DEVELOPING NUTRIENT TARGETS TO CONTROL BENTHIC CHLOROPHYLL LEVELS IN STREAMS - A CASE-STUDY OF THE CLARK-FORK RIVER

Approaches for assessing the effects of lowering nutrients on periphyt on biomass in streams and rivers are poorly developed in contrast to t hose for lakes. Here we present two complementary approaches to assess target nutrient concentrations in streams, given desired mean and max imum standing crops of benthic algal chlorophyll. In the first approac h. a reference portion or reach of the river that typically exhibits a cceptable levels of benthic chlorophyll is identified (i.e. seasonal m ean and maximum values do not exceed desirable levels), and the target levels for instream nutrient concentrations are defined by mean nutri ent levels in the reference region. In the second approach, regression and graphical analyses of a large stream database are used to identif y acceptable levels of instream total N and total P. The first approac h supplies site-specific nutrient targets, whereas the second places n utrient control into a broader, more comparative perspective. In order to link these target concentrations to specific nutrient control meas ures, we describe a spreadsheet model that can be used to translate ch anges 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, Monta na. We suggest that, in general, maintenance of mean instream total N concentrations below 350 mu g l(-1) and total P below 30 mu g l(-1) wi ll result in mean benthic algal chlorophyll a density below nuisance l evels of 100 mg m(-2) in most streams. (C) 1997 Elsevier Science Ltd.