Nutrients are elements that are essential for plant growth, including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), sulfur (S), and silicon (Si). N, P, and K are considered primary nutrients, and N and P are the major limiting nutrients in most aquatic environments.

When considering candidate causes, N and P are evaluated to determine the trophic status (or relative nutrient condition) of freshwater systems. In lakes, streams, large rivers, and estuaries, trophic status may be expressed in terms of oligotrophy (low nutrients, minimally productive), mesotrophy (moderate nutrients, moderately productive), or eutrophy (high nutrients, highly productive).

In most cases, nutrients are not proximate stressors for aquatic communities. Although certain forms of N [i.e., unionized ammonia (NH₃), nitrite (NO₂^-) and, in some cases, nitrate (NO₃^-)] may be toxic, these effects are considered in the ammonia and unspecified toxic chemicals modules. Nutrients have indirect adverse effects on aquatic communities through their effects on primary production, the growth and accumulation of plant and algal biomass, and the species composition of algae (i.e., phytoplankton in lakes or periphyton in streams) and other plant assemblages (Figures 1 and 2); Dodds and Welch 2000).

Increasing primary production and changes in plant species composition can be proximate causes of effects on consumers (i.e., macroinvertebrates and fish) by:

• Altering food resources: the amount of food resources, their type (e.g., living plant and algal biomass versus detritus),or their palatability (e.g., changes in cell size in algae for filter-feeding animals);
• Altering habitat structure: changes in benthic interstitial space, ease of movement across benthic surfaces or through the water column and availability of macrophytes as habitat for some species and life stages; and
• Algal toxins: some algae characteristic of eutrophic conditions can be toxic to fish, invertebrates, and even humans.

These increases in primary production and plant and algal biomass can affect other physical and chemical characteristics of the water body, such as pH and dissolved oxygen, which in turn may be proximate stressors for the aquatic community. Algal growth and biomass accumulation associated with excess nutrients also hinder recreation, fishing, hunting, and aesthetic enjoyment of waterbodies and may interfere with drinking water treatment and use of water by industrial facilities.

Checklist of sources, site evidence and biological effects

This module addresses excess nutrients as a stressor leading to plant assemblage changes and other proximate stressors (e.g., low dissolved oxygen); toxic effects of nutrients are considered in the ammonia and unspecified toxic chemicals modules. Nutrients should be listed as a candidate cause when potential human sources and activities, site observations, or observed biological effects support portions of the source-to-impairment pathways in the conceptual diagram for nutrients (Figure 3). This diagram and some of the other information also may be useful in Step 3: Evaluate Data from the Case.

The checklist below will help you identify key data and information useful for determining whether to include nutrients among your candidate causes. The list is intended to guide you in collecting evidence to support, weaken, or eliminate excess nutrients as a candidate cause. For more information on specific sources and activities, site evidence, and biological effects listed in the checklist, click on checklist headings or go to the When to List tab of this module.

Consider listing nutrients as a candidate cause when the following sources and activities, site evidence, and biological effects are present:

Consider these commonly associated candidate causes when listing nutrients as a candidate cause:

• Dissolved oxygen
• Temperature
• Suspended and bedded sediments
• pH
• Ammonia toxicity
• Pathogens
• Co-migrating contaminants

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