THE POTENTIAL USE OF WETLANDS TO REDUCE PHOSPHORUS EXPORT FROM AGRICULTURAL CATCHMENTS

Natural and artificial wetlands have the potential to reduce phosphoru s (P) loads from dispersed agricultural runoff and from point sources in the Peel-Harvey catchment, Western Australia. Small experimental sy stems containing wetland plants and substrate have shown significant r emoval of P from inflowing water, the proportion of P removed being de pendent on P concentration and flow rate of water through the system. The use of artificial wetlands to treat diffuse agricultural runoff is limited by the highly seasonal runoff typical of this Mediterranean c limate, while use at point sources has so far been unsuccessful becaus e compounds from the effluent clog the wetland 'filters'. Treatment at point sources may well be feasible after further research. Natural we tlands in the catchment absorb P received in runoff from farmland and, in the absence of any outflow channels to the drainage system, confin e this P within the boundaries of the wetland. Disturbance to wetlands may reduce their efficiency in absorbing nutrients and may release P stored in the vegetation and sediment to the water. The conservation o f natural wetlands is recommended to maximise nutrient retention in th e catchment.