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Particulate organic matter (POM) plays an important role in biological pumping as a source of energy and nutrients in aquatic systems, as well as being the mechanism for algal bloom formation. However, research on its sources and composition, particularly the research on the contribution of algae in the bloom season, is still insufficient. In this study, the sources and composition of the POM in the surface water of Lake Taihu during the algal bloom season were quantitatively analysed. Dual stable carbon (delta C-13) and nitrogen (delta N-15) isotopes revealed that the algae were not the only sources of POM during algal blooms, and their contributions were inconsistent in the different lake regions. On average, algae made up 57.2% of the POM in Lake Taihu. Due to wind farms and nutrient runoff, algal POM was mainly concentrated in Meiliang Bay, Gonghu Bay and Central Taihu. The proportions of algal POM in the western estuary area and East Taihu were the smallest with a minimum of 10.8%. The proportion of terrestrial POM in the surface water and vertical sections was 18.3% and 40.0%, respectively. Furthermore, the highest value was 76.8%, which accounted for a large proportion in the algal bloom season. The terrestrial POM that came from rivers was mainly concentrated in the western part of Lake Taihu, while the resuspension of sediments also increased the proportion of terrestrial sources in the surface water. Affected by the distribution of the vegetation in Lake Taihu, the proportion of macrophyte POM in East Taihu was higher than that of algal sources. Although the contribution of endogenous POM might further increase due to the dual effects of climate change and eutrophication, our isotopic evidence suggested that terrestrial POM cannot be ignored, even in the algal bloom season.