Developing nutrient targets to control benthic chlorophyll levels in streams: A case study of the Clark Fork River

Abstract

Approaches for assessing the effects of lowering nutrients on periphyton biomass in streams and rivers are poorly developed in contrast to those for lakes. Here we present two complementary approaches to assess target nutrient concentrations in streams, given desired mean and maximum standing crops of benthic algal chlorophyll. In the first approach, a reference portion or reach of the river that typically exhibits acceptable levels of benthic chlorophyll is identified (i.e. seasonal mean and maximum values do not exceed desirable levels), and the target levels for instream nutrient concentrations are defined by mean nutrient levels in the reference region. In the second approach, regression and graphical analysis of a large stream database are used to identify acceptable levels of instream total N and total P. The first approach supplies site-specific nutrient targets, whereas the second places nutrient control into a broader, more comparative perspective. In order to link these target concentrations to specific nutrient control measures, we describe a spreadsheet model that can be used to translate changes in external loading by point sources into predicted new instream nutrient concentrations. These quantitative methods are applied here to the control of nuisance algal growth in the Clark Fork River, Montana. We suggest that, in general, maintenance of mean instream total N concentrations below 350 μg l −1 and total P below 30 μg l −1 will result in mean benthic algal chlorophyll a density below nuisance levels of 100 mg m −2 in most streams.