Carbon and nitrogen isotopic compositions of particulate organic matter and biogeochemical processes in the eutrophic Danshuei Estuary in northern Taiwan Kon-Kee Liu a,b, ⁎ , Shuh-Ji Kao c , Liang-Saw Wen d , Kuan-Lun Chen d a Institute of Hydrological Sciences, National Central University, Taiwan, Republic of China b National Center for Ocean Research, Taiwan, Republic of China c Research Center for Environmental Changes, Academia Sinica, Taiwan, Republic of China d Institute of Oceanography, National Taiwan University, Taiwan, Republic of China Received 30 August 2006; received in revised form 16 April 2007; accepted 17 April 2007 Available online 23 May 2007 Abstract The Danshuei Estuary is distinctive for the relatively short residence time (1–2 d) of its estuarine water and the very high concentration of ammonia, which is the dominant species of dissolved inorganic nitrogen in the estuary, except near the river mouth. These characteristics make the dynamics of nitrogen cycling distinctively different from previously studied estuaries and result in unusual isotopic compositions of particulate nitrogen (PN). The δ 15 N PN values ranging from - 16.4‰ to 3.8‰ lie in the lower end of nitrogen isotopic compositions (- 16.4 to +18.7‰) of suspended particulate matter observed in estuaries, while the δ 13 C values of particulate organic carbon (POC) and the C/N (organic carbon to nitrogen) ratios showed rather normal ranges from - 25.5‰ to - 19.0‰ and from 6.0 to 11.3, respectively. There were three major types of particulate organic matter (POM) in the estuary: natural terrigenous materials consisting mainly of soils and bedrock-derived sediments, anthropogenic wastes and autochthonous materials from the aquatic system. During the typhoon induced flood period in August 2000, the flux-weighted mean of δ 13 C POC values was - 24.4‰, that of δ 15 N PN values was +2.3‰ and that of C/N ratio was 9.3. During non-typhoon periods, the concentration-weighted mean was - 23.6‰ for δ 13 C POC , - 2.6‰ for δ 15 N PN and 8.0 for C/N ratio. From the distribution of δ 15 N PN values of highly polluted estuarine waters, we identified the waste-dominated samples and calculated their mean properties: δ 13 C POC value of - 23.6 ± 0.7‰, δ 15 N PN value of - 3.0 ± 0.1‰ and C/N ratio of 8.0 ± 1.4. Using a three end- member mixing model based on δ 15 N PN values and C/N ratios, we calculated contributions of the three major allochthonous sources of POC, namely, wastes, soils and bedrock-derived sediments, to the estuary. Their contributions were, respectively, 83%, 12% and 5% under non-typhoon conditions, and 9%, 63% and 28% under typhoon conditions. The autochthonous POM had the most varied isotopic compositions, encompassing the full ranges of δ 13 C POC (- 25.5 to - 19.1‰), δ 15 N PN (- 16.4–3.8‰) and C/N ratio (6.0–11.3). The heavy end of the carbon isotopic composition reflected the typical marine condition and the lower end the estuarine condition, which probably had elevated concentrations of dissolved inorganic carbon with low δ 13 C values due to input from decomposition of organic matter. The lack of isotopically heavy PN, as found in larger estuaries, was attributed to isotopically light starting materials, namely, anthropogenic wastes, the slow phytoplankton growth within the estuary and the rather short residence time; the latter two factors made 15 N enrichment during ammonia consumption very limited. The most isotopically light Science of the Total Environment 382 (2007) 103 – 120 www.elsevier.com/locate/scitotenv ⁎ Corresponding author. Institute of Hydrological Sciences, National Central University, Jungli, Taoyuan, 32001, Taiwan, Republic of China. Tel.: +886 3 422 3354; fax: +886 3 422 2894. E-mail address: kkliu@ncu.edu.tw (K.-K. Liu). 0048-9697/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2007.04.019