Occurrence and quantification of biogenic phosphorus pools at benthic and pelagic lake interfaces

Phosphorus is an essential macronutrient and, together with nitrogen, often the limiting factor for primary production in lakes. Increased anthropogenic phosphorus inputs have made eutrophication one of the most pressing threats to water bodies worldwide. In stratified lakes, phosphorus transport across limnic boundary layers is of particular importance, as the location and extent of phosphorus availability determines its ecological impact. This work investigated the effects of two biogenic phosphorus pools at the pelagic and benthic boundary layers on the internal phosphorus cycle of stratified lakes in three studies: (1) the effect of the deep-living cyanobacterium Planktothrix rubescens on phosphorus sedimentation, (2) the seasonal formation and release of polyphosphate in the uppermost sediment layer, and (3) the antagonistic effect of iron on polyphosphate formation in lake sediments and activated sludge. Since 2018, P. rubescens has become the dominant phytoplankton species in Lake Stechlin. Particulate phosphorus bound in P. rubescens was quantified via its characteristic pigment phycoerythrin using high-resolution vertical profiles. Planktothrix rubescens constituted approximately 20–30% of euphotic zone phosphorus during summer stratification and 10–20% of total water body phosphorus during winter circulation. Since its mass occurrence of P. rubescens, phosphorus sedimentation per biomass decreased by ~70%. The reduced sedimentation rates of phosphorus (and organic material) can be explained by the active buoyancy control of P. rubescens. Planktothrix rubescens thus forms a mechanism by which phosphorus is retained in the euphotic zone. At the sediment-water interface, polyphosphate forms a variable biogenic phosphorus pool in benthic microorganisms. These microorganisms intracellularly accumulate or release polyphosphate depending on environmental conditions, especially redox conditions. Field studies at three stratified lakes confirmed pronounced seasonal polyphosphate dynamics in the uppermost sediment layer: polyphosphate was formed at the beginning of summer stratification when oxygen was still available and phosphate concentrations in the porewater where high, while it was degraded under anoxic conditions. A comparative study of lake sediments and activated sludge demonstrated that iron antagonistically suppresses polyphosphate formation due to the high phosphorus-binding capacity of amorphous iron hydroxides. These results highlight the significant role of biogenic phosphorus pools at limnic boundary layers in the internal phosphorus cycle of stratified lakes. Furthermore, the reduced sedimentation by P. rubescens likely affects various biogeochemical processes in the hypolimnion and at the sediment-water interface. The antagonistic effect of iron on polyphosphate is also practically relevant, as chemical and biological phosphorus removal in wastewater treatment are often applied in tandem, offering optimisation potential.