Effects of Toxic Chemicals and Other Pollutants on Aquatic Ecosystems - Freshwater Ecology: Concepts and Environmental Applications - page 277

When acid precipitation reaches the ground, it can react with the ter-
restrial ecosystem. If sufficient base is present, the acid will be neutralized.
The ability of a soil or water body to absorb acidity without a change in
pH is called buffering capacity . The most common material that confers
the ability to resist changes in pH is the bicarbonate in limestone. The bi-
carbonate equilibrium (discussed in Chapter 12) leads to neutralization of
the acid and release of CO 2 . Watersheds and aquatic systems that have a
significant amount of limestone have a high buffering capacity and are able
to resist the effects of acid precipitation.

Biological Effects of Acidification
Acid rain has major effects on biological systems ranging from altered
microbial activity to the ability of fish to survive and reproduce (Table
14.2). Habitats that are naturally acidic include acid peat bogs ( Sphagnum
bogs) and blackwater swamps (Benner et al., 1989). Lakes and streams in
watersheds dominated by such bogs or swamps can be relatively acidic.
Some geothermal springs are very acidic and have very distinct microbial
communities associated with them. Much of our understanding of the ef-
fects of long-term acidification on aquatic ecosystems derives from the
study of naturally acidic habitats. Amazingly, an iron-oxidizing archae-
bacterium isolated from acid mine drainage has been demonstrated to have
the ability to grow at pH 0 (Edwards et al., 2000).
One of the basic ecosystem influences of acidification is the lowered
rate of decomposition mediated by microbes. Microbes from the naturally
acidic Okefenokee Swamp are able to metabolize low-molecular-weight
carbon compounds at rates comparable to those of nearby neutral wet-
lands (Benner et al., 1989). However, the microbes in acidic habitats are
less able to metabolize recalcitrant cellulose and lignin, although some de-
gree of adaptation to the acids does occur (Fig. 14.7). Inhibition of micro-
bial activity by low pH leads to greater rates of deposition of organic ma-
terial and may partially explain the stable existence of acidic depositional

TABLE 14.2 Influences of Decreasing pH on Several Groups of Aquatic

Organisms a

Approximate

Organism or process pH value

Most mayflies disappear 6.5

Phytoplankton species decline/green filamentous periphyton dominate 6

Most mollusks disappear 5.5-6

Waterfowl breeding declines 5.5

Bacterial decomposition slows/fungal decomposition predominates 5

Salmonid reproduction fails/aluminum toxicity increases 5

Most amphibia disappear 5

Caddis flies, stoneflies, and Megaloptera disappear 4.5-5

Beetles, bugs, dragonflies, and damselflies disappear 4.5

Most adult fish harmed 4.5

a Modified from Jeffries and Mills (1990).