Variations of the Absorption of Chromophoric Dissolved Organic Matter in the Pearl River Estuary

Environmental contextEstuaries play an important role in global carbon cycling in terms of transforming dissolved organic matter (DOM). We describe the molecular composition and spatial distribution of DOM in the Pearl River Estuary, an area severely impacted by anthropogenic activities, and show how DOM composition gradually changes with salinity. The results will help our understanding of the sources and transformations of anthropogenic DOM discharged to the coastal seas. AbstractThe Pearl River is the second-largest river in China in terms of water discharge and brings enormous amounts of nutrients and terrestrial organic matter to the South China Sea, which makes the Pearl River Estuary (PRE) highly eutrophic. However, the molecular composition and distribution of dissolved organic matter (DOM) in the PRE have scarcely been investigated. In this study, solid-phase extraction (SPE) was performed to collect DOM samples from PRE along a salinity gradient. The samples were characterised by negative-ion electrospray ionisation (ESI) Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to analyse their molecular composition and spatial distribution. The FT-ICR MS results showed that the terrestrial organic matter was gradually diluted and/or degraded during the migration from the river to the coastal ocean. Furthermore, both sulfur containing and unsaturated molecules were highly abundant in the upper stream samples, which indicated that anthropogenic input might be another important source of the assigned DOM in PRE. A group of bio-refractory molecules, characterised as carboxylic-rich alicyclic-like molecules, was found to accumulate with the increase of salinity. The composition of the SPE-DOM showed a gradual variation with the salinity and spatial changes; however, the variation was slightly different from those in pristine estuaries. This study demonstrates that the molecular composition of DOM is crucial for elucidating its source and transformation in an estuary.

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

Biogeosciences Discussions ◽

10.5194/bgd-7-2889-2010 ◽

2010 ◽

Vol 7 (2) ◽

pp. 2889-2926 ◽

Cited By ~ 2

Author(s):

B. He ◽

M. Dai ◽

W. Huang ◽

Q. Liu ◽

H. Chen ◽

...

Keyword(s):

Organic Matter ◽

Organic Carbon ◽

Surface Sediments ◽

River Estuary ◽

Pearl River Estuary ◽

Outer Shelf ◽

Pearl River ◽

Terrestrial Organic Matter ◽

The Pearl River Estuary ◽

The Pearl River

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 by a variety of techniques, including elemental (C and N), stable carbon isotopic (δ 13C) composition, as well as molecular-level analyses. Total organic carbon (TOC) content was 1.61±1.20% 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. δ13C values ranged from −25.11‰ to −21.28‰ across the studied area, with a trend of enrichment seaward. The spatial trend in C/N ratios mirrored that of δ13C, with a substantial decrease in C/N ratio from 10.9±1.3 in the Lingdingyang Bay surface sediments to 6.5±0.09 in the outer shelf surface sediments. 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, suggesting that the relative abundance of total carbohydrate was fairly constant in TOC. Total neutral sugars as detected by the nine major monosaccharides (lyxose, rhamnose, ribose, arabinose, fucose, xylose, galactose, mannose, and glucose) yielded between 4.0 and 18.6 mg (100 mg OC)−1 in the same sediments, suggesting that a significant amount of carbohydrates were not neutral aldoses. The bulk organic matter properties, isotopic composition and C/N ratios, combined with molecular-level carbohydrate compositions were used to assess the sources and accumulation of terrestrial organic matter in the Pearl River Estuary and the adjacent northern South China Sea shelf. Results showed a mixture of terrestrial riverine organic carbon with in situ phytoplankton organic carbon in the areas studied. Using a two end-member mixing model based on δ13C values and C/N ratios, we estimated that the terrestrial organic carbon contribution to the surface sediment TOC was ca. 57±13% for Lingdingyang Bay, 19±2% for the inner shelf, which decreased further to 4.3±0.5% on the outer shelf. The molecular composition of the carbohydrate in surface sediments also suggested that the inner estuary was rich in terrestrial-derived carbohydrates but that the contribution of terrestrial-derived carbohydrates decreased offshore. Terrestrial organic carbon accumulation flux was estimated as 1.37±0.92×1011 g yr−1 in Lingdingyang Bay, which accounted for 37±25% of the terrestrial organic carbon transported to the Bay. The burial efficiency of terrestrial organic matter was markedly lower than that of suspended particulate substance (~71%) suggesting that the riverine POC undergoes significant degradation and replacement during transportation through the estuary.

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