USGS Publications Warehouse

Abstract

Excessive nutrient (phosphorus and nitrogen) loss from watersheds is frequently associated with degraded water quality in streams. To reduce this loss, agricultural performance standards and regulations for croplands and livestock operations are being proposed by various States. In addition, the U.S. Environmental Protection Agency is establishing regionally based nutrient criteria that can be refined by each State to determine whether actions are needed to improve a stream's water quality. More confidence in the environmental benefits of the proposed performance standards and nutrient criteria will be possible with a better understanding of the biotic responses to a range of nutrient concentrations in different environmental settings. The U.S. Geological Survey and the Wisconsin Department of Natural Resources collected data from 240 wadeable streams throughout Wisconsin to: 1) describe how nutrient concentrations and biotic-community structure vary throughout the State; 2) determine which environmental characteristics are most strongly related to the distribution of nutrient concentrations; 3) determine reference water-quality and biotic conditions for different areas of the State; 4) determine how the biotic community of streams in different areas of the State respond to changes in nutrient concentrations; 5) determine the best regionalization scheme to describe the patterns in reference conditions and the responses in water quality and the biotic community; and 6) develop new indices to estimate nutrient concentrations in streams 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. For total nitrogen (N) and suspended chlorophyll (SCHL) concentrations and water clarity, regional variability in reference conditions and in the responses in water quality to changes in land use are best described by subdividing wadeable streams into two categories: streams in areas with high clay-content soils (Environmental Phosphorus Zone 3, EPZ 3) and streams throughout the rest of the State. The regional variability in the response in total phosphorus (P) concentrations is also best described by subdividing the streams into these two categories; however, little consistent variability was found in reference P concentrations in streams throughout the State. Reference P concentrations are smilar throughout the State (0.03-0.04 mg/L). Reference N concentrations are divided into two categories: 0.6-0.7 mg/L in all streams except those in areas with high clay-content soils, where 0.4 mg/L is more appropriate. Reference SCHL concentrations are divided into two categories: 1.2-1.7 ?g/L in all streams except those in areas with high clay-content soils, where 1.0 ?g/L may be more appropriate. Reference water clarity is divided into two categories: streams in areas with high clay-content soils with a lower reference water clarity (Secchi tube depth, SD, of about 110 cm) and streams throughout the rest of the State (SD greater than or equal to about 115 cm). For each category of the biotic community (SCHL and benthic chlorophyll a concentrations (BCHL), periphytic diatoms, macroinvertebrates, and fish), a few biotic indices were more related to differences in nutrient concentrations than were others. For each of the indices more strongly related to nutrient concentrations, reference conditions were obtained by determining values corresponding to the worst 75th percentile value from a subset of minimally impacted streams (streams having reference nutrient concentrations). By examining the biotic community in streams having either reference P or N concentrations but not both, the relative importance of these two nutrients was determined. For SCHL, P was the more important limiting nutrient; however, for BCHL and all macroinvertebrate indices, it appears that N was the more important nutrient when concentrations were near reference concentrations. For other diatom indices and all fish indices, small additions of P or N appear to have little effect on these communities when nutrient concentrations are near reference conditions. Concentrations of P and N in streams increase as the percentage of agricultural land increases. Concentrations of P increase more quickly and concentrations of N increase more slowly in response to increasing percentages of agriculture in areas with high clay-content soils than do streams in the rest of the State. The response in water clarity is similar in streams throughout the State; however, the streams in areas with high clay-content soils have poorer reference water clarity, and, therefore, as the percentage of agriculture increases, their clarity remains lower than in streams in areas with other soil types. As nutrient concentrations increase, many biotic indices change. This result indicates that these nutrients have direct or indirect effects on the composition of the biotic community. Thresholds were identified at which a small change in nutrient concentrations results in a relatively large change in the biotic communities. The thresholds in the response to changes in P concentrations range from about 0.04 mg/L for BCHL, to 0.06-0.07 mg/L for diatom and fish indices, to about 0.09 mg/L for macroinvertebrate indices. The thresholds in the response to changes in N concentrations range from 0.5 mg/L for the fish indices and one macroinvertebrate index to about 0.9-1.2 mg/L for the diatom and other macroinvertebrate indices. Most of the biotic indices had a wedge-shaped response to increases in nutrient concentrations. At relatively low nutrient concentrations, the biotic indices ranged widely, but at relatively high concentrations, the indices generally were poor. The wedge-shaped distribution indicates that at low nutrient concentrations, factors other than nutrients often limit the health of biotic communities, whereas, at high nutrient concentrations, nutrients and factors correlated with high nutrient concentrations are the predominant factors. The biotic communities that are present in a stream reflect the overall ecological integrity; therefore, they integrate the effects of many different stressors and thus provide a broad measure of their aggregate effect. Nutrient concentrations by themselves explained from about 6 to 13 percent of the total variance in the components of the biotic communities or from about 14 to 23 percent of the explained variance. Nutrient concentrations were most important in affecting SCHL concentrations and macroinvertebrate communities, and least important in affecting BCHL, periphytic diatoms, and fish-community structure. For each component of the biotic community, nutrients by themselves only explained a small part of the overall variance; about half of the variance could not be explained by the variables examined in this study and about one-third of the explained variance could not be assigned to single categories of environmental characteristics. By use of a combination of four biotic indices, two new multiparameter indices (Biotic Index of total Phosphorus, BIP, and Biotic Index of total Nitrogen, BIN) were developed to estimate P and N concentrations in streams from biotic data collected in streams. These multiparameter models estimated high and low nutrient concentrations equally well. The BIP predicted P concentrations better than the BIN predicted N concentrations. The difference in the accuracy of these indices was consistent with biotic indices being more correlated with P concentrations than with N concentrations. This result suggests that P is more important than N in affecting most biotic communities as nutrient concentrations increase above reference concentrations. Although specific mechanisms of how nutrients affect the biota in wadeable streams were not examined in this study, the results indicate that nutrients are important in controlling the biotic health of streams. Although the biotic-community structure represents the overall ecological integrity of the stream, 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, especially those in agricultural areas, are well above the concentrations where thresholds in the response were found to occur; therefore, small reductions in nutrient concentrations in these streams 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 community, further the beneficial ecological functioning of most streams, and improve the quality of downstream nutrient-limited receiving waters.