ABSTRACT Planktothrix rubescens is a toxin‐producing filamentous cyanobacterium commonly found in temperate stratified lakes. Planktothrix rubescens regulates its buoyancy through intracellular gas vesicles and typically accumulates in the metalimnion during summer stratification, forming substantial biomass within a thin layer. Planktothrix rubescens can reach high biomass levels, performs luxury phosphorus uptake and likely influences the overall phosphorus budget of lakes; however, its role as a biogenic phosphorus reservoir and its effects on vertical distribution and cycling of P in lakes have not been explored yet. We obtained high‐resolution vertical probe and particulate phosphorus profiles from the metalimnetic P. rubescens peak during summer stratification in Lake Stechlin. Based on the linear relationship between particulate phosphorus and phycoerythrin, the characteristic red pigment of P. rubescens , we quantified the P. rubescens ‐related biogenic phosphorus pool. Additionally, we used long‐term phosphorus sedimentation and phytoplankton biomass data from Lake Stechlin (2009–2024) to calculate the phosphorus sedimentation per total and non‐ P. rubescens biomass comparing periods with and without P. rubescens dominance in the phytoplankton communities. Notably, up to 20% of total P in the water column of Lake Stechlin was bound to P. rubescens biomass. In recent years with dense P. rubescens blooms, P sedimentation per phytoplankton biomass decreased by approximately 70% compared to a reference period without P. rubescens dominance. In contrast, P sedimentation in relation to the non‐ P. rubescens biomass did not change significantly over the observation period (2009–2024). Our results suggest that the mass development and resulting dominance of P. rubescens reduce the sedimentation rates of P and alter the internal P cycle by retaining P in the water column. We conclude that the phytoplankton species composition can crucially influence the internal P cycle in lakes.
Schröder et al. (Wed,) studied this question.