Preliminary study on the effect of cascade dams on organic matter sources of sediments in the middle Lancang–Mekong River

Abstract

Carbon and nitrogen isotopes in sediments collected from the Manwan and Dachaoshan Reservoirs, which were created by cascade dams along the Lancang–Mekong River, were measured to preliminarily analyse their organic matter sources. Sediment samples were collected from the Manwan and Dachaoshan Reservoirs twice during both the rainy and dry seasons by using a gravitational bottom sampler. One set of samples was collected from the river centres of each reservoir, and the other set was collected from the river edges, 5 m from each bank. Sediments were divided by wet sieving into three classes of water-stable aggregates based on sizes, including macroaggregates, microaggregates, and silt and clay. At the same time, terrestrial plants (Phragmites) and surface soil samples (0–5 cm) were collected from the river bank. The total organic carbon (TOC), total nitrogen (TN), stable carbon isotope (δ13C), and stable nitrogen isotope (δ15N) were measured for sediments, soils, and plants to preliminarily analyse the sources of organic matter in the sediments. Because of the presence of dams, a greater amount of the fine fraction was deposited in the reservoirs, which adsorbed and preserved more organic matter. Although terrestrial sources of organic matter dominate in riverine systems, aquatic sources have been shown to act as new organic matter sources in the Manwan Reservoir. Input from C3 plants was the major new organic matter source in the Dachaoshan Reservoir, which also had strong inputs from aquatic sources. In the dry season, C3 plant input was significantly reduced and less fine fraction was found in the sediments. The high nitrogen isotope values of the sediments collected during the dry season indicated that strong organic matter decomposition decreased the storage of organic matter. The cascade dams permanently changed the aquatic environment and modified regional carbon flows resulting in the diversification of organic matter sources. The major source of sedimentary organic matter was terrestrial, including from plants, soils, and particle organic matter, with the new organic matter absorbed by macroaggregates primarily stemming from aquatic organisms and C3 plants. Samples collected during the dry season exhibited strong decomposition, which further weakened the preservation of organic carbon in sediments.