Long-term response of periphyton and macrophytes to reduced municipal nutrient loading to the Bow River (Alberta, Canada)

The biomass of periphyton and aquatic macrophytes (Potamogeton vaginatus and Potamogeton pectinatus) in the Bow River was sampled over 16 years to assess the response of these plants to improved phosphorus (1982-1983) and nitrogen removal (1987-1990) at Calgary's two municipal wastewater treatment plants. These improvements in treatment reduced total phosphor-us loading to the Bow River by 80%, total ammonia loading by 53%, and nitrite + nitrate loading by 50%. No change in periphytic biomass was detected after enhanced phosphorus removal where total dissolved phosphorus (TDP) in river water remained relatively high (10-33 mug.L-1). However, periphytic biomass declined at sites further downstream with TDP < 10 μg.L-1. Regression analysis predicted that nuisance periphyton biomass (>150 mg.m(-2)) occurred at TDP > 6.4 mug.L-1 (95% confidence interval: 1.9-7.6 mug.L-1). Macrophyte biomass was inversely correlated with discharge and was lower during high-discharge years. Biomass also declined following enhanced nutrient removal, with the greatest decrease following reduced nitrogen discharge. These results provide the first evidence for a response of periphyton and aquatic macrophytes to enhanced nutrient removal from municipal wastewater.