Nitrate Enrichment and Fish Community Nitrate Tolerance: Assessing Biologically Based Criteria

Fish community and nitrate data were collected at stream sites along nutrient enrichment gradients in two contrasting settings: an agricultural basin in Indiana in the Eastern Corn Belt ecoregion and an urban basin in the metropolitan Atlanta, Georgia area in the Piedmont ecoregion.  Nationally-derived nitrate tolerance indicator values (TIVs) were assigned to each species, and average TIVs were determined for fish communities at each site.  Models were used to predict fish species expected to occur at a site under minimally disturbed conditions and average TIVs were determined for each expected community.  Fish community nitrate tolerance was assessed based on the ratio of observed-to-expected (O/E) TIVs.  Fish community nitrate tolerance increased (O/E > 1) significantly with increased nitrate concentrations in both basins.  Regression equations indicated that a 0% increase in fish community nitrate tolerance (O/E = 1) in the Atlanta area corresponded to a concentration of 0.19 mg/L (compared to a USEPA nitrate criterion of 0.18 mg/L for Piedmont streams) and 0.31 mg/L (compared to a nitrate criterion of 1.60 mg/L for Eastern Corn Belt streams) in the agricultural basin.  Empirical relationships between fish community nitrate tolerance and nitrate concentrations provide valuable information regarding biologically based nitrate criteria.