In the 13 years following the reduction of phosphorus (as total dissolved phosphorus) in the Bow River, periphyton biomass declined downstream. Within 10 km of the effluent, however, there was no change in periphyton biomass. Downstream, at average summer TDP concentrations less than 10 µg/L, average maximum periphyton biomass was less than 100 mg chlorophyll a/m2. That biomass is below nuisance levels (~150 mg chlorophyll a/m2). Researchers found that periphyton biomass was correlated with concentrations of TDP, but not of TP. The overall reduction in nutrients resulted in changes to the species composition of periphyton, with a near disappearance of Cladophora and other filamentous algae. A reduction in N was associated with a decline in macrophytes (e.g., pond weeds) within 30 km downstream of the effluent, from more than 2,000 g/m2 to less than ~200 g/m2. The researchers concluded that biomass levels below nuisance levels could be achieved at TDP concentrations of ~10–15 µg/L. They noted that recycling of nutrients from sediments might affect the time it takes for impacts from nutrient reductions to be seen. The researchers also developed a model to simulate spatial and temporal biomass and nutrient concentrations in cobble/gravel-bed rivers during summer low-flow conditions. The model compared favorably to empirical data (USEPA 2000b).
Reference:
USEPA. 2000b. Nutrient Criteria Technical Guidance Manual: Rivers and Streams. EPA-822-B-00-002. U.S. Environmental Protection Agency, Office of Water, Office of Science and Technology, Washington, DC. Accessed October 2016. https://nepis.epa.gov/Exe/ZyPDF.cgi/20003CVP.PDF?Dockey=20003CVP.PDF.