Most organisms have a well defined range of pH
tolerance. If the pH falls below the tolerance range, death will
occur due to respiratory or osmoregulatory failure (Kimmel, 1983). Low
pH causes a disturbance of the balance of sodium and chloride ions in
the blood of aquatic animals. At low pH, hydrogen ions may be taken
into cells and sodium ions expelled (Morris et al., 1989). Mayflies
are one of the most sensitive groups of aquatic insects to low pH; stoneflies
and caddisflies are generally less sensitive to low pH. Mayflies and
stoneflies that normally live in neutral water experience a greater
loss of sodium in their blood when exposed to low pH than do acid-tolerant
species of stoneflies, such as Leuctra and Amphinemura, whose sodium
uptake is only slightly reduced by low pH (Sutcliffe and Hildrew, 1989).
Acid waters typically have fewer species and a lower abundance and biomass
of macroinvertebrates than near-neutral pH waters. Attempts have been
made to specifically identify limiting factors, and two factors investigated
are interruption of the food chain and direct effects of low pH levels
on aquatic life. Macroinvertebrates are often grouped by their feeding
habits, and assemblages of invertebrates in acidified waters appear
to be related to food availability. The fauna
of low pH streams is usually composed of shredders (organisms that eat
leaves that fall into the stream), collectors (organisms that filter
or gather particles of organic matter from the water), and predators.
Low pH tends to eliminate species that feed on algae (scrapers or grazers).
Low pH may inhibit growth of bacteria which help break down leaves to
make them more easily digestible and which also serve as a food source.
These observations led early investigators to theorize that low pH levels
reduced the food sources for invertebrates, thereby indirectly reducing
their numbers. This is partially true; however, more recent studies
have shown that direct effects of low pH on aquatic life are more critical
than indirect effects on food sources (Rosemond et al., 1992).
Cooper and Wagner (1973) studied the distribution of fish in Pennsylvania
streams affected by acid mine drainage. They found fish species were
severely impacted at pH 4.5 to 5.5; ten species showed some tolerance
to pH 5.5 or less; 38 species were found at pH 5.6 to 6.4; and 68 species
were found only at pH greater than 6.4 (Table 4.1). They found that
a pH of 4.5 and total acidity of 15 mg/L accounted for complete loss
of fish in 90% of streams studied. Although no concentrations of metals
were taken into account, Cooper and Wagner indicated that the absence
of fish in acidified waters can be related to dissolved metals at certain
pH levels. They also indicated that sulfates,
a major constituent of acid mine drainage, did not become toxic to fish
until concentrations exceeded the saturation level of several thousand
mg/L.
The primary causes of fish death in acid waters is loss of sodium ions
from the blood and loss of oxygen in the tissues (Brown and Sadler,
1989). Acid water also increases the permeability of fish gills to water,
adversely affecting gill function. Ionic imbalance in fish may begin
at a pH of 5.5 or higher, depending on the tolerance of the species;
severe anoxia will occur below pH 4.2 (Potts and McWilliams, 1989).
Low pH that is not directly lethal may adversely affect fish growth
rates and reproduction (Kimmel, 1983).
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