Carbon and nitrogen isotopic compositions of particulate organic matter and biogeochemical processes in the eutrophic Danshuei Estuary in northern Taiwan

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 δ15NPN 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 δ13C 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 δ13CPOC values was − 24.4‰, that of δ15NPN 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 δ13CPOC, − 2.6‰ for δ15NPN and 8.0 for C/N ratio. From the distribution of δ15NPN values of highly polluted estuarine waters, we identified the waste-dominated samples and calculated their mean properties: δ13CPOC value of − 23.6 ± 0.7‰, δ15NPN value of − 3.0 ± 0.1‰ and C/N ratio of 8.0 ± 1.4. Using a three end-member mixing model based on δ15NPN 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 δ13CPOC (− 25.5 to − 19.1‰), δ15NPN (− 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 δ13C 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 15N enrichment during ammonia consumption very limited. The most isotopically light PN likely originated from phytoplankton incorporating 15N-depleted nitrate near the river mouth, where ammonia inhibition of nitrate uptake probably stopped.