Sources and Pollution Characteristics of Sedimentary Organic Matter in a Small Urban Lake in Eastern Hubei Province: A Case Study of Cihu Lake

Small urban lakes are among the most anthropogenically impacted aquatic systems, making it critical to understand the sources and pollution characteristics of sedimentary organic matter (OM) to elucidate eutrophication mechanisms. However, quantitative source apportionment in small urban lakes has been insufficiently explored. This study investigated a typical urban lake, Cihu Lake, located in eastern Hubei, China. Total organic carbon (TOC), total nitrogen (TN), C/N ratios, and stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N) in surface sediments were analyzed, in combination with the organic pollution index (OI) and the MixSIAR Bayesian mixing model, to systematically characterize OM sources and pollution features. The results showed that sedimentary OM in Cihu Lake was highly enriched (OI=1.50-3.87), with higher pollution levels observed in the southern basin compared to the northern basin. The δ¹³C values and C/N ratios indicated a pronounced algal signature in sedimentary OM, while δ¹⁵N suggested the influence of external nitrogen inputs. Quantitative results from MixSIAR revealed that sewage and soil organic matter were the dominant sources, contributing 40.6% and 31.3%, respectively. Overall, sedimentary OM in Cihu Lake exhibited a coupled pattern of external input dominance and enhanced internal production, whereby external organic inputs not only constitute the primary material source but also stimulate in-lake primary production, thereby amplifying autochthonous OM accumulation.