Phosphorus Flux across the Sediment-Water Interface in a Drainage Ditch Receiving Water from Livestock Farms in Kasaoka, Japan

Abstract

Internal loading of phosphorus (P) in drainage ditches may be sufficient to enhance eutrophication in aquatic systems under certain conditions. This study aimed to determine the effects of dissolved oxygen (DO) concentrations on P flux across the sediment-water interface. The study was conducted in a drainage ditch that receives discharge from livestock farms. The diffusion in situ was estimated according to Fick’s first law equation using P concentrations in pore water and the overlying water. The overlying water and intact sediment cores were taken at the same location and time and incubated under anaerobic or aerobic conditions to determine the effects of DO on P fluxes based on the time-course of changes of P concentrations in water columns for 21 days in the laboratory. The diffusion of P across the sediment-water interface in the field was 9.79 ± 0.60 mg/(m² day) from sediment to the overlying water. In the laboratory, the fluxes of P were 1.76 ± 0.48 mg/(m² day) from sediment to overlying water under anaerobic conditions and 1.57 ± 1.05 mg/(m² day) from water into sediment under aerobic conditions. The upward diffusion of P under anaerobic conditions may be related to the reduction of iron-bound P. These results highlighted the role of oxygen in regulating P diffusion at the sediment-water interface. Accordingly, the low oxygen concentration in the drainage ditch was a reason for P release from sediment.