Chinese Journal of Oceanology and Limnology Vol. 32 No. 1, P. 162-173, 2014 http://dx.doi.org/10.1007/s00343-014-3102-x Key nitrogen biogeochemical processes revealed by the nitrogen isotopic composition of dissolved nitrate in the Changjiang River estuary, China* YU Haiyan (于海燕) 1, 2 , YU Zhiming (俞志明) 1, ** , SONG Xiuxian (宋秀贤) 1 , CAO Xihua ( 曹西华) 1 , YUAN Yongquan ( 袁涌铨) 1 , LU Guangyuan ( 卢光远) 1 , LIU Lili ( 刘丽丽) 1 1 Key Laboratory of Marine Ecology and Environmental Sciences , Institute of Oceanology , Chinese Academy of Sciences , Qingdao 266071 , China 2 University of Chinese Academy of Sciences , Beijing 100049 , China Received Mar. 18, 2013; accepted in principle May 17, 2013; accepted for publication Jun. 17, 2013 © Chinese Society for Oceanology and Limnology, Science Press, and Springer-Verlag Berlin Heidelberg 2014 Abstract Seasonal variations in the nitrogen isotopic composition of dissolved nitrate (δ 15 NO 3 ) were investigated in the surface waters of the Changjiang River estuary in 2009 and 2010. δ 15 NO 3 varied between -4.6‰ and 8.9‰ with changes in temperature, salinity, dissolved oxygen concentrations, and the composition of the dissolved inorganic nitrogen pool. In February, biological processes decreased because of low temperature, and the mean δ 15 NO 3 near the river mouth was 2.4‰. In May, δ 15 NO 3 was the highest in the surface waters among all seasons. Analysis on the conservative mixing revealed assimilation, and this finding is supported by positive relationship between Chl a and δ 15 NO 3 . The fractionation factor of assimilation was estimated to be 2.0‰ by the Rayleigh equation. Nitrification was supported based on the mixing behaviors in November 2010 and the low δ 15 NO 3 values in May and November 2009. The high ammonium concentrations in the adjacent marine area and positive relationships between total organic nitrogen and δ 15 NO 3 in November 2010 indicated that mineralization was taking place. Keyword: assimilation; nitrification; mineralization; eutrophication 1 INTRODUCTION Coastal marine ecosystems are seriously influenced by the increasing pressure of eutrophication and pollution (Vidal et al., 1999), and anthropogenic input is regarded as the main reason for coastal eutrophication. In the Changjiang River estuary, large amounts of nutrients were carried into the estuary. In addition, anthropogenic nutrient loads (e.g. nitrate) have increased by over tenfold and have been continuously increasing for the past two decades (Yan et al., 2003). The resultant eutrophication has had extremely negative impacts on the local ecosystem, including enhanced harmful algal blooms (Gao and Song, 2005; Tang et al., 2006), accelerated occurrences of anoxia and hypoxia in the bottom waters (Chen et al., 2007), and reduced biodiversity and alteration of community structure (Chen et al., 2007). Thus, it is important to study the nitrogen source and the biogeochemical processes in the Changjiang River estuary. δ 15 NO 3 has recently been applied as a useful technique for understanding marine biogeochemistry and the marine nitrogen cycle since nitrate is the main form of the marine inorganic nitrogen pool (Altabet, 2006). δ 15 NO 3 is governed by the mixing of external input and internal processes that either consume or produce nitrate. Nitrogen entering the Changjiang River estuary has many sources that are characterized by unique δ 15 NO 3 values. Higher proportions of nitrate from animal waste and sewage resulted in a heavier δ 15 NO 3 * Supported by the National Natural Science Foundation of China (No. 41276116), the Fund for Creative Research Groups by NSFC (No. 41121064), and the National Basic Research Program of China (973 Program) (No. 2010CB428706) ** Corresponding author: zyu@qdio.ac.cn