Effluent-dominated streams. Part 1: Presence and effects of excess nitrogen and phosphorus in Wascana Creek, Saskatchewan, Canada



Regina, Saskatchewan, Canada (population 190,400) treats its sewage at a modern sewage treatment plant (STP) on Wascana Creek. In the winter, treated sewage effluent makes up almost 100% of stream flow. Four surveys conducted from 2005 to 2007, in differing seasons, indicated significantly higher nitrogen (N) and phosphorus (P) concentrations at sites downstream of the STP compared to an upstream control site. Downstream, Wascana Creek is N hypersaturated (total dissolved N >3 mg/L) and soluble reactive phosphorus (SRP) makes up a greater percentage of total P (TP). Diminished nutrient retention capacities for both N and P are directly attributable to STP effluent. Creek SRP concentrations are less than estimates of equilibrium P concentrations (EPCo), indicating that creek sediments may be a source of P, further exacerbating hypereutrophic ambient SRP concentrations. As well, NO2 + NO3-N concentrations far surpass World Health Organization limits for drinking water (10 mg/L) and sensitive taxa, while NH3-N, NH4-N, and NO2 + NO3-N exceed Canadian Water Quality Guidelines for Protection of Aquatic Life and those for the U.S. Environmental Protection Agency. High NH4-N concentrations may be responsible for depressions not only in algal biomass and production observed downstream but reductions in primary to bacterial production ratios (PP:BP). In spring and fall, these reductions push PP:BP from net autotrophy to heterotrophy. The Wascana Creek study highlights the considerable problems associated with excess nutrients in effluent-dominated ecosystems (EDS). It also underlines the need for better controls on NH4-N additions from STPs in such EDS, especially in a day and age when freshwater supplies are dwindling and negative effects of climate change are expected. Environ. Toxicol. Chem. 2011;30:496–507. © 2010 SETAC