Impacts of algal community structure on algae-derived phosphorus concentrations in Lake Taihu, China, from 2021 to 2025

Abstract Phosphorus control remains a critical challenge for managing eutrophication in Lake Taihu, yet quantitative assessments of algae-derived phosphorus (ADP) and its response to community shifts remain scarce. Based on monthly data from 20 entire-lake stations during 2021–2025 and an experimentally determined cellular phosphorus content for Microcystis spp. (1.46×10 − 10 mg/cell), we estimated cell-specific phosphorus for 113 algal species by cell-volume conversion and quantified ADP and phylum-level contributions. Annual mean total algal density decreased from 1.2×10 8 cells/L in 2021 to 6.9×10 7 cells/L in 2025, with marked turnover: Microcystis spp. fell from 83.6% to 32.4% of total density, while diatoms, cryptophytes and chlorophytes rose significantly. Annual mean ADP ranged from 0.015 to 0.022 mg/L (33.4% of TP) and was dominated by cyanobacteria, diatoms and chlorophytes. Cyanobacteria supplied over 65% of ADP during 2021–2022, declining markedly after 2023, while diatom and chlorophyte contributions rose. Notably, an algal rebound in December 2024 (density up 79.3% year-on-year) coincided with a 25.0% ADP decrease, because a large gain in small-celled cyanobacteria was outweighed by a much smaller loss of large-celled diatoms containing seven to eight times more phosphorus per cell. These findings show that ADP is a key component of TP in Lake Taihu and is governed by two mechanistically distinct controls: algal biomass (quantity) and the community-weighted cell-specific phosphorus content set by the assemblage cell-size structure (quality), which can act in opposite directions. Algal community structure thus acts as an active regulator of lake phosphorus and must be considered alongside algal density in phosphorus management.