Abstract: To investigate the distribution, source and conversion of low molecular weight organic acids (LMWOAs) in coastal seawater, the distribution characteristics of composition, concentration and conversion rates of LMWOAs in the Jiaozhou Bay were systematically investigated during November 2022. Correlation analyses were applied to explore the relationship between different components of LMWOAs and the factors influencing their conversion rates. The results showed that during autumn the concentrations of lactic acid (LA), acetic acid (AA), formic acid (FA) and the total concentration of these three acids (TOA) in the Jiaozhou Bay were within the ranges of 0 to 0.55, 0.94 to 3.39, 0 to 3.35, and 1.24 to 7.19 μmol·L⁻¹, respectively. Average concentration of AA accounted for 63.99% of TOA. High concentrations areas of LMWOAs were predominantly observed near the estuaries and the bay entrance, and the overall concentrations of LMWOAs in the surface seawater were consistently higher than those in the bottom layer, indicating their sources originated from human activities. The results from enrichment culture experiments revealed that the seawater of the Jiaozhou Bay exhibited a net consumption of LMWOAs during autumn. Their conversion rates showed complex variations in different stations and different water layers with characteristics of locality and diversity. A negative correlation was observed between the microbial conversion rates of the three LMWOAs and their corresponding non-biological conversion rates. Microbial activities in the seawater from different stations could produce and consume LMWOAs at different rates, primarily resulting in net consumption, and microorganisms were found to play a critical role in controlling the conversion rates of LMWOAs in the seawater of the bay. In the absence of photochemical processes, non-biological mechanisms produced and consumed LMWOAs at differing rates among various stations, predominantly resulting in net production, as important source of LMWOAs in the bay. The enzymes released by microorganisms played crucial roles on controlling the conversion of LMWOAs. During autumn, in the seawater of the Jiaozhou Bay, AA and FA shared similar biological origins with similar metabolic processes, and their microbial conversion rates were obviously influenced by water temperature, salinity and nutrient levels. In the seawater of the Jiaozhou Bay, several factors promoted the production of LMWOAs in autumn, including decreased water temperature, decreased salinity and concentrations of dissolved oxygen, increased concentrations of nutrients, and input from surrounding rivers.