Understanding phosphorus recycling in eutrophic lakes is the basis for pollution control. In this study, we analyzed phosphorus fractions, phosphate oxygen isotopes (δ 18O p) in iron-bound phosphorus (Fe–P), authigenic phosphorus (Auth-P), detrital phosphorus (Detr-P), and physicochemical indexes at the water–sediment interface to study the recycling of internal phosphorus during the annual cyanobacterial life stage in a eutrophic lake, Lake Chaohu, China. Increased Fe–P in sediments from the cyanobacterial decline to the benthic stage indicated the deposition of newly formed iron-bound phosphorus and the enriched δ 18O Fe – P indicated that these phosphates were absorbed to ferric oxides derived directly from the cyanobacterial cells (enriched δ 18O p). From benthic to the initial growth stage, depletion of δ 18O Fe – P indicated that these phosphates were derived mainly from the hydrolysis of organic phosphorus (depleted δ 18O p). Increased Fe–P in sediments from the initial growth stage to algal blooms indicated a net deposition of phosphorus, and the depletion of δ 18O Fe – P indicated similar phosphates from the hydrolysis of organic phosphorus. From benthic to the blooms stage, enrichment of δ 18O Auth-P suggested phosphates from cyanobacterial remains or transformation of bioavailable phosphorus. Stable Detr-P and δ 18O Detr-P in sediments during the cyanobacterial life stage indicated the resistance of Detr-P to biogeochemical alteration. Spatially, δ 18O Fe – P was more depleted in the western lake than in the middle and eastern lakes, likely due to its high level of Fe–P from domestic sewage, with newly formed Fe–P (enriched δ 18O Fe – P) only contributing minor proportions. The δ 18O Auth-P in the eastern lake was depleted, indicating that the hydrolysis of organic phosphorus served as a phosphate source. Enriched δ 18O Detr-P indicated that more phosphates from domestic sewage (enriched δ 18O p) contributed to the formation of Detr-P in West Lake Chaohu. The present study provides novel insight into the recycling of internal phosphorus in eutrophic lakes. [Display omitted] • Internal phosphorus recycling during the cyanobacterial life cycle was studied. • Various phosphorus fractions at the sediment–water interface were analyzed. • Enrichment of δ 18O Fe – P indicated phosphate release from the cyanobacterial cells. • Depletion of δ 18O Fe – P indicated phosphate release from the hydrolysis of organic P. • Stable δ 18O Detr-P indicated the resistance of Detr-P to biogeochemical alteration. [ABSTRACT FROM AUTHOR]