NUTRIENT DYNAMICS IN RIVERBEDS - THE IMPACT OF SEWAGE EFFLUENT AND AQUATIC MACROPHYTES

To determine the impact of nutrient loading from a sewage treatment plant and from aquatic macrophytes on riverbed chemistry, a 6-month (May-November) study was undertaken in the South Saskatchewan River, Saskatchewan, Canada at five sites located upstream and downstream of a municipal sewage treatment plant outfall and with differing biomasses of aquatic macrophytes. Nitrogen and phosphorus concentrations in the sediment-bound and porewater pools of the riverbed were greatest and porewater dissolved oxygen concentrations were lowest at the site with highest open-water nutrient concentrations (118 and 553 mu g/l TP and TDN) and aquatic macrophyte biomass (205 g/m(2)). Sites receiving little or no sewage effluent (23-60 mu g/l TP and 221-325 mu g/l TDN) had the lowest porewater and sediment-bound nutrient concentrations if no macrophytes were present, whereas concentrations were one-third to seven-fold greater if macrophytes were present in moderate abundance (135 g/m(2)). Our results showed that effluent loading and aquatic macrophytes may cause significant changes in the chemistry of riverbed sediments and suggest that for shallow slow-flowing rivers, benthic nutrient exchanges represent a critical component in water quality modeling.