Response of benthic soluble reactive phosphorus transfer rates to step changes in flow velocity

Purpose The purpose of this study was to investigate the responses of the benthic soluble reactive phosphorus (SRP) transfer rate to step changes in the flow velocity of the overlying water using laboratory experiments and a non-steady-state numerical model. Materials and methods Laboratory experiments were conducted using a rectangular recirculating flume. After pre-incubation of sediments in a cavity of the experimental flume for 2 days, the step responses of the SRP transfer rate to sudden increases in the flow velocity were examined under anaerobic conditions. The benthic SRP transfer rates were obtained from the rate of increase in the SRP concentration of the overlying water. We also analysed the response using a newly constructed numerical model that consists of a one-dimensional diffusion model of the diffusive boundary layer (DBL) and a biochemical model of the sediment, in which oxygen, SRP, ferrous iron and nitrate were model variables. The non-steady-state calculation was performed to reproduce the experiments after the step change in the flow velocity. Results and discussion The experiments revealed a rapid increase in the SRP concentration in the overlying water that continued for approximately 5 min after the step change in flow velocity and was followed by a lower, steady increase in the SRP concentration. The model analyses also demonstrated that a step increase in flow velocity leads to a drastic enhancement of the SRP transfer rate within a few minutes. The abrupt increase in the transfer rate was due to the rapid transport of SRP that had accumulated in the DBL and to enhanced diffusion caused by a temporal increase in the SRP concentration gradient in the DBL. The modelled results for the SRP transfer rate were in agreement with the experimental results. Conclusion The temporal increase in SRP transfer was due to rapid SRP transport caused by intensified diffusion following a decrease in DBL thickness. The model-based results for the response of the SRP transfer rate were in agreement with the experimental results. Therefore, our model can simulate the response of the SRP concentration profile near the sediment-water interface to temporal variations in flow velocity.