Identifying the external N and Hg inputs to the estuary ecosystem based on the triple nitrate isotopic information

Abstract

Estuarine regions are affected by a complex interplay of diverse pollution sources. We evaluated the supply and sources of nitrogen (N) and mercury (Hg) in the Geum Estuary, located on the eastern side of the Yellow Sea and the western coast of Korea. Thus, we employed triple isotope proxies (δ15NNO3, Δ17ONO3 and δ18ONO3) of nitrate (NO3−) combined with observed concentrations and conservative mixing between river and ocean waters to improve the isotope finger-printing methods. Based on the triple isotopic information, we determined that the N pool in the Geum Estuary was primarily influenced by the Yellow Sea water, followed by Geum River discharge (terrestrial origin), and then atmospheric deposition. The influence of the river was also found to be greater for Hg than that of the atmosphere. The triple isotope proxies revealed that both the riverine and the atmospheric inputs have been affected by human activities, specifically septic wastes and fossil fuel burning. From the inner estuary towards offshore region, the influence of the river diminishes, thus increasing the relative impact of the atmosphere. Moreover, the estuarine isotope proxies showed a significant influence of N assimilation in winter (February) and nitrification in spring (May).