Temporal-spatial distribution of stable carbon isotope of organic matter and source analysis in the Changjiang Estuary
长江口潮滩有机质稳定碳同位素时空分布与来源分析

Changjiang Estuary, as a typical transitional zone between land and ocean, receives large quantity of organic materials from the Yangtze River and the ocean. In addition, the input of local materials such as detritus of macrophytes, photoplankton and benthic organisms makes it more complicated to trace the source and fate of organic matter in the estuarine and coastal tidal flat. The objective of this study is to trace the potential source of organic matter and the contribution of local environment to the organic materials by investigating the stable carbon isotopes in the organic constituents from the southern tidal flats of Changjiang Estuary. According to the features of physical geographical environment and the discrepancy in the effects of human activities in the study area, 9 typical sampling stations were selected along the southern Changjiang estuarine and coastal line. By measuring the stable carbon isotopes of organic matter in the winter (February 2006) and summer (August 2006) seasons of low and relatively high discharge, we analyzed the probable sources of organic matter from suspended particles and surface sediment. It is found that the ratios of stable carbon isotope in SPOM were respectively -25.8 ~-23.4‰ and -25.1‰~-22.9‰ in February and August, with the character of more negative in February. The spatial change of carbon isotopes displayed the trend of more positive in salt water than in freshwater areas, suggesting the mixture of marine and riverine organic matter. Due to the seasonal change of water discharge and marine phytoplankton blooming, terrigenous inputs exceeded marine inputs in winter and the reverse results have been found in summer. The ratios of stable carbon isotope in surface sediment ranged from -25.0 to -20.4 and -24.7 to -19.5 respectively in February and August. There was no obvious seasonal difference observed in isotopic compositions and the spatial change was similar to SPOM, suggesting the OM in sediment coming from the deposition of suspended particles. However, the autochthonous materials had also certain influence on the stable carbon isotopic compositions in organic matter. The input of sewage and tributary water impacted the carbon isotope in SPOM while the buried marsh macrophytes and benthic microalgaes contributed to the sedimentary organic matter.