In Montana’s Clark Fork River, river bottom algae concentrations are having a negative effect on beneficial uses. Stakeholders have joined in a voluntary nutrient reduction program (VNRP) to develop a 10-year plan to reduce algae concentrations. To determine appropriate targets for algae (as chlorophyll a), researchers sampled current chlorophyll a levels in waters in which nuisance conditions occur and performed a literature review.

Target chlorophyll a levels have been set at <100 mg/m2 (the average over the growing season) and 150 mg/m2 (the maximum). The concentrations will be reevaluated over time as more information becomes available about the association between chlorophyll a concentrations and water quality problems. The VNRP committee also examined actions that would most likely reduce algal concentrations and concluded that reductions in nutrient concentrations would have the most impact. They identified TN and TP targets needed to meet the algal target (300 ppb TN and 39 ppb TP in the middle river, and 20 ppb TP in the upper river) (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.

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