Spatial and seasonal dynamics of organic carbon in physically fractioned sediments associated with dam construction in the middle Lancang-Mekong River

Abstract

Purpose Dam construction alters hydrological conditions, impacting the sedimentary environment in the reservoir and modifying the regional flow of carbon. This study examined the spatial and seasonal dynamics of organic carbon in sizefractioned sediments to identify the role of physical protection (e.g. burial, aggregation) for preserving carbon and the potential impact of a dam on the mineralization of sedimentary organic carbon in the dammed Lancang-Mekong River. Materials and methods Sediment samples were collected from the Manwan Reservoir, China, in the rainy and dry seasons using a gravitational bottom sampler. One sample was collected from the centre of the river and two samples were collected on the river edges, 5 m away from each bank. Sediments were divided into three size classes of waterstable aggregates by wet sieving, including macroaggregates, microaggregates and silts. The total organic carbon (TOC), labile organic carbon (LOC), total nitrogen (TN) and stable carbon isotope (δ 13 C) were measured to analyse the dynamic features of organic carbon in the sediments. Results and discussion The preservation of organic carbon in the sediments collected from the main stream decreased with an increase in distance from the Manwan dam, while it remained consistently high in the tributaries. The majority of the sediments collected during the rainy season preserved more TOC and TN than the samples collected during the dry season. Because of the increase in the proportion of sands in macroaggregates and microaggregates, the TOC contents decreased. In contrast, silts and clays were not affected and performed well in protecting organic carbon. The sediments in the lacustrine area and forepart of the transition area, with lower δ 13 C and higher C/N values, suggested a mixture of aquatic and agro-ecosystem inputs and led to high proportions of LOC in TOC. Conclusions Dam construction reinforced the carbon sink and role of physical protection in the Manwan reservoir by enhancing fine fraction deposition and organic carbon inputs, especially during the rainy season. The sediments dominated by silts and clays absorbed and preserved more organic carbon than the samples dominated by sands. The majority of the organic carbon buried in the lacustrine area, the forepart of the transition area and the tributaries was protected by microaggregates, silts and clays. With the development of fisheries and agriculture, increasing amounts of labile organic carbon buried in sediments may lead to an increase in the potential for carbon mineralization.

Characteristics of sediment affecting monomethylmercury accumulation in benthic fish of the Mekong Delta

“…Based on 3 yr of geochemical survey results, we found that 1) sediment MMHg concentration tends to be higher at high TOC sites; 2) organic carbon sources of sediment and benthic fish are distinctive between river and estuary regions, but MMHg concentrations of sediment, SPM, and food web organisms are not distinguishable between the two regions; 3) the BSCF of MMHg is mostly determined by the trophic level of benthic fish rather than the sediment characteristics; and 4) a relatively low risk of MMHg exposure, via consumption of delta fish, to the local residents is expected. The current levels of TOC in Mekong Delta sediment are relatively low (0.2–1.5%), but multiple dam constructions are reinforcing the organic carbon sedimentation rate with increasing fine fractions (i.e., silts and clays) of sediment, especially in rainy seasons (Zhao et al ). In addition to dam construction, expanded shrimp aquaculture associated with salinity intrusion in the low‐lying delta area is currently enhancing the labile TOC supply to the surface sediment.…”

Determination of Total Organic Carbon Concentration in Surficial Sediments of Sungai Pinang, Penang, Malaysia

“…This is because the organic matter is regulating the mobility of those pollutants in sediment [6,7,8]. In addition, some researchers are calculating the fraction of TOC with total sulfur while some other researchers determine the relationship between TOC and total nitrogen for the purpose of evaluation of the type of sedimentary environment and elucidate the dynamic of organic matter in sediment [2,3,9]. Hence, TOC is one of the important chemical sediment quality measurements that has been widely used in environmental evaluation analysis [1,8].…”

“…Surficial sediment is acting as the sink of TOC and other chemical pollutants as it is able to accumulate all the substances that have settled down on the bottom of water column, including particulate organic carbon and sediment particles which is attached with organic carbon substance [3,5,9]. Thus, surficial sediment is capable of reflecting the latest deposited pollution history [2,10].…”

“…In addition, previous study by Saad et al [13] had concluded that the water quality of Sungai Pinang had degraded and was classified under V showing a very high level of chemical oxygen demand (COD) which was 1219.33 mg/L, very low in dissolved oxygen (DO) which account for < 1.0 mg/L and the temperature (°C) of water was not categorized in normal range. Apart of that Sungai Pinang is surrounded by heavy industrial and urban areas, the river mouth is an important channel in the distribution of carbon matters [9,14]. Thus, the surficial sediment of Sungai Pinang is exposure to various chemical compound contamination which is contained in the bottom of the water.…”