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

To determine the impact of nutrient loading from a sewage treatment pl ant and from aquatic macrophytes on riverbed chemistry, a 6-month (May -November) study was undertaken in the South Saskatchewan River, Saska tchewan, Canada at five sites located upstream and downstream of a mun icipal sewage treatment plant outfall and with differing biomasses of aquatic macrophytes. Nitrogen and phosphorus concentrations in the sed iment-bound and porewater pools of the riverbed were greatest and pore water dissolved oxygen concentrations were lowest at the site with hig hest 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 mac rophytes 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 macrophyte s may cause significant changes in the chemistry of riverbed sediments and suggest that for shallow slow-flowing rivers, benthic nutrient ex changes represent a critical component in water quality modeling.