Influence of nutrients, habitat, and streamflow on indicators of eutrophication in agricultural streams: Applications for developing nutrient criteria

The effects of varying nutrient concentrations on stream eutrophication were examined in three diverse agricultural regions of the United States. Sites within each study area were selected along a nutrient concentration gradient while minimizing natural variation. Seventy wadeable sites were selected with 18 to 30 sites per study area with nutrients, habitat, algal biomass, and macrophyte/macroalgae cover sampled during a single summer period in 2006 or 2007. Continuous stream stage and water temperature were collected at each site for 30 days prior to sampling to evaluate antecedent conditions. Wide ranges of concentrations were found for total nitrogen (TN, 0.07-9.61 mg/L) and total phosphorus (TP, 0.002-0.361 mg/L) across all sites. Biotic measures of stream eutrophication including periphytic (RCHL) and sestonic (SCHL) chlorophyll-a, and aquatic macrophyte (AQM) growth were not strongly related to concentrations of TN or TP; R² values ranged from 0.02 to 0.32. Model performance generally did not improve when applied to individual study units. An index combining RCHL, SCHL, and AQM is offered as an alternative to evaluate eutrophic status. A significant multiple regression model (R² = 0.50, P < 0.001) identified TN, percent canopy, median riffle depth, and daily change in stage as important variables in predicting index scores.