Ecological health in the Nation's streams

The Quality of Our Nation’s Waters

Aquatic biological communities, which are collections of organisms, are a direct measure of stream health because they indicate the ability of a stream to support life. This fact sheet highlights selected findings of a national assessment of stream health by the National Water-Quality Assessment (NAWQA) Program of the U.S. Geological Survey (USGS). The assessment was unique in that it integrated the condition of three biological communities—algae, macroinvertebrates, and fish—as well as measures of streamflow modification, pesticides, nutrients, and other factors. At least one biological community was altered at 83 percent of assessed streams, and the occurrence of altered communities was highest in urban streams. Streamflows were modified at 86 percent of assessed streams, and increasing severity of streamflow modification was associated with increased occurrence of altered biological communities. Agricultural and urban land use in watersheds may contribute pesticides and nutrients to stream waters, and increasing concentrations of these chemicals were associated with increased occurrence of altered biological communities.

The occurrence of altered biological communities was higher in urban than agricultural settings (89 percent versus 79 percent of assessed sites, fig. 1). All three communities— algae, macroinvertebrates, and fish—were altered in 22 percent

Mixed use

of assessed streams. A biological community was classified as altered at a stream site if the numbers and types of organisms in it were substantially different from its natural potential as estimated from regional reference sites. The high occurrence of altered communities suggests that stream health is threatened by a wide variety of land and water use across the Nation.

All sites

The presence of unaltered biological communities in agricultural and urban watersheds suggests that it is possible to maintain stream health despite substantial human activities in a watershed. Within each land-use setting, altered biological communities varied widely in the degree to which they differed from their natural potential. This finding suggests that the effects of agricultural and urban land use on stream health are not uniform across the Nation, but depend on factors such as local land-use practices, climate, and topography.

Assessments limited to a single biological community are likely to underestimate the influence of land and water use on stream health.

Integrated assessments of algal, macroinvertebrate, and fish communities revealed twice as many altered streams in agricultural and mixed-use settings compared to singlecommunity assessments. Multicommunity assessments increase the likelihood of detecting reduced stream health because species in different communities have unique vulnerabilities to manmade changes in their physical and chemical surroundings.

Reduced stream health is associated with manmade modifications to physical and chemical factors that often result from land and water use.

Maintenance of stream health requires that physical and chemical properties of streams remain within the bounds of natural fluctuations. Many naturally occurring species have low tolerances to changes in their physical or chemical environment. When physical and chemical properties of streams are shifted beyond their natural ranges, vulnerable species may be eliminated, ultimately reducing stream health.

Manmade modifications to key physical and chemical factors that control stream health are extensive, occurring in all types of land-use settings. Recent assessments by NAWQA, the U.S. Environmental Protection Agency, State agencies, and others have documented the importance of habitat modification, chemical contaminants, and nonnative species in reducing stream health across the Nation. This fact sheet focuses on the importance of modified flows, excess nutrients, and pesticides to the health of streams and rivers.

Streamflow Modification

Annual high or low flows were modified in 86 percent of the streams assessed across the Nation (fig. 2). Natural fluctuations of flows are critical to stream health because they build and maintain physical habitats, influence physical and chemical characteristics of water, and provide important life-stage cues for aquatic organisms. Streamflows are modified by a variety of land- and water-management activities, including reservoir storage and releases, stream diversions, subsurface tile drainage, groundwater withdrawals, wastewater inputs, and removal of vegetated land cover in the watershed.

Biological communities were altered more frequently in streams with modified flows. The occurrence of altered fish communities increased 12–40 percent in streams with increasing severity of depleted high flows (fig. 3). This association between biological alteration and streamflow modification was evident even after controlling for the influence of other factors that affect biological communities, such as nutrients, salinity, and land cover (Carlisle and others, 2010).

Figure 3. The occurrence of altered fish communities increased in streams with increasingly depleted high flows. Reservoirs, diversions, and other manmade changes to streams and their watersheds modify natural streamflows that are critical to the life stages of aquatic animals.

Excess Nutrients

Excess concentrations of nutrients (also known as “nutrient enrichment”) are widespread in the Nation’s streams (Dubrovsky and others, 2010; U.S. Environmental Protection Agency, 2013). A variety of sources can contribute nutrients to streams, such as wastewater and industrial discharges, fertilizer and manure applications to agricultural land, runoff from urban areas, and atmospheric sources. Nutrient sources and resulting concentrations in streams vary across the Nation on the basis of regional differences in agricultural practices, urban land use, and natural factors such as climate and geology (Dubrovsky and others, 2010).

With increasing nutrient enrichment in stream water, the occurrence of altered algal communities increased from 21 to 39 percent (fig. 4). Biological alteration associated with elevated nutrient levels was most pronounced for algal communities, likely because of the direct link between nutrient availability and algal growth and reproduction. Alteration of algal communities is typified by changes in the types of algae—such as diatoms—that are predominant in a stream. Some types of diatoms thrive in streams with elevated nutrients and are referred to as “eutrophic diatoms.” These species were a predominant part of algal communities in streams with elevated nutrients throughout the Nation (fig. 5). Deleterious effects to aquatic animals occur when excess nutrients cause excessive growths of algae and other aquatic plants, which consume oxygen in the water as they grow, die, and decompose. These effects, however, can vary from one stream to another as a result of differences in streamflow, amount of riparian shading, and water clarity (Dubrovsky and others, 2010).

