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

Abstract

Abstract. Organic matter in surface sediments from the upper reach of the Pearl River Estuary and Lingdingyang Bay, as well as the adjacent northern South China Sea shelf was characterized using a variety of techniques, including elemental (C and N) ratio, bulk stable organic carbon isotopic composition (δ 13 C), and carbohydrate composition analyses. Total organic carbon (TOC) content was 1.21 ± 0.45% in the upper reach, down to 1.00±0.22% in Lingdingyang Bay and to 0.80 ± 0.10% on the inner shelf and 0.58 ± 0.06% on the outer shelf. δ 13 C values ranged from −25.1‰ to −21.3‰ in Lingdingyang Bay and the South China Sea shelf, with a trend of enrichment seawards. The spatial trend in C/N ratios mirrored that of δ 13 C, with a substantial decrease in 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 the estuarine and shelf sediments. Total neutral sugars, as detected by the nine major monosaccharides (lyxose, rhamnose, ribose, arabinose, fucose, xylose, galactose, mannose, and glucose), were between 4.0 and 18.6 mg (100 mg OC) −1 in the same sediments, suggesting that significant amounts of carbohydrates were not neutral aldoses. Using a two end-member mixing model based on δ 13 C values and C/N ratios, we estimated that the terrestrial organic 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 in the surface sediments also suggested that the inner estuary was rich in terrestrially derived carbohydrates but that their contribution decreased offshore. A relatively high abundance of deoxyhexoses in the estuary and Correspondence to: M. Dai (mdai@xmu.edu.cn) shelf indicated a considerable bacterial source of these carbohydrates, implying that sediment organic matter had undergone extensive degradation and/or transformation during transport. Sediment budget based on calculated regional accumulation rates showed that only ∼50% of the influxes of terrestrial organic carbon were accumulated in the estuary. This relatively low accumulation efficiency of terrestrial organic matter as compared to the total suspended solids (accumulation efficiency ∼73%) suggested significant degradation of the terrestrial organic carbon within the estuarine system after its discharge from the river. This study demonstrated that the combination of the bulk organic matter properties together with the isotopic composition and molecular-level carbohydrate compositions can be an efficient way to track down the source and fate of organic matter in highly dynamic estuarine and coastal systems. The predominance of terrestrially originated organic matter in the sediment and its generally low accumulation efficiency within the estuary is not surprising, and yet it may have important implications in light of the heavy anthropogenic discharges into the Pearl River Estuary during the past thirty years.

The composition and flux of particulate and dissolved carbohydrates from the Rhone River into the Mediterranean Sea

“…11 %) most likely suggest a phytoplanktonic influence in POM (see also the previous discussion). Other studies have found an enrichment of galactose in the particulate material in a coastal environment and/or sediments where primary productivity is generally high (Cowie and Hedges, 1984;Bergamaschi et al, 1999;He et al, 2010).…”

“…The origins of carbohydrates in both the POM and DOM pools were further assessed using the carbohydrate compositional patterns, similarly to previous investigations (Cowie and Hedges, 1984;da Cunha et al, 2002;Guggenberger et al, 1994;He et al, 2010). Because glucose composition may vary with sources, the following calculations were performed on a glucose-free basis.…”

“…PCA has been successfully used in previous studies to assess the origin or the degradation status of organic matter (Panagiotopoulos and Sempéré, 2005b;He et al, 2010) and was performed using the individual carbohydrate concentrations (i.e. glucose, and mannose) as well as the POC and DOC data ( Table 1).…”

“…(Ittekkot, 1988). This classification was based on the carbohydrate and amino acid measurements in POM, which are generally considered to be labile compounds (susceptible to bacterial degradation), and because of their quantitative significance in POM (Moran and Hodson, 1989;Wakeham et al, 1997;Panagiotopoulos and Sempéré, 2005b and references therein; He et al, 2010). We Our results showed that LPOC was mainly distributed between TSM concentrations of 0-15 mg l −1 (52 % of the data) and 15-50 mg l −1 (29 % of the data).…”

Source, composition, and environmental implication of neutral carbohydrates in sediment cores of subtropical reservoirs, South China

“…They comprise about 20-40 wt % of plankton (Parsons et al, 1984), more than 40 wt % of bacteria (Moers et al, 1993), and more than 75 wt % of vascular plants (Moers et al, 1993). Due to their high biological reactivity and availability, carbohydrates are preferentially utilized by heterotrophic organisms (e.g., bacteria and fungi) in water columns (Hernes et al, 1996;Khodse et al, 2007), leading to the preservation of some refractory structural carbohydrates in sediments Burdige et al, 2000;Jensen et al, 2005;He et al, 2010). Moreover, the compositional signature of structural carbohydrates depends more on planktonic sources than the diagenetic pathway (Hernes et al, 1996).…”