Recycling of internal phosphorus during cyanobacterial growth and decline in a eutrophic lake in China indicated by phosphate oxygen isotopes

Understanding phosphorus recycling in eutrophic lakes is the basis for pollution control. In this study, we analyzed phosphorus fractions, phosphate oxygen isotopes (δ¹⁸Oₚ) 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 δ¹⁸OFₑ–P indicated that these phosphates were absorbed to ferric oxides derived directly from the cyanobacterial cells (enriched δ¹⁸Oₚ). From benthic to the initial growth stage, depletion of δ¹⁸OFₑ–P indicated that these phosphates were derived mainly from the hydrolysis of organic phosphorus (depleted δ¹⁸Oₚ). Increased Fe–P in sediments from the initial growth stage to algal blooms indicated a net deposition of phosphorus, and the depletion of δ¹⁸OFₑ–P indicated similar phosphates from the hydrolysis of organic phosphorus. From benthic to the blooms stage, enrichment of δ¹⁸OAᵤₜₕ₋P suggested phosphates from cyanobacterial remains or transformation of bioavailable phosphorus. Stable Detr-P and δ¹⁸ODₑₜᵣ₋P in sediments during the cyanobacterial life stage indicated the resistance of Detr-P to biogeochemical alteration. Spatially, δ¹⁸OFₑ–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 δ¹⁸OFₑ–P) only contributing minor proportions. The δ¹⁸OAᵤₜₕ₋P in the eastern lake was depleted, indicating that the hydrolysis of organic phosphorus served as a phosphate source. Enriched δ¹⁸ODₑₜᵣ₋P indicated that more phosphates from domestic sewage (enriched δ¹⁸Oₚ) 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.