Figure 4. The occurrence of altered biological communities increased in streams with greater nutrient enrichment—a pattern that was most pronounced for algal communities.

Pesticides

Pesticides are detected in stream water in all land-use settings and typically reflect patterns of use in the watershed (Gilliom and others, 2006). Although pesticide concentrations are highly variable seasonally and from year to year, they may reach levels that are potentially harmful to biological communities, particularly in agricultural and urban streams (Gilliom and others, 2006).

The occurrence of altered macroinvertebrate communities increased from 20 to 42 percent in streams with greater potential toxicity of pesticide mixtures (fig. 6). This association is expected because stream macroinvertebrate communities are mainly composed of insects, and the most frequently detected— and potentially toxic—pesticides were insecticides (chlorpyrifos, carbaryl, and diazinon), which are designed to kill insects. After controlling for nutrients, salinity, habitat, and land use, streams with insecticide levels that exceeded aquatic-life benchmarks had 12 percent fewer macroinvertebrate taxa than streams without insecticide exceedances (Yuan and others, 2009). These findings suggest that pesticides contribute to reduced stream health in agricultural and urban streams, and support a growing body of literature documenting the adverse effects of pesticides on aquatic biological communities.

Figure 6. The occurrence of altered macroinvertebrate communities increased in streams where potential toxicity of dissolved pesticide mixtures was higher. Concentrations of widely used insecticides often occur in stream water at levels that are harmful to macroinvertebrate communities, which are dominated by aquatic insects. Baseline for graph was established from 132 streams where no pesticides were detected.

Management Implications

Assessments that are limited to a single biological community are likely to underestimate the effects of land and water use on stream health.

Water quality is not independent of water quantity because flows are a fundamental part of stream health. Because flows are modified in so many streams and rivers, there are many opportunities to enhance stream health with targeted adjustments to flow management.

Efforts to understand the causes of reduced stream health should consider the possible effects of nutrients and pesticides, in addition to modified flows, particularly in agricultural and urban settings.

Stream health is often reduced due to multiple physical and chemical factors. Understanding how these multiple factors influence biological communities is essential in developing effective management strategies aimed at restoring stream health. Improving assessments of stream health

National Aquatic Bioassessment Database

The USGS BioData Retrieval System provides access to biological community and physical habitat data collected by USGS scientists from streams across the Nation. Data from more than 15,000 fish, aquatic macroinvertebrate, and algal community samples can be accessed at https://aquatic.biodata.usgs.gov/.

References

Carlisle, D.M., Wolock, D.M., and Meador, M.R., 2010, Alteration of streamflow magnitudes and potential ecological consequences: A multiregional assessment: Frontiers in Ecology and the Environment, v. 9, p. 264–270.

Dubrovsky, N.M., Burow, K.R., Clark, G.M., Gronberg, J.M., Hamilton P.A., Hitt, K.J., Mueller, D.K., Munn, M.D., Nolan, B.T., Puckett, L.J., Rupert, M.G., Short, T.M., Spahr, N.E., Sprague, L.A., and Wilber, W.G., 2010, The quality of our Nation’s waters —Nutrients in the Nation’s streams and groundwater, 1992–2004: U.S. Geological Survey

Gilliom, R.J., Barbash, J.E., Crawford, C.G., Hamilton, P.A., Martin, J.D., Nakagaki, N., Nowell, L.H., Scott, J.C., Stackelberg, P.E., Thelin, G.P., and Wolock, D.M., 2006, Pesticides in the Nation’s streams and ground water, 1992–2001: U.S. Geological Survey Circular 1291, 172 p.

National Reference Site Database

The USGS and the Western Center for Monitoring & Assessment of Freshwater Ecosystems compiled biological data from reference-quality sites across the Nation. These data can be accessed at http://www.cnr.usu.edu/wmc/htm/data.U.S. Environmental Protection Agency, 2013, National rivers and streams assessment, 2008–2009: A collaborative survey, Draft: U.S. Environmental Protection Agency, Office of Wetlands, Oceans and Watersheds; Office of Research and Development, EPA/841/D-13/001, 110 p., accessed May 8, 2013, at http://water.epa.gov/type/rsl/monitoring/riverssurvey/upload/NRSA0809_Report_Final_508Compliant_130228.pdf.

Yuan, L.L., Pollard, A.I., and Carlisle, D.M., 2009, Using propensity scores to estimate the effects of insecticides on stream invertebrates from observational data: Environmental Toxicology and Chemistry, v. 28, p. 1518–1527.

For Additional Information Diatoms of the United States

This online taxonomic guide and ecological resource provides identification information about diatoms of the United States. Expert contributors continue to add taxonomic information to the database, including high-quality images, morphological descriptions, and key features of each species. For many species, environmental response plots and geographic distribution maps are also included. This guide can be accessed at http://westerndiatoms.colorado.edu.

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