NH3 is the principal form of toxic ammonia. It has been reported toxic to fresh water organisms at concentrations ranging from 0.53 to 22.8 mg/L. Toxic levels are both pH and temperature dependent. Toxicity increases as pH decreases and as temperature decreases. Plants are more tolerant of ammonia than animals, and invertebrates are more tolerant than fish. Hatching and growth rates of fishes may be affected. In the structural development, changes in tissues of gills, liver, and kidneys may also occur. Toxic concentrations of ammonia in humans may cause loss of equilibrium, convulsions, coma, and death.
Ammonia levels in excess of the recommended limits may harm aquatic life. Ammonia toxicity is thought to be one of the main causes of unexplained losses in fish hatcheries. Although the ammonia molecule is a nutrient required for life, excess ammonia may accumulate in the organism and cause alteration of metabolism or increases in body pH. Different species of fish can tolerate different levels of ammonia but in any event, less is better. Rainbow trout fry can tolerate up to about 0.2 mg./l while Hybrid striped bass can handle 1.2 mg/l.
Fish may suffer a loss of equilibrium, hyperexcitability, increased respiratory activity and oxygen uptake, and increased heart rate. At extreme ammonia levels, fish may experience convulsions, coma, and death. Experiments have shown that the lethal concentration for a variety of fish species ranges from 0.2 to 2.0 mg/l. Trout appear to be most susceptible of these fish and carp the least susceptible.
At higher levels (>0.1 mg/liter NH3) even relatively short exposures can lead to skin, eye, and gills damage. Slightly elevated ammonia levels falling within the acceptable range may adversely impact aquatic life. Fish may experience a reduction in hatching success; reduction in growth rate and morphological development; and injury to gill tissue (i.e., hyperplasia), liver, and kidneys. Hyperplasia-the gill filaments are swollen and clumped together, reducing the fish's ability to 'breath'.
Elevated levels can also lead to ammonia poisoning by suppressing normal ammonia excrement from the gills. If fish are unable to excrete this metabolic waste product there is a rise in blood-ammonia levels resulting in damage to internal organs. The fish response to toxic levels would be lethargy, loss of appetite, laying on the pond bottom with clamped fins, or gasping at the water surface if the gills have been affected. Because this response is similar to the response to poor water quality, parasite infestations and other diseases.
Experiments have shown that exposure to un-ionized ammonia concentrations as low as 0.002 mg/l for six weeks causes hyperplasia of gill lining in salmon fingerlings and may lead to bacterial gill disease.