USGS NAWQA

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The U.S. Geological Survey’s National Water-Quality Assessment (NAWQA) Program is assessing the effects of nutrient enrichment (nitrogen and phosphorus) on stream ecosystems in agriculturally dominated landscapes. This study provides nationally consistent and comparable data and analyses of nutrient conditions, including how these conditions vary as a result of natural and human-related factors, and how nutrient conditions affect biological communities and ecological processes in streams. More information on the study is available in USGS Fact Sheet 118-03.

The national study includes eight agriculturally influenced study units that encompass a diversity of agricultural practices including crop types, irrigation practices, and fertilizer application. The streams themselves range from channelized open systems with agricultural practices immediately adjacent to the stream margin, to canopy covered natural streams with extensive riparian corridors.

Information from this study will benefit many stakeholders, including the U.S. Environmental Protection Agency (USEPA), States and partners involved in the development of nutrient criteria to protect the aquatic health of streams in different geographic regions. Findings will also benefit the U.S. Department of Agriculture, along with local, State, and regional resource and conservation managers who deal with ever changing demands on agricultural lands and water resources.

Objectives of the nutrient effects study

1. Determine the interaction of nutrients and biological communities (algae, invertebrates, and fish) in streams, and how these interactions are modified by the agricultural setting, including watershed characteristics, stream habitat, and other natural factors.
2. Assess the interrelations among nutrients, algal biomass, and stream metabolism, defined as the gain and loss in dissolved oxygen associated with photosynthesis and plant and animal respiration.
3. Determine the transport, chemical transformation, and retention of nutrients (referred to as "nutrient cycling") in the water column and surrounding sediments.
4. Test existing ecological indicators of nutrient enrichment and develop new models for the prediction of ecological effects.

Why are nutrients important?

Nitrogen and phosphorus are essential for healthy plant and animal populations in streams, with each water body requiring a specific balance of nutrients to maintain aquatic life. However, elevated concentrations of nutrients can lead to excessive, often unsightly, growth of aquatic plants. Excessive aquatic plant growth reduces dissolved oxygen in water and alters stream habitat, both of which are critical for fish and other aquatic life. These problems occur over the full range of surface waters, from small streams to receiving coastal waters where they can threaten fish and shellfish that are economically and ecologically important. Overgrowth of aquatic plants can clog water-intake pipes and filters and can interfere with recreational activities, such as fishing, swimming, and boating. Subsequent decay of aquatic plans can result in foul odors and taste. An improved understanding of the effects nutrient enrichment in small streams is particularly important because these systems can be particularly effective at processing nutrients along with being highly sensitive to nutrient enrichment.