A changing nitrogen source for phytoplankton in the central Yellow Sea revealed by Compound-Specific Isotope Analysis of amino acids

Abstract

Recently, gradual increase of anthropogenic nitrogen input into the marine environment has been focused as one of the widespread impact on global ocean. Major components of anthropogenic nitrogen deposition are usually derived from nitrogen fertilizers and fossil fuels, which have been explosively increased by massive agriculture system and urbanization. Nitrogen isotope ratio (δ N) has provided source information of anthropogenic nitrogen from local to surrounding areas, and even ocean environment. Compound-specific isotope analysis approach has been developed as an improved tool to trace nitrogen source with independence from baseline variation in the nitrogen isotope ratio. In this study, we have measured compound-specific isotope analysis of nitrogen within amino acids (CSIAAAs) to understand historical record of nitrogen source variation during approximately two centuries from sediment core samples, where are the sites in the central Yellow Sea. Since 1990s the δ N values of phenylalanine (δ N ), as an indicator of background nitrogen, showed significantly lower, close to 0‰ than the previous times. Recent depletion of nitrogen isotope ratios may be related with apparently enhanced atmospheric nitrogen deposition as a new nitrogen source in the central Yellow Sea. In addition, nitrogen isotope ratios of phytoplankton were determined by using a combination of δ N and δ N as two types (trophic and source) of amino acids in the present study. We found temporal variability of nitrogen isotope ratios of phytoplankton recorded in the sediments of central Yellow Sea, suggesting recent changing nitrogen source through the increased availability of dissolved inorganic nitrogen during a couple of decades