Phosphorus dynamics in Australian lowland rivers

In freshwater systems, phosphorus is adsorbed predominantly to clay within the sediments. Assuming a linear adsorption/desorption isotherm, rapid equilibrium adsorption, and transport by molecular diffusion, estimates are derived for (a) the rates of exchange between the adsorbed phosphorus pool in the sediments and the dissolved pool in the water column and (b) time scales to re-establish equilibrium after a step change in the water column phosphorus concentration. For oxic sediments, the time scale is of the order of tens of days. Anoxic release is much faster; the time scale is tens of minutes. The release of treated sewage at Narrabri abruptly raises the phosphorus concentration in the Namoi River. The concentration only returns to its original level 10-20 km downstream. A sediment adsorptive-uptake model underestimates the downstream phosphorus uptake rates. An alternative model, based on biotic uptake by Cladophora, describes reality better. It treats phosphorus transfer as controlled by physical transport processes and by the phosphorus uptake capacity of the biota. We show also that carp resuspension is faster than diffusion (6 v. 28 days) in restoring phosphorus concentrations in the water column after perturbation by rapid algal drawdown.