Acid Rain

The community of microscopic algae (or phytoplankton) of lakes is quite diverse in species. Non-acidic, oligotrophic (i.e., unproductive) lakes in a temperate climate are usually dominated by golden-brown algae and diatoms, while acidic lakes are typically dominated by dinoflagellates, cryptomonads, and green algae.

An important experiment was performed in a remote lake in Ontario, in which sulfuric acid was added to slowly acidify the entire lake, ultimately to about pH 5.0 from the original pH of 6.5. During this whole-lake acidification, the phytoplankton community changed from an initial domination by golden-brown algae to dominance by green algae. There was no change in the total number of species, but there was a small increase in algal biomass after acidification because of an increased clarity of the water.

In some acidified lakes the abundance of larger plants (called macrophytes) has decreased, sometimes accompanied by increased abundance of a moss known as Sphagnum. In itself, proliferation of Sphagnum can cause acidification, because these plants efficiently remove cations from the water in exchange for H⁺, and their mats interfere with acid neutralizing processes in the sediment.

Zooplankton are small crustaceans living in the water column of lakes. These animals can be affected by acidification through: (1) the toxicity of H⁺ and associated metals ions, especially Al³⁺; (2) changes in their phytoplankton food; and (3) changes in predation, especially if plankton-eating fish become extirpated by acidification. Surveys have demonstrated that some zooplankton species are sensitive to acidity, while others are more tolerant. In general, higher-pH lakes are richer in zooplankton species. For example, a survey of lakes in Ontario found 9-16 species with three to four dominants at pH greater than pH 5, but only 1-7 species with one to two dominants at more acidic pHs.

In the whole-lake experiment mentioned previously, the abundance of zooplankton increased by 66-93% after acidification, a change attributed to an increase in algal biomass. Although there was little change in dominant species, some less common species were extirpated.

Fish are the best-known victims of acidification. Loss of populations of trout, salmon, and other species have occurred in many acidified freshwaters. A survey of 700 Norwegian lakes, for example, found that brown trout were absent from 40% of the water bodies and sparse in another 40%, even though almost all of the lakes had supported healthy fish populations prior to the 1950s. Surveys during the 1930s in the Adirondack Mountains of New York found brook trout in 82% of the lakes. However, in the 1970s fish did not occur in 43% of 215 lakes in the same area, including 26 definite extirpations of brook trout in re-surveyed lakes. This dramatic change paralleled the known acidification of these lakes. Other studies documented the loss of fish populations from lakes in the Killarney region of Ontario, where there are known extirpations of lake trout in 17 lakes, while smallmouth bass have disappeared from 12 lakes, largemouth bass and walleye from four, and yellow perch and rock bass from two.

Many studies have been made of the physiological effects of acidification on fish. Younger life-history stages are generally more sensitive than adults, and most losses of fish populations can be attributed to reproductive failure, rather than mortality of adults (although adults have sometimes been killed by acid-shock episodes in the springtime).

There are large increases in concentration of certain toxic metals in acidic waters, most notably ions of aluminum. In many acidic waters aluminium ions can be sufficient to kill fish, regardless of any direct effect of H⁺. In general, survival and growth of larvae and older stages of fish are reduced if dissolved aluminium concentrations are larger than 0.1 ppm, an exposure regularly exceeded in acidic waters. The most toxic ions of aluminium are Al³⁺ and AlOH²⁺.

Although direct effects of acidification on aquatic birds have not been demonstrated, changes in their habitat could indirectly affect their populations. Losses of fish populations would be detrimental to fish-eating waterbirds such as loons, mergansers, and osprey. In contrast, an increased abundance of aquatic insects and zooplankton, resulting from decreased predation by fish, could be beneficial to diving ducks such as common goldeneye and hooded merganser, and to dabbling ducks such as the mallard and black duck.

Trees killed by acid rain in the Great Smoky Mountains. JLM Visuals. Reproduced by permission.