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

Abstract

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 size-fractioned 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. 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 water-stable 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 (δ13C) were measured to analyse the dynamic features of organic carbon in the sediments. 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 δ13C and higher C/N values, suggested a mixture of aquatic and agro-ecosystem inputs and led to high proportions of LOC in TOC. 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.