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 equi librium adsorption, and transport by molecular diffusion, estimates are der ived 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 scale s to re-establish equilibrium after a step change in the water column phosp horus 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 minu tes. The release of treated sewage at Narrabri abruptly raises the phosphor us concentration in the Namoi River. The concentration only returns to its original level 10-20 km downstream. A sediment adsorptive-uptake model unde restimates the downstream phosphorus uptake rates. An alternative model, ba sed on biotic uptake by Cladophora, describes reality better. It treats pho sphorus transfer as controlled by physical transport processes and by the p hosphorus uptake capacity of the biota. We show also that carp resuspension is faster than diffusion (6 v. 28 days) in restoring phosphorus concentrat ions in the water column after perturbation by rapid algal drawdown.