In a two-part study of Wisconsin streams, the U.S. Geological Survey (USGS) and Wisconsin Department of Natural Resources (WDNR) collaborated to collect information in support of numeric nutrient criteria development. The researchers sampled 240 wadeable streams between 2001 and 2003 and 40 nonwadeable streams in 2003, and chose sites in each ecoregion to try to represent a wide range of nutrient concentrations. Streams were sampled monthly for discharge and water quality from May through October, while benthic algae and diatoms were sampled once during that period. Data on macroinvertebrate and fish populations were obtained from surveys that had been conducted in the previous 5 years. Phosphorus loading was estimated using monthly mean discharge data and phosphorus concentrations as reported by permitted dischargers.

For each site, the drainage basin was digitized and geological information system data were used to characterize environmental conditions of the watershed. Geospatial data included land use and land classification, soil conditions via the USSOILS dataset from the State Soil Geographic (STATSGO) database, annual air temperature and precipitation from the National Climatic Data Center (NCDC), slope via digital elevation models from USGS, and average annual runoff.

Reference conditions for phosphorus, nitrogen, chlorophyll, and water clarity were estimated using a multiple linear regression approach. Reference values for biotic indices were determined by the 75th percentile approach of minimally impacted sites. Also, a multiple linear regression instead of the 25th percentile approach was used to relate water quality to anthropogenic characteristics (Robertson et al. 2006).

Reference:

Roberston, D.M., D.J. Graczyk, P.J. Garrison, L. Wang, G. LaLiberte, and R. Bannerman. 2006. Nutrient Concentrations and their Relations to the Biotic Integrity of Wadeable Streams in Wisconsin. Professional Paper 1722. U.S. Geological Survey. Reston, VA. Accessed October 2016. https://pubs.usgs.gov/pp/pp1722/pdf/PP_1722.pdf.

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