Sources and accumulation of organic carbon in the Pearl River Estuary surface sediment as indicated by elemental, stable carbon isotopic, and carbohydrate compositions

摘要

Organic matter in surface sediments from the upper reach of the Pearl RiverEstuary and Lingdingyang Bay, as well as the adjacent northern South ChinaSea shelf was characterized using a variety of techniques, includingelemental (C and N) ratio, bulk stable organic carbon isotopic composition(δ13C), and carbohydrate composition analyses. Total organiccarbon (TOC) content was 1.21±0.45% in the upper reach, down to1.00±0.22% in Lingdingyang Bay and to 0.80±0.10% on theinner shelf and 0.58±0.06% on the outer shelf. δ13Cvalues ranged from ?25.1‰ to ?21.3‰ in Lingdingyang Bay and the SouthChina Sea shelf, with a trend of enrichment seawards. The spatial trend inC/N ratios mirrored that of δ13C, with a substantial decreasein C/N ratio offshore. Total carbohydrate yields ranged from 22.1 to 26.7 mg(100 mg OC)?1, and typically followed TOC concentrations in theestuarine and shelf sediments. Total neutral sugars, as detected by the ninemajor monosaccharides (lyxose, rhamnose, ribose, arabinose, fucose, xylose,galactose, mannose, and glucose), were between 4.0 and 18.6 mg (100 mgOC)?1 in the same sediments, suggesting that significant amounts ofcarbohydrates were not neutral aldoses. Using a two end-member mixing modelbased on δ13C values and C/N ratios, we estimated that the terrestrialorganic carbon contribution to the surface sediment TOC was ca. 78±11% for Lingdingyang Bay, 34±4% for the inner shelf, and 5.5±1% for the outer shelf. The molecular composition of the carbohydrate inthe surface sediments also suggested that the inner estuary was rich interrestrially derived carbohydrates but that their contribution decreasedoffshore. A relatively high abundance of deoxyhexoses in the estuary andshelf indicated a considerable bacterial source of these carbohydrates,implying that sediment organic matter had undergone extensive degradationand/or transformation during transport. Sediment budget based on calculatedregional accumulation rates showed that only ~50% of the influxesof terrestrial organic carbon were accumulated in the estuary. Thisrelatively low accumulation efficiency of terrestrial organic matter ascompared to the total suspended solids (accumulation efficiency ~73%)suggested significant degradation of the terrestrial organic carbonwithin the estuarine system after its discharge from the river. This studydemonstrated that the combination of the bulk organic matter propertiestogether with the isotopic composition and molecular-level carbohydratecompositions can be an efficient way to track down the source and fate oforganic matter in highly dynamic estuarine and coastal systems. Thepredominance of terrestrially originated organic matter in the sediment andits generally low accumulation efficiency within the estuary is notsurprising, and yet it may have important implications in light of the heavyanthropogenic discharges into the Pearl River Estuary during the past thirtyyears.