<0.1 0.2-0.5 0.6-1.0 1.1-1.5 1.6-2.0 units of diatom inferred pH decrease a

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presence of soot from the burning of fossil fuels supports the industrial origin of the increased acidity (Last 1989). The corollary, that decreased acid emissions from industrial activities should lead to a reduction in lake acidity is perhaps reflected in rising pH values in some lakes in Britain and Scandinavia, but the data are as yet too limited to provide conclusive results (Last 1989; Mason 1990).

Figure 4.8 Aluminium concentration in relation to pH for 20 lakes near Sudbury, Ontario

Figure 4.8 Aluminium concentration in relation to pH for 20 lakes near Sudbury, Ontario

Global Environmental Issues
Source: From Harvey (1989)

Some uncertainties over the relationship between industrial emissions and acid rain will always remain, because of the complexity of the environment, and the variety of its possible responses to any input. There can be no doubt, however, that acid rain does fall, and when it does its effects on the environment are often detrimental.

In addition to reduced pH values, acidic lakes are characterized by low levels of calcium and magnesium and elevated sulphate levels. They also have above-normal concentrations of potentially toxic metals such as aluminium (Brakke et al. 1988)(see Figure 4.8). The initial effect of continued acid loading varies from lake to lake, since waterbodies differ in their sensitivity to such inputs. Harmful effects will begin to be felt by most waterbodies when their pH falls to 5.3 (Henriksen and Brakke 1988), although damage to aquatic ecosystems will occur in some lakes before that level is reached, and some authorities consider pH 6.0 as a more appropriate value (Park 1987). Whatever the value, once the critical pH level has been passed, the net effect will be the gradual destruction of the biological communities in the ecosystem.

Most of the investigations into the impact of acid rain on aquatic communities have involved fish populations. There is clear evidence, from areas as far apart as New York State, Nova Scotia, Norway and Sweden, that increased surface water acidity has adverse effects on fish (Baker and Schofield 1985). The processes involved are complex and their effectiveness varies from species to species (see Figure 4.9). For example, direct exposure to acid water may damage some species. Brook trout and rainbow trout cannot tolerate pH levels much below 6.0 (Ontario: Ministry of the Environment 1980), and at 5.5 smallmouth bass succumb (LaBastille 1981). The salmonid group of fish is much less tolerant than coarser fish such as pike and perch (Ontario: Ministry of the Environment 1980). Thus as lakes become progressively more acid, the composition of the fish population changes.

The stage of development of the organism is also important (see Figure 4.10). Adult fish, for example, may be able to survive relatively low pH values, but newly hatched fry, or even the spawn itself, may be much less tolerant (Ontario: Ministry of the Environment 1980). As a result the fish population in acid lakes is usually wiped out by low reproductive rates even before the pH reaches levels which would kill mature fish (Jensen and Snekvik 1972).

Fish in acid lakes also succumb to toxic concentrations of metals, such as aluminium, mercury, manganese, zinc and lead, leached from the surrounding rocks by the acids. Many acid lakes, for example, have elevated concentrations of aluminium (Brakke et al. 1988), which has been recognized as a particularly potent toxin (Cronan and Schofield 1979). The toxic effects of aluminium are complex, but in lakes where the pH has fallen below 5.6 it is commonly present in sufficient quantity to kill fish (Stokes et al. 1989). The fish respond to the presence of the aluminium by producing mucus which clogs the gills, inhibiting breathing and causing a breakdown of their salt regulation systems

Figure 4.9 The impact of acid rain on aquatic organisms population structure

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Survival Basics

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