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

Abstract

Seasonal changes in the nitrogen stable isotope ratio of NO 3 − ( δ N NO 3 15 ) were investigated in a temperate eutrophic coastal embayment in Japan. Wide variation of δ N NO 3 15 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, δ N NO 3 15 showed positive exponential and linear relationships with NO 3 − concentrations and fractions, respectively. The minima of δ N NO 3 15 were observed in spring and autumn when the NO 3 − fraction in DIN was small, while the maximum was in summer when the NO 3 − fraction was large. These results indicate that nitrification was the dominant process that determined δ N NO 3 15 variation. The δ N NO 3 15 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 ( ε nit) 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 NO 3 − 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.