Biotic influences on chemical fluxes and sediment-water exchanges in sediment deposits
This is the first study undertaken in a controlled environment to understand the kinetics of the release of soluble reactive phosphorus and copper from sediments of natural systems with an associated biofilm, and to identify which of the size compartments affected those fluxes most. It was found that of the sediment size fractions in the system, the stones that had a substantial biofilm growth attached had the greatest influence. Differences in the responses were observed between the sediment size fractions and the two sites, where contaminant concentrations varied. The equilibrium phosphate concentration and a phosphorus transfer index were used to establish if there was a net uptake or release of phosphorus by the sediment size at the time of sampling. The sediment having a biofilm and associated particulate material resulted in a greater flux than fine sediment, which does not support a filamentous biomass. The kinetic results imply a different mechanism than diffusion being involved. It was demonstrated that both gravel and stone substrates can have an important control over the release of soluble reactive phosphorus due to their role as firm substrate for a biofilm growth. Changes in the steady-state concentration of dissolved copper suggest that the bed sediment is responding to reduced river water concentration and setting a new steady-state. The kinetics of the reaction of the sediment to copper were of a similar order, and rate constants increased through the season, but were of a similar magnitude for both sites. The differences in the n and rate constant values indicate a difference in the mechanisms (i.e. the order of the kinetics) of uptake of copper through the seasons. The kinetics are described by a rate law which yields a method of estimating the flux to the sediment for recovery time after a pollution incident and how far downstream the copper concentration would remain elevated after a pollution incident or sediment.