Title
Algal response to reductions in nutrient loading in the Clark Fork River
Abstract
In the 1980s, much of the Clark Fork River was impaired by nuisance algae as a result of excess nutrients. By the 1990s, stakeholders developed a Voluntary Nutrient Reduction Plan (VNRP) which was accepted as meeting the requirement for a TMDL. The goals of the VNRP were to reduce loads of Total Nitrogen (TN) and Total Phosphorus (TP) to the river, reduce instream nutrient levels to summer targets, which are below nutrient saturation breakpoints, and to reduce algae levels (Chlorophyll a and Ash-free dry mass) to summer targets. In 2004, the Missoula Wastewater Treatment Plant (the river’s largest point source of nutrients) received a major upgrade, and by 2010 nutrient reduction procedures there became more consistent and reliable. Over the past 30 years, TN and TP loads to the river from the Missoula WWTP have been reduced by 70 and 85% respectively, even as the number of households and businesses served have increased by 14%.
A study of 17 years of summer data from 1998-2014, covering 383 km of the Clark Fork, shows that nutrient reduction efforts have produced improvements at some sites, and have at least prevented significant increases in algal and nutrient levels elsewhere. Despite a 20% increase in population in Missoula over the decade of the VNRP, TP and TN loads have decreased due to improvements in the town’s wastewater treatment plant. TP concentrations downstream of the WWTP fell below the VNRP target of 39 μg/l, and TN concentrations met targets of 300 μg/l by 2007. The study found that, in order to control nuisance algae levels, nutrient levels should be decreased below saturation breakpoints or even to natural background levels.
Start Date
24-4-2015 12:00 PM
End Date
24-4-2015 2:00 PM
Document Type
Poster
Algal response to reductions in nutrient loading in the Clark Fork River
In the 1980s, much of the Clark Fork River was impaired by nuisance algae as a result of excess nutrients. By the 1990s, stakeholders developed a Voluntary Nutrient Reduction Plan (VNRP) which was accepted as meeting the requirement for a TMDL. The goals of the VNRP were to reduce loads of Total Nitrogen (TN) and Total Phosphorus (TP) to the river, reduce instream nutrient levels to summer targets, which are below nutrient saturation breakpoints, and to reduce algae levels (Chlorophyll a and Ash-free dry mass) to summer targets. In 2004, the Missoula Wastewater Treatment Plant (the river’s largest point source of nutrients) received a major upgrade, and by 2010 nutrient reduction procedures there became more consistent and reliable. Over the past 30 years, TN and TP loads to the river from the Missoula WWTP have been reduced by 70 and 85% respectively, even as the number of households and businesses served have increased by 14%.
A study of 17 years of summer data from 1998-2014, covering 383 km of the Clark Fork, shows that nutrient reduction efforts have produced improvements at some sites, and have at least prevented significant increases in algal and nutrient levels elsewhere. Despite a 20% increase in population in Missoula over the decade of the VNRP, TP and TN loads have decreased due to improvements in the town’s wastewater treatment plant. TP concentrations downstream of the WWTP fell below the VNRP target of 39 μg/l, and TN concentrations met targets of 300 μg/l by 2007. The study found that, in order to control nuisance algae levels, nutrient levels should be decreased below saturation breakpoints or even to natural background levels.