Sources of organic matter in the sediments of Baoxiang river reservoir, Dianchi watershed

Abstract

Baoxiang river reservoir, a semi-natural water body, is characteristised by eutrophication, which is significantly harmful to regional human health and sustainable development. The sediments are sources and sinks of nutrients. In addition to providing important habitats for aquatic organisms, sediments play a significant role in determining the overall environmental quality of a watershed. Organic carbon in sediments plays an important role in release and transformation of aquatic pollutants, which is one of the main causes of eutrophication. In this study,profiles of TOC, C/N ratio, δ13C and grain sizes in the sediments of Baoxiang river reservoir, Dianchi watershed were used to demonstrate the sources and change of sedimentary organic matter and explore reservoir eutrophication sensitivity to human activities.Considering the characteristics of topography and human activities of the study area, with the help of GPS(Global Positioning System), two reservoir bottom sediments(A and B) profiles were collected during dry period. Our results showed that the sources of sedimentary organic matter were different in the different sampling locations. C/N ratios in the sediment A varied from 2.93 to 11.73, with bulk δ13C(‰) values ranging from-23.66‰ to-21.78‰. This indicated that organic matters mainly from aquatic organisms and terrestrial input contributed little to the sedimentary organic matters. The concentrations of organic matters were influenced significantly by the grain sizes. The concentrations of sedimentary organic matters and total nitrogen in the sediment B varied from 14.62 to 24.93 g ·kg-1, and 2.11 to 3.3 g·kg1, respectively,higher than that of the sediment A, and with bulk δ13C(‰) values of-28.36‰ to-26.36‰.The grain sizes were mainly composed of clay and silt. This demonstrated that organic matters in the sediment B came mainly from terrestrial input, which is an indication of the main effect of human activities, not the sediment grain size.Based on the volume percentage of sediment in each size fraction, we found that grain size in the sediment A is classified as thick, indicating poor sorting and strong water movement. The sediment B is characteristized with fine particles, with clay and fine sand as the main components and low water flow. Correlation analysis between sediment grain size and organic carbon contents showed that they were highly correlated in the sediment A. Synchronous change was found in mean sediment grain size and precipitation.