment

Seasonal changes in the nitrogen stable isotope ratio of NO3−(δNNO315) were investigated in a temperate eutrophic coastal embayment in Japan. Wide variation of δNNO315 from −10.4 to 9.3‰ was found along with changes in temperature, dissolved oxygen concentrations, and the composition of the dissolved inorganic nitrogen (DIN) pool. Throughout the year, δNNO315 showed positive exponential and linear relationships with NO3− concentrations and fractions, respectively. The minima of δNNO315 were observed in spring and autumn when the NO3− fraction in DIN was small, while the maximum was in summer when the NO3− fraction was large. These results indicate that nitrification was the dominant process that determined δNNO315 variation. The δNNO315 minima in spring and autumn suggested that hydrographic properties, such as low water temperature and short residence time of the water, suppressed the nitrifying bacterial activity in these seasons. An apparent nitrogen isotope discrimination (εnit) by nitrification was estimated as −15 to −25‰, which is similar to or slightly higher than previous laboratory estimates of marine nitrifiers. Furthermore, isotope results at the bay center over a year suggest that most of the NO3− was regenerated by in situ nitrification in the water-column. This implies that large amounts of regenerated nitrogen would lead the eutrophication in Ise Bay.