Carbon and nitrogen isotopic compositions of particulate organic matter and biogeochemical processes in the eutrophic Danshuei Estuary in northern Taiwan
Introduction
The Danshuei River (Fig. 1), also known as the Tamshui River, is the largest river in northern Taiwan, where the population density (2380 persons km− 2) is the highest in Taiwan. The high population brings a heavy loading of anthropogenic wastes to the river. Until 1990, a large fraction of sewage and surface runoff was discharged into the river with no or primitive treatment due to the inadequate sewer system. Although a sewage treatment plant with a planned capacity of 1.32 Mt d− 1 was built in the 1990s, its operation never reached its goal because of flawed design and engineering problems (Lo, 2001). Consequently, the Danshuei Estuary is highly eutrophied and polluted (Wen et al., 2007). At some monitoring stations, the mean concentration of dissolved oxygen in surface water dropped to 97 μM, and mean total organic carbon loading rose to 500 μM, mean total nitrogen loading to 570 μM, and mean concentration of total dissolved copper to 60 mgL− 1 (Sun and Peng, 2001).
The purpose of this study was to trace the origin of particulate organic matter (POM) in the Danshuei Estuary. POM is especially important in the estuarine system because of its capacity to carry trace metals and hydrophobic organic pollutants (Santschi et al., 1997, Ko and Baker, 2004), its high oxygen demand and its potential to release nutrient elements upon degradation. Carbon and nitrogen isotopes have long been used as indicators of different sources of organic matter in aquatic environments (Peters et al., 1978, Wada et al., 1987, Cifuentes et al., 1988, Kao and Liu, 2000, Sigleo and Macko, 2002). The combination of these isotopic signatures or the dual isotope composition has proven useful in differentiating the origins of POM in estuarine and coastal environments (Rau et al., 1981, Cifuentes et al., 1988, McClelland and Valiela, 1998). For this study, we try to understand the biogeochemical processes that control the isotopic compositions and distinguish the highly polluted Danshuei Estuary from others. We also attempt to determine the contributions from major external sources of POM to the estuary using carbon and nitrogen isotopic compositions and geochemical characteristics.
Section snippets
Sampling and area description
Samples were collected from the Danshuei Estuary (Fig. 1) on five occasions between August 2000 and March 2001 (Table 1) covering a wide range of flow conditions, from nearly base flow (March 2001) to flooding (August 2000). For the latter, water samples were collected during and after the invasion of Typhoon Bilis at the Chung-Yang Bridge (Fig. 1). A bucket was lowered from the bridge to collect the water sample 2–5 times every day during the period from 22 August to 24 August and once per day
Results
All sampling stations are within the tidal reach of the estuary. During the sampling occasions, except the typhoon period in August 2000, the salinity front with 50% seawater in the surface water fluctuated between Station T5 at ebb tide and Station T1 at flood tide (Fig. 1). The mean water temperature was 22.3 ± 0.5 °C in September 2000, 19.9 ± 1.0 °C in December 2000 and 20.2 ± 0.7 °C in March 2001 (http://wqshow.epa.gov.tw/). We first report results obtained on non-typhoon occasions as the
Discussion
The origin of organic matter may be traced through geochemical characteristics, such as C/N ratio and isotopic compositions (Table 2). Observations from other estuarine systems are shown for comparison (Table 2 and Fig. 4). The geochemical characteristics of potential end-members from the watershed–river–estuary system are shown in Table 3 as references. In addition to the known end-members, we discuss other possible sources of organic materials and the processes that may be responsible for the
Conclusions
The major sources of organic matter in the suspended particulate matter in the estuary include anthropogenic wastes, soils, bedrock-derived sediments and phytoplankton produced POM. The majority (29 out of 49) of non-typhoon samples and all (n = 15) but one typhoon samples were dominated by allochthonous POM. Using a three end-member mixing model, we found the average contributions of different sources to the allochthonous POC in the estuary: 83% from anthropogenic waste, 12% from soils and 5%
Acknowledgments
This manuscript has benefited greatly from thorough and constructive comments provided by Dr. J. Barth and an anonymous reviewer and useful input from Dr. Ming-Hsu Li. The main support of this study came from Academia Sinica and the National Science Council of Taiwan. This is NCU-IHS contribution no. 58.
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