Source and Biogeochemical Distribution of Organic Matter in Surface Sediment in the Deep Oligotrophic Lake Fuxian, China

The source, quantity and quality of sedimentary organic matter (SOM) were investigated in the surface sediments of Lake Fuxian, a deep oligotrophic lacustrine system in China. Granulometry, biochemical organic composition, bulk organic proxies and their stable isotopes were determined in the surface sediments (0–4 cm). The values of δ13C, δ15N and atomic ratio of total organic carbon to total nitrogen (TOC/TN) indicated that the sediments in the large partial lake were influenced by autochthonous organic matter. The concentrations of TOC, protein (PRT) and total hydrolysable amino acids may be mainly modulated by phytodetritus sinking from euphotic zones since they correlated significantly with chlorins. Otherwise, the lack of correlation between chlorins concentrations and carbohydrate (CHO) and lipid (LIP) indicated that the latter may have an additional terrestrial source. The highest sediment quantity was found in the deepest station NC owing to its fine sediment. Stations near northwestern shore accumulated more SOM than other littoral stations, which was in accordance with sewage discharge strength. Degradation quality indices, such as chlorin index, degradation index, PRT/CHO, and LIP/CHO, were in general agreement in showing the degraded status of SOM in Lake Fuxian. No clear spatial patterns were found in sediment degradation quality, which may be influenced by bottom oxygen concentration in the deep stations.