Abstract: Fatty acids, key lipid components of membranes, hold significant value as bio-indicators and show great potential in biogeochemical research. However, their distribution in the particulate matter remains poorly understood in the South China Sea (SCS). This study offers a detailed analysis of fatty acids in surface suspended particulate matter of the SCS during summer, identifying over 30 fatty acid compounds. Based on statistical analysis and existing knowledge, these fatty acids are categorized into five sources: fresh phytoplankton, phytoplankton detritus, bacteria, zooplankton, and terrestrial higher plants. Fresh phytoplankton produces unsaturated fatty acids, especially polyunsaturated fatty acids (PUFA), which can be attibuted to diatoms, dinoflagellates and cyanobacteria. These fatty acids are relatively rich in the southwest and west areas of the SCS, affected by summer upwelling, indicating abundant fresh phytoplankton, particularly diatoms. In contrast, the northeastern area, with high temperature and salinity, is dominated by short-chain saturated fatty acids from phytoplankton detritus. Zooplankton levels are closely tied to those of fresh phytoplankton and phytoplankton detritus, respectively. Bacterial fatty acids show an inverse distribution to phytoplankton unsaturated fatty acids, reflecting a shift between primary production and decomposition in the ecosystem—primary production is dominant in the southwest and west, while microbial respiration is more active in the central and southeast areas. Terrestrial long-chain fatty acids are minimal and distributed in a summer current-driven pattern, decreasing from the southwest to the northeast. This study indicates that fatty acids, as effective molecular biomarkers, can effectively indicate the input of different biogenic sources into the SCS, and may help uncover the marine ecological structure in a large spatial scale.

Key words: Fatty acid, Biomarker, Sea surface suspended particulate matter, Plankton, South China Sea.