CADDIS Volume 2: Sources, Stressors & Responses
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Authors: G.W. Suter II, S.M. Cormier,
K. Schofield, M. Bowersox, H. Latimer
Ammonia (NH3) is a common toxicant derived from wastes (Figure 1), fertilizers, and natural processes. Ammonia nitrogen includes both the ionized form (ammonium, NH4+) and the unionized form (ammonia, NH3). An increase in pH favors formation of the more toxic unionized form (NH3), while a decrease favors the ionized (NH4+) form. Temperature also affects the toxicity of ammonia to aquatic life. Ammonia is a common cause of fish kills, but the most common problems associated with ammonia relate to elevated concentrations affecting fish growth, gill condition, organ weights, and hematocrit (Milne et al. 2000). Exposure duration and frequency strongly influence the severity of effects (Milne et al. 2000).
Ammonia in sediments typically results from bacterial decomposition of natural and anthropogenic organic matter that accumulates in sediment. Sediment microbiota mineralize organic nitrogen or (less commonly) produce ammonia by dissimilatory nitrate reduction. Ammonia is especially prevalent in anoxic sediments because nitrification (the oxidation of ammonia to nitrite [NO2-] and nitrate [NO3-]) is inhibited. Ammonia generated in sediment may be toxic to benthic or surface water biota (Lapota et al. 2000).
Ammonia also exerts a biochemical oxygen demand on receiving waters (referred to as nitrogenous biological oxygen demand or NBOD) because dissolved oxygen is consumed as bacteria and other microbes oxidize ammonia into nitrite and nitrate. The resulting dissolved oxygen reductions can decrease species diversity and even cause fish kills. Additionally, ammonia can lead to heavy plant growth (eutrophication) due to its nutrient properties (see the Nutrients module). Conversely, algae and macrophytes take up ammonia, thereby reducing aqueous concentrations.
This module addresses ammonia as a proximate stressor with a toxic mode of action (its nutrient properties are considered under Nutrients). It 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 ammonia (Figure 2). 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 ammonia among your candidate causes. The list is intended to guide you in collecting evidence to support, weaken, or eliminate ammonia 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 ammonia as a candidate cause when the following sources and activities, site evidence, and biological effects are present:
- Municipal waste treatment outfalls
- Septic seepage
- Industrial point sources
- Agricultural and urban runoff (fertilizer)
- Manure application
- Concentrated animal feeding operations
- Landfill leachate
- Atmospheric sources
- Riparian devegetation
- Slow-moving or stagnant water
- High density of fish
- Presence of organic waste
- Foul odor
- Presence of organic suspended solids or floc
- Alkaline, anoxic, or warm water
- High plant production (e.g., algal blooms)
- Reduction or absence of ammonia-sensitive species
- Physiological effects (e.g., decreased nitrogen excretion, decreased oxygen binding to hemoglobin)
- Behavioral effects (e.g., loss of equilibrium, hyperexcitability, increased breathing)
- Morphological effects (e.g., proliferation of gill lamellae, reduction of lymphoid tissue in the spleen, lesions in blood vessels, mucus secretion)
- Organismal and population effects (e.g., decreased growth and abundance, mass mortality)
Consider contributing, modifying, and related factors as candidate causes when ammonia is selected as a candidate cause:
- Temperature: Toxicity of ammonia (as total ammonia) increases with an increase in temperature (U.S. EPA 1999).
- pH: The concentration and resulting toxicity of NH3 increases as pH increases, although less NH3 is required to produce toxic effects at lower pH (IPCS 1986, Wurts 2003).
- Dissolved oxygen: Oxygen is consumed as NH3 is oxidized (nitrification), and low oxygen levels increase NH3 levels by inhibiting nitrification.
- Season: Total ammonia-nitrogen concentration in surface waters tends to be lower during summer than during winter, due to uptake by plants and decreased ammonia solubility at higher water temperatures (IPCS 1986).
- Ionic strength: Tolerance to NH3 can increase with an increase in ionic strength or salinity (Sampaio et al. 2002).
- Sediments: Fine sediments tend to generate ammonia due to low oxygen levels and high organic matter.
Consider not listing (eliminating) ammonia as a candidate cause when you have evidence from your site:
- Ammonia concentrations measured continuously over time at the site show that concentrations are the same as or less than ammonia at sites where biological impairment is not observed (lack of co-occurrence).
- All life stages of ammonia-sensitive species are present at the site.