Dynamic organic matter sources drive intact polar lipids recently deposited in surface sediments from the Yangtze River Estuary and adjacent sea

Lipids, especially the intact polar lipids, serve as valuable biological information for biogeochemical studies. However, the distributions of sedimentary lipids and their controlling factor in dynamic estuarine systems remain poorly constrained. Here we used an untargeted lipidomics approach to analyze lipid composition in surface sediments across freshwater, turbidity and inner shelf zones in the Yangtze River estuary. A total of 852 lipids from 28,378 molecular features were annotated, covering 35 subclasses based on LIPIDMAPS framework. The most abundant lipids, including subclasses free fatty acids, triacylglycerols, ceramides, diacylglyceryl trimethylhomoserines, and phosphatidylcholines, accounted for up to 71.4 %. Multivariate analysis reveals a significant effect of total organic carbon δ¹³C values (δ¹³C-TOC) and grain size on lipid distribution, suggesting that sedimentary organic matter sources and hydrodynamic conditions are key controlling factors on spatial lipid heterogeneity. An apparent seawards increase in δ¹³C-TOC values (−24.7 − −20.4 ‰) indicates contribution of phytoplankton-derived organic matter to sedimentary TOC from the adjacent sea area. However, most putatively eukaryotic lipids, including phospholipids, glycerolipids and betaine lipids, exhibit no distinct seawards pattern. This pattern is likely attributed to the integrative effect between lipid degradation of riverine phytoplankton and replenishment of marine phytoplankton with the resuspension of sediment. In contrast, most ceramides, especially phytosphingosine-based ceramides, show a substantially decreasing trend seawards and a strongly negative association with δ¹³C-TOC (r < −0.8, p < 0.001), probably indicating a terrestrial origin. This study highlights the lipid diversity under the complex marine dynamics and the potential use of lipids for tracing the biogeochemical cycling of polar lipids in the ocean.