Nutrient enrichment of northern rivers in response to pulp mill and municipal discharges

Abstract

To evaluate the effects of pulp mill and municipal sewageeffluents on the nitrogen (N) and phosphorus (P) status of northernCanadian rivers, the Northern River Basins Study required an integratedresearch and assessment program consisting of field observation andexperimentation. Analysis of monitoring data collected over 3–13 yshowed that on an annual basis, pulp mills contributed 22% of theP and 20% of the N load discharged from the Wapiti to the Smokyriver, and 6 to 16% of the P load and 4 to 10% of the Nload in the Athabasca River. Despite these low contributions, N and Pconcentrations were elevated below pulp mill discharges on all threerivers during the low discharge period of September to April. Insitu experiments conducted with nutrient diffusing substrata showedthat periphyton biomass was maintained at low levels by insufficient Pin the upper reaches of the Athabasca River and insufficient N+P inthe Wapiti River upstream of point-source discharges. In contrast,effluent loading from pulp mill and sewage inputs alleviated nutrientlimitation downstream of major discharges on both rivers. Experimentsconducted in artificial streams located beside the Athabasca Riverupstream of the first pulp mill showed that P addition increasedperiphyton biomass (expressed as chlorophyll a content) suchthat biomass increased sharply at low P concentrations (2–5µg/L soluble reactive P) and approached saturation at 35µg/L soluble reactive P. As a result of recommendations fromthis integrated program of river monitoring and experimentation,nutrient loading has been reduced to the most affected river(Wapiti-Smoky) and federal and provincial departments of environment arereviewing loading limits for other river reaches.