Cyanobacterial organic matter dynamics and nitrate inflow: key drivers of phosphorus fluctuations in Lake Taihu (2012–2023)

The influence of cyanobacterial blooms and their decomposition on phosphorus concentration in lacustrine systems has garnered considerable academic attention. However, the mechanisms through which organic acids released during cyanobacterial accumulation, in conjunction with high nitrogen inputs, drive fluctuations in total phosphorus (TP) concentration in Lake Taihu remain unexplored. In this study, organic matter from decomposed bloom-forming cyanobacteria and nitrate were sequentially added to sediments to evaluate their effects on the fluctuation of TP concentration. Statistical analysis revealed that from 2012 to 2023, the total nitrogen (TN) concentration in Lake Taihu decreased in tandem with declining TN levels in inflowing rivers. In contrast, the TP concentration in the lake water exhibited significant fluctuations despite a continuous decrease in TP loading from the inflowing rivers. High-density bloom-forming cyanobacteria were found to enhance phosphorus release from sediments due to Fe(III) reduction to Fe(II), whereas nitrate–nitrogen (NO₃⁻–N) appeared to promote phosphorus assimilation in sediments under low cyanobacterial density. Microcosm experiments demonstrated that cyanobacterial organic matter significantly enhanced phosphate release from sediments. Thus, cyanobacterial organic matter may have facilitated a shift in phosphorus dynamics from a sink to a source in the sediments of the western lake region of Lake Taihu before 2019. Conversely, from 2019 to 2023, NO₃⁻–N inputs from rivers stimulated Fe(II) oxidation, leading to phosphate binding with Fe(III) and enhancing phosphorus retention in sediments. This process reversed phosphorus from a source to a sink in sediments. These findings reveal a bidirectional source–sink switch regulated by cyanobacterial bloom intensity and nitrate loading, providing a mechanistic explanation for TP fluctuations in Lake Taihu between 2012 and 2023. The study offers broader insights into internal phosphorus cycling and eutrophication management in shallow lakes.