Effects of low-level eutrophication on ecological integrity of rivers in the Rocky Mountain National Parks of Canada

Nutrient availability has long been considered one of the most important factors regulating production of benthic algae in oligotrophic rivers yet empirical models relating benthic algal abundance to nutrient availability have rarely yielded accurate predictions. The aim of this research was to improve the ability to predict patterns of benthic algal abundance in oligotrophic rivers by improving the understanding of processes that regulate abundance. In situ experiments combined with surveys of physical, chemical and biological attributes of oligotrophic mountain rivers confirmed that small amounts of anthropogenic phosphorus (P, 0.1--5.6mug/L total phosphorus) adversely impacted the ecological integrity of Rocky Mountain Rivers. Downstream of municipal wastewater treatment plants (MWWTPs), abundance of benthic algae and benthic macroinvertebrates (BMIs) increased by 4- to 30-fold. In sites upstream of MWWTPs, benthic communities were dominated by thin diatom films and mayfly scrapers but downstream abundance of diatoms and chironomids increased. Epilithic abundance was proximately controlled by P availability if light was not limiting but ultimately controlled by variation in temperature and river discharge, and was highest downstream of MWWTPs in autumn. However, at a given P concentration, there were greater increases in abundance of benthic biota downstream of MWWTPs than upstream. The variable response of benthic biota was attributed to high P bioavailability in wastewater effluent. Upstream of MWWTPs, algal accrual was limited by the availability of P, and organisms in the epilithon could cleave P with metabolically expensive enzymes. Downstream, P limitation of epilithon decreased or was eliminated, epilithic organisms did not produce phosphatase enzymes, metabolic costs decreased and therefore, more algal growth per unit P occurred. As a consequence of decreased P limitation of benthic algae downstream of MWWTPs, P content of epilithon increased. Increased abundance of BMIs downstream of MWWTPs was attributed to improved food quality as a result of increased epilithic P content rather than increased food quantity. Measuring the degree of both epilithic nutrient limitation and producer-consumer nutrient imbalance will improve the ability to predict the magnitude of effects from eutrophication of oligotrophic rivers, and will help elucidate the causes and consequences of various patterns and processes.