Wednesday, March 9, 2011

Essay on Acid Rain

Short Essay on Acid Rain and its Effects

Humans produce about 160% of the natural emissions of sulphur. This is compared with about 5-10% in the case of carbon dioxide and nitrogen (Krebs, 2001). Combustion of fossil fuels, according to Krebs (2001) has ‘altered the sulphur cycle more than any of the other nutrient cycles’.

Sulphur is quickly oxidized in the atmosphere to sulphate (SO4) and redeposited rapidly on land or in the oceans. One clear manifestation of this alteration of the sulphur cycle is the widespread problem of acid rain (Krebs, 2001).

In areas uncontaminated by either industrial emissions or calcareous dust, precipitation usually has a pH value close to 5.0 (Schindler, 1998). Distilled water, which contains no carbon dioxide, has a neutral pH of 7.0. Liquids with a pH less than 7.0 are acid, and those with a pH greater than 70 are alkaline ( Uncontaminated rainfall usually has a slightly acidic pH because it contains small amounts of both weak and strong acids of natural origin. In most areas within several hundred kilometers of industrial emissions precipitation has a pH value of much less than 5.0 (Schindler, 1998). Around Washington D.C. for example, average rain pH is between 4.2 and 4.4 (


Schindler (1998) says; ‘it is now clear that acid rain has already caused widespread acidification of many aquatic ecosystems in the north-eastern United States, Canada, Norway, Sweden, and the United Kingdom’. Many fish invertebrates are sensitive to acidification, with some disappearing at pH values as high as 6.0 (Schindler, 1998).

Aquatic environments vary widely in their sensitivity to acidification. When water moves through limestone or dolomite rocks and soils, it picks up calcium and magnesium carbonates. These minerals neutralize incoming acids. The result is that streams, rivers, and lakes with high carbonate concentrations have a large neutralizing capacity and typically are not susceptible to acidification. In areas where water moves through carbonate-poor rocks and soils, streams, rivers, and lakes have a lower neutralizing ability and become acidified when the acidic materials in them exceed the neutralizing capacity. This results in a drop in pH (May and Eilers, 1987).

Waters with differing pH may be similar in appearance, yet the pH differentials influence the organisms within them rather significantly (May and Eilers, 1987). Acid rains’ earliest and most dramatic effects on fish populations were recorded in southern Norway in the 1920’s, where declines in salmon runs were correlated to increasing river acidity. Today, seven formerly important salmon streams in southern Norway have lost their runs, and a survey of over 2,000 lakes indicated that approximately one-third experienced fish population declines between 1940 and 1971 (Swenson and May, 1987).

The loss of fish in North America was first seen in the La Cloche Mountain lakes of Ontario. These lakes received acid deposition from a large copper-nickel smelter. In the 1970’s studies showed that 388 fish populations might have been lost from some 55 of these lakes. However, it was not possible to tell if changes in pH alone caused this because besides becoming acidic, these lakes showed increases in heavy metals from the smelter that can also harm fish. More recent research has documented the loss of fish populations from a smaller number of acidified lakes not influenced by the smelter (Swenson and May, 1987).

According to Krebs (2001), ‘the clearest effects of acid precipitation are in Scandinavia and eastern Canada’. He notes that ‘In Canada, lakes containing trout have been the principle focus for research on the effects of acid rain. Lake trout disappear in lakes once the pH falls below 5.4’.

In 1985 Schindler and his colleagues described the effects of eight years gradual, manipulated acidification on a small lake ecosystem in Ontario. The value of pH was slowly decreased from 6.8 to 5.0. One of the parameters used to monitor the lake ecosystem was the condition of trout. There were few distinguishable chemical changes and little biological change in the baseline survey between 1974 and 1976. In 1980 the condition of the lake trout had declined. The trout continued to be affected; spawning behavior changed in 1982 and in 1983 their condition was very poor. There was also evidence of cannibalism amongst the trout (Spellerberg, 1991).

Several studies have identified direct effects on gill function, reproduction, and early life stages of fish in aquatic environments. Special cells in the gills of fish, called ionocytes or chloride cells, control the concentration of chemicals in the blood. They collect ions such as chloride and sodium from the water, and transfer them to the blood where they must be kept at higher concentrations. Changes in these cells seem to be one way by which acidification affects fish populations (Swenson and May, 1987).

The cells become pitted as pH decreases. Chloride cells can be destroyed as a thick slime may cover the gill surface. Fish can no longer keep the excess hydrogen ions, formed by acidity, out of the blood. Sodium and Chloride ions can’t be retained either. Young fish are more sensitive to this than adult fish; for example adult brook trout may survive at pH 3.5 to 4.5, whereas brook trout fry may die at pH 4.5 to 6.5 (Swenson and May, 1987)

The combination of acidic water and metal contamination is a special problem for the sensitive embryo and fry stages of some species. Brook trout and Atlantic salmon are good examples. The development period coincides with the spring snowmelt that brings the lowest pH readings. If acid melt waters dissolve aluminium from the soils, the fry can be killed by the combination of the stresses even when the waters acidity alone would not kill them. The combined effects of aluminium concentrations and acidity may kill the young but not the adults. A reproductive failure like this results in a slow population decline over time, as with the example of lake trout (Swenson and May, 1987).

Reduced levels of fish reproductive hormones have also been associated with low pH. Female fish in one study of acidified waters showed a higher percentage of eggs that do not develop and are reabsorbed (Swenson and May, 1987).

Sensitivity to acidification differs enormously among fish species. Some evidence suggests that in some cases sensitivity relates to the indirect effects of acidification. These include the formation of dense mats on the lake bottom, destroying plants and other organisms required as food (Swenson and May, 1987).

The occurrence of higher concentration of mercury in fish from acid lakes is an important indirect effect of lake acidification. Some scientists believe that higher mercury levels require a greater level of feeding from the fish. This can be a problem in low pH lakes where food is scarce (Swenson and May, 1987).

Most biological research has emphasised the impact on fish. However, fish depend on other organisms for food and shelter. More recent research has focused on the impact of acid deposition on these aquatic organisms, including invertebrates (May and Eilers, 1987). Schindler (1998) says, ‘the early disappearance of organisms at lower trophic levels may cause starvation to stress large predatory fish well before direct toxic action of the hydrogen ions is evident’.

Schindler’s 1985 report into a manipulated ecosystem described earlier suggests that more common fish species (such as lake trout) are not sensitive, reliable indicators of early damage due to acidification. The damage at lower trophic levels would, it was predicted, cause almost complete extinction of the trout within a decade (Spellerberg, 1991).

As suggested earlier, organisms have a ‘range of tolerance’ that is broader for those more tolerant of acidity and narrower for those more sensitive. Chemical reactions critical to maintain life take place most efficiently at the optimum pH. How organisms are affected depends upon their size, life history stage, time of year, and other factors. Such direct effects of increasing acidity are, in general, more harmful to the simpler components of the ecosystem. More mature of complex organisms are increasingly capable of regulating their internal body chemistry (May and Eilers, 1987).

Many invertebrates, such as zooplankton, that swim in the open water of lakes, are microscopic. Just like fish these are also subject to the effects of acidification. May and Eilers (1987) of the University of Wisconsin revealed several patterns after an extensive review of North American and European studies:

  1. There are fewer invertebrate species found in lower pH waters. This reduction is most noticeable over the pH interval from 6.1 to 5.2.
  2. Some groups of organisms are particularly sensitive to acidic conditions. These organisms could give early indications of acidification.
  3. Species within these groups indicate large variability in response to acidity, as do some individuals within certain species.

Many species of organisms would disappear before most fish are affected. Though tolerance levels vary widely, the number of surviving species declines noticeably as water becomes more acidic (May and Eilers, 1987).

As a summary it is clear that acidification affects both fish and invertebrates, directly and indirectly. Some species have a higher tolerance to acidic environments than others, and some organisms within a population have a higher tolerance than others within the same population.

Attention must now turn to reducing acid rain, and the recovery of lakes and streams. The recent rate of acidification of lakes is slower than once predicted, in part the result of decreased sulphur oxide emissions