Response of Algal Biomass to Large-Scale Nutrient Controls in the Clark Fork River, Montana, United States

Nutrient pollution is an ongoing concern in rivers. Although nutrient targets have been proposed for rivers, little is known about long-term success of programs to decrease river nutrients and algal biomass. Twelve years of summer data (1998-2009) collected along 383km of the Clark Fork River were analyzed to ascertain whether a basin-wide nutrient reduction program lowered ambient total nitrogen (TN) and total phosphorus (TP) concentrations, and bottom-attached algal biomass. Target nutrient and algal biomass levels were established for the program in 1998. Significant declines were observed in TP but not TN along the entire river. Downstream of the city of Missoula, TP declined below a literature-derived TP saturation breakpoint and met program targets after 2005; TN was below targets since 2007. Algal biomass also declined significantly below Missoula. Trends there likely relate to the city's wastewater facility upgrades, despite its 20% population increase. Upstream of Missoula, nutrient reductions were less substantial; still, TP and TN declined toward saturation breakpoints, but no significant reductions in algal biomass occurred, and program targets were not met. The largest P-load reduction to the river was from a basin-wide phosphate laundry detergent ban set 10years before, in 1989. We document that nutrient reductions in rivers can be successful in controlling algal biomass, but require achievement of concentrations below saturation and likely close to natural background.