Devlopment of regional macroinvertebrate and nutrient criteria to assess nutrient enrichment in wadeable streams and rivers in Tennessee

D.H. Arnwine.Water Pollution Control, Tennessee Department of Environment, 7th Floor L&C Annex, 401 Church Street, Nashville, TN 37243

Elevated nutrient concentrations affect over 1500 stream miles in Tennessee. Sources of nutrient enrichment include livestock, wastewater treatment plants, urban run-off and fertilizers. Under certain environmental conditions, elevated nutrient levels can affect the benthic community.

The Tennessee Department of Environment and Conservation (TDEC) has developed both nutrient and macroinvertebrate criteria to assess impairment in wadeable streams and rivers. An ecoregion reference stream approach was used. Ten years of regional reference data were compiled to define background levels of total phosphorus and nitrate+nitrite. Macroinvertebrate data from the reference streams were used to develop a multi-metric index for each of 15 bioregions.

Nutrient criteria were set at the 90th percentile in each of 25 ecoregions or group of similar ecoregions. Since nutrients are generally considered a secondary stressor rather than a direct toxicant, in-stream nutrient levels that exceeded background levels were not deemed sufficient evidence of impairment. TDEC developed a method to use both the causative variable (nitrate+nitrite or total phosphorus) and a response variable (composition of the macroinvertebrate community) to determine whether elevated nutrient concentrations have resulted in changes to the benthic population.