Aquatic plants dominate the sources of long-chain n-alkanes in floodplain lakes in the middle and lower reaches of the Yangtze River

It has been generally accepted that long-chain n-alkanes found in lacustrine sequences are mainly derived from terrestrial plants, which is the basis for tracking paleoenvironmental changes associated with terrestrial plants. However, this application may be complicated by the possible contribution of long-chain n-alkanes from nonemergent aquatic plants. To explore this issue, a sedimentary core spanning the past 150 years from Lake Longgan in the middle and lower reaches of the Yangtze River, together with terrestrial plant and soil samples collected around the lake, was analyzed for molecular distributions and delta C-13 values of n-alkanes. Results show that n-alkanes with different chain lengths have similar sources, as evidenced by the strong positive correlations in concentrations (r(2) > 0.88, p < 0.001) and delta C-13 values (r(2) > 0.79, p < 0.001) among n-alkane homologs. The delta C-13(alk) values (especially n -C-29 and n -C-31) indicated that n-alkanes have a mixed source, with non-emergent aquatic plants having a greater influence in shallow lakes in the middle and lower reaches of the Yangtze River (MLYR). Results from the MixSIAR model further reveal that non-emergent aquatic plants contribute a large amount of mid-chain n-alkanes and almost half (34.6% similar to 57.7%) of the long-chain n-alkanes. These findings suggest that long-chain n-alkanes in lacustrine sequences should be used with caution when attempting to reflect terrestrial vegetation succession and the associated climate changes.