Central Columbia Plateau - Yakima River Basin NAWQA Study - Publications

Environmental factors affecting benthic algae in the Palouse River basin, Idaho and Washington

By Mark D. Munn, U.S. Geological Survey, WRD, Tacoma, WA 98402

Abstract

The role of nutrients in eutrophication is a major water quality issue in many western streams. One of the goals of the USGS National Water Quality Assessment (NAWQA) Program is to assess how land use practices influence nutrients in streams. The objective of this study was to determine what physical and chemical factors most influence the growth of benthic algae in the Palouse River, a system affected by three land use practices: forest, urban and dryland farming. Unglazed ceramic tiles were placed at 16 sites for an eight-week colonization period; benthic algae were removed from replicate tiles and analyzed for chlorophyll a. Physical and water quality parameters were determined during the study. The concentrations of nutrients and chlorophyll a differed between the Palouse River and the South Fork Palouse River, a tributary which receives the majority of its flow from a water treatment plant. Chlorophyll a in the Palouse River ranged from 0.7 to 14 mg/0.1 m², with concentrations increasing downstream from the forested headwaters to the dryland farming region. Multiple regression results indicated that nitrogen was the limiting nutrient, with solar energy, water temperature, and water velocity influencing algal growth on a more local, reach-specific scale. In the South Fork, chlorophyll a varied from 5.0 mg/0.1 m² above the treatment plant to 37 mg/0.1m² directly below the treatment plant; benthic algae and nitrogen decreased substantially with distance downstream, whereas phosphorous remained constant. None of the physical or nutrient parameters measured were correlated with algal growth in the South Fork system; nutrient concentrations were elevated to levels that likely were not limiting.