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Stable carbon and nitrogen isotopic compositions of high molecular weight dissolved organic matter from four U.S. estuaries |
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| Authors: | Xu-Chen Wang Mark A Altabet Robert F Chen |
| Institution: | 1 Department of Environmental, Coastal and Ocean Sciences, University of Massachusetts at Boston, Boston, MA 02125-3393, USA 2 Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, China 3 School for Marine Science and Technology, University of Massachusetts at Dartmouth, New Bedford, MA 02744-1221, USA |
| Abstract: | High molecular weight dissolved organic matter (HMW-DOM) represents an important component of dissolved organic carbon (DOC) in seawater and fresh-waters. In this paper, we report measurements of stable carbon (δ13C) isotopic compositions in total lipid, total hydrolyzable amino acid (THAA), total carbohydrate (TCHO) and acid-insoluble “uncharacterized” organic fractions separated from fourteen HMW-DOM samples collected from four U.S. estuaries. In addition, C/N ratio, δ13C and stable nitrogen (δ15N) isotopic compositions were also measured for the bulk HMW-DOM samples. Our results indicate that TCHO and THAA are the dominant organic compound classes, contributing 33-46% and 13-20% of the organic carbon in HMW-DOM while total lipid accounts for only <2% of the organic carbon in the samples. In all samples, a significant fraction (35-49%) of HMW-DOM was included in the acid-insoluble fraction. Distinct differences in isotopic compositions exist among bulk samples, the compound classes and the acid-insoluble fractions. Values of δ13C and δ15N measured for bulk HMW-DOM varied from −22.1 to −30.1‰ and 2.8 to 8.9‰, respectively and varied among the four estuaries studied as well. Among the compound classes, TCHO was more enriched in 13C (δ13C = −18.5 to −22.8‰) compared with THAA (δ13C = −20.0 to −29.6‰) and total lipid (δ13C = −25.7 to −30.7‰). The acid-insoluble organic fractions, in general, had depleted 13C values (δ13C = −23.0 to −34.4‰). Our results indicate that the observed differences in both δ13C and δ15N were mainly due to the differences in sources of organic matter and nitrogen inputs to these estuaries in addition to the microbial processes responsible for isotopic fractionation among the compound classes. Both terrestrial sources and local sewage inputs contribute significantly to the HMW-DOM pool in the estuaries studied and thus had a strong influence on its isotopic signatures. |
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