Nutrient Concentrations and Their Relations to the Biotic Integrity of Nonwadeable Rivers in Wisconsin

Excessive nutrient [phosphorus (P) and nitrogen (N)] input from point and nonpoint sources is frequently associated with degraded water quality in streams and rivers. Point-source discharges of nutrients are fairly constant and are controlled by the U.S. Environmental Protection Agency's (USEPA) National Pollutant Discharge Elimination System. To reduce inputs from nonpoint sources, agricultural performance standards and regulations for croplands and livestock operations are being proposed by various States. In addition, the USEPA is establishing regionally based nutrient criteria that can be refined by each State to determine whether actions are needed to improve water quality. More confidence in the environmental benefits of the proposed performance standards and nutrient criteria would be possible with improved understanding of the biotic responses to a range of nutrient concentrations in different environmental settings. To achieve this general goal, the U.S. Geological Survey and the Wisconsin Department of Natural Resources collected data from 282 streams and rivers throughout Wisconsin during 2001 through 2003 to: (1) describe how nutrient concentrations and biotic-community structure differ throughout the State, (2) determine which environmental characteristics are most strongly related to the distribution of nutrient concentrations and biotic-community structure, (3) determine reference conditions for water quality and biotic indices for streams and rivers in the State, (4) determine how the biotic communities in streams and rivers in different areas of the State respond to differences in nutrient concentrations, (5) determine the best regionalization scheme to describe the patterns in reference conditions and the corresponding responses in water quality and the biotic communities (primarily for smaller streams), and (6) develop algorithms to estimate nutrient concentrations in streams and rivers from a combination of biotic indices. The ultimate goal of this study is to provide the information needed to guide the development of regionally based nutrient criteria for Wisconsin streams and rivers. In this report, data collected, primarily in 2003, from 42 nonwadeable rivers are used to describe nutrient concentrations and their relations to the biotic integrity of rivers in Wisconsin. In a separate report by Robertson and others (2006a), the data collected from 240 wadeable streams are used to describe these relations in streams in Wisconsin. Reference water-quality conditions for nonwadeable rivers were found to be similar throughout Wisconsin (approximately 0.035 milligrams per liter (mg/L) for total P (TP), 0.500 mg/L for total N (TN), 4 micrograms per liter for suspended chlorophyll a (SCHL), and greater than 110 centimeters for Secchi-tube depth (SD)). For each category of the biotic community (SCHL, macroinvertebrates, and fish), a few indices were more strongly related to differences in nutrient concentrations than were others. For the indices most strongly related to nutrient concentrations, reference conditions were obtained with a regression approach, from values corresponding to the worst 75 th -percentile value from a subset of minimally impacted streams (streams having reference nutrient concentrations), and from the best 25 th -percentile value of all the data. Concentrations of TP and TN in nonwadeable rivers increased as the percentage of agricultural land in the basin increased; these increases resulted in increased SCHL concentrations and decreased SDs. The responses in SDs and SCHL concentrations to changes in nutrient concentrations were similar throughout most of the State except in rivers in the southeastern part, where SCHL concentrations were lower than would be expected given their nutrient concentrations. Rivers in the southeastern part of the State had high concentrations of total suspended sediment compared to the SCHL concentrations. Many biotic indices responded to increases in nutrient concentrations, which indicates that nutrients have direct or indirect effects on the composition of the biotic community. Higher nutrient concentrations and poorer biotic index scores, indicative of poorer water quality, were found in agricultural areas in the southern half of the State. Most of the biotic indices were more strongly related to changes in TP concentrations than to changes in TN concentrations. Many of the responses to changes in nutrient concentrations were nonlinear and, therefore, thresholds or breakpoints were identified where a small change in nutrient concentrations corresponded to a relatively large change in the biotic communities. The thresholds in the responses to changes in TP concentrations ranged from 0.03 to 0.15 mg/L, whereas thresholds to changes in TN concentrations ranged from about 0.5 to 2.0 mg/L. The thresholds for many of the biotic responses were only slightly higher than the reference TP concentrations estimated for rivers throughout the State. The biotic communities in a river reflect its overall ecological integrity; they integrate the effects of many different stressors and thus provide a broad measure of the stressors' aggregate effect. Nutrient concentrations by themselves, however, explained only 1―11 percent of the total variance in the components of the biotic communities or about 2―25 percent of the explained variance. Nutrient concentrations were most important in affecting SCHL concentrations. Three biotic indices were combined to create two new multiparameter indices [Biotic Index of total Phosphorus (BIP) and Biotic Index of total Nitrogen (BIN)] to estimate TP and TN concentrations from biotic data collected in the rivers. The BIP predicted TP concentrations better than the BIN predicted TN concentrations (63 and 51 percent of the variances, respectively). The difference in the accuracy of these indices was consistent with biotic indices that were more correlated with TP concentrations than with TN concentrations. This result indicates that P is more important than N in affecting most biotic communities in rivers. Distributions of water quality and biotic indices for nonwadeable rivers, in general, were similar to those found for wadeable streams, with best conditions in the northern (forested) part of the State. The main differences between wadeable streams and nonwadeable rivers include: nonwadeable rivers had a smaller range in nutrient concentrations (less extreme concentrations, especially lower maximum concentrations), although median concentrations were similar; nonwadeable rivers had higher percentages of P and N in particulate forms; nonwadeable rivers had SCHL concentrations that were higher and had a stronger relation with nutrient concentrations; most biotic indices in nonwadeable rivers were more strongly related to nutrient concentrations; most biotic indices in nonwadeable rivers had a less consistent wedge-shaped response to changes in nutrient concentrations (the wedge-shaped response in wadeable streams resulted from biotic indices that ranged widely at low nutrient concentrations, but were consistently poor at high nutrient concentrations); and the biota in nonwadeable rivers had a slightly larger range in the thresholds in the responses to changes in TP concentrations. Although specific mechanisms of how nutrients affect the biota in wadeable streams and nonwadeable rivers were not examined in this study, the results indicate that nutrients are important in controlling their biotic health. Although the biotic-community structure represents the overall ecological integrity of the stream or river, nutrients alone explained only a small part of the variance in the biotic community. Therefore, it is difficult to predict the exact result of reducing nutrient concentrations without also modifying the factors typically associated with high nutrient concentrations. Nutrient concentrations in many streams and rivers, especially in agricultural areas, are well above the threshold concentrations; therefore, small reductions in nutrient concentrations in these streams and rivers are not expected to have large effects on the biotic community. Even with these limitations, however, it is expected that reducing nutrient concentrations will improve the biotic communities of most streams and rivers, improve their beneficial ecological functioning, and improve the quality of downstream nutrient-limited receiving waters.