Unraveling the sources of organic matter in suspended particulates and sediment in a closed inland lake using stable isotope fingerprinting

Suspended particulates and sediment are significant reservoirs of organic matter (OM) in lakes, and tracking the formation of suspended particulate organic matters (SPOMs) and sediment organic matters (SOMs) is the key to understanding the environmental behavior of OM and the carbon cycling of lake ecosystems. However, few studies have simultaneously focused on the sources of SPOM and SOM in closed inland lakes to reveal their differences and implications for water quality. The current study investigated the sources of SPOM and SOM in Daihai Lake, a typical closed inland lake in northern China, based on stable carbon (δ¹³C) and nitrogen (δ¹⁵N) isotopic compositions, during spring, summer, and autumn. The results showed that δ¹³C_POC and δ¹⁵N_PN (where POC and PN denote particulate organic carbon and nitrogen, respectively) of SPOM varied from −30.99‰ to −21.71‰ and 1.43‰–9.47‰, respectively. SPOM mainly originated from sewage, soil, and phytoplankton, with average contributions of 29.5%, 27.7%, and 19%, respectively, and each source of SPOM showed low spatial variation. However, the contribution of phytoplankton showed a decreasing trend from spring to summer, while the contribution of soil showed the opposite change. δ¹³C_TOC and δ¹⁵N_TN (where TOC and TN denote total organic carbon and nitrogen, respectively) of SOM varied from −26.41‰ to −23.99‰ and 3.3‰–7.66‰, respectively. Soil and sewage were the major sources of SOM, with average contributions of 43.3% and 27.8%, respectively, and each source showed small temporal and spatial variations. The differences between sources of SPOM and SOM revealed that phytoplankton-derived SPOM was easily degraded, whereas sewage- and soil-derived SPOM tended to deposit in the lake sediment. Additionally, the impact of SPOM on water quality significantly exceeded that of SOM. The source characteristics of SPOM were correlated with eutrophication and salinity indicators in the water, which have important implications for water quality. This isotopic evidence revealed that exogenous inputs were the main sources of OM in closed inland lakes, but there were some differences in the source characteristics between SPOM and SOM.