Microbial control of sediment phosphorus release along a river-floodplain gradient

Sediment phosphorus release, also referred to as internal loading,  typically delays the response of eutrophic waters to reductions in external nutrient inputs. Internal loading is particularly relevant in shallow waterbodies like floodplain lakes with high sediment-to-water ratios. Traditionally, sediment phosphorus release has largely been explained by the biogeochemical interactions of iron, phosphorus, and oxygen. However, in sediments with a limited availability of iron but high organic content, the direct release of phosphorus from microbial mineralisation is the major mechanism behind internal loading. This is particularly the case in floodplains where benthic microbial functional diversity and corresponding activity play a pivotal role in sediment phosphorus release. Lateral hydrological connectivity further modulates sediment nutrient fluxes and microbial processes by altering biogeochemical conditions. Although the importance of microbe-organic matter interactions for phosphorus dynamics has been recognised, they are often not considered when assessing sediment phosphorus release.Here, we analyse the trajectory of potential sediment phosphorus release as well as dissolved carbon and nitrogen concentrations along a river-floodplain gradient of the River Elbe (Germany) from April to September 2024. Specifically, we link the dynamics of nutrients to dissolved organic matter quality and quantity, extracellular enzyme release, metabolic carbon diversity and further sediment biogeochemical parameters. Our findings reveal a general decrease in dissolved phosphorus concentrations from the river to the floodplain backwaters. However, in the periodically disconnected waterbody, we observed unexpectedly high soluble reactive phosphorus concentrations (~0.5 mg L⁻¹) following hydrological isolation, coinciding with elevated benthic extracellular phosphatase and β-glucosidase activity. Further linkages between the prevalent dissolved organic matter components, microbial mineralisation and microbial functional diversity were analysed and will be presented. Our results contribute to the mechanistic understanding of how microbial mineralisation processes modulated by hydrological connectivity shape sediment phosphorus release in river-floodplain systems.