Acceptable nutrient concentrations in agriculturally dominant landscapes: A comparison of nutrient criteria approaches for Nebraska rivers and streams

Developing numeric nutrient criteria for streams has been exceedingly complex due to uncertain background concentrations and uncertain impacts to water quality. In this study, I empirically examine the utility of multiple criteria for total nitrogen (TN) and total phosphorus (TP) criteria for rivers and streams of Nebraska, USA using: (1) whole-population nutrient percentiles; (2) reference stream percentiles; (3) model predicted estimates from relationships between nutrients and anthropogenic land usage; and (4) stressor–response modeling of nutrients and invertebrate and fish assemblages. I predicted that criteria developed for Nebraska would be greater than the criteria recommended by the US Environmental Protection Agency for the associated nutrient ecoregions because the new criteria would be derived from streams only within this agriculturally-dominated state. Also, I predicted that criteria based on responses of biota would be higher than those based on the frequency distribution of nutrient data because biota have been filtered by their ability to endure higher nutrient concentrations. The percentage of rowcrop agriculture was responsible for the vast majority of land usage, and TN and TP were predictable by rowcrop. Nutrient ecoregions did not appear to be particularly useful in Nebraska. EPA recommendations for TP were lower than any of the new criteria we developed (from 113 to 599 μg/l) and those for TN were almost always lower than new criteria (from 552 to 2352 μg/l). Relationships of nutrients to biotic integrity were weak or non-existent; TN was not among the best three predictors of invertebrate or fish metrics and TP was only a good predictor of the pollution tolerance of fish, and this criterion was much higher than EPA recommendations (599 μg/l) and criteria based on percentiles and land usage. I discuss the applicability of criteria to agriculturally-intense streams in Nebraska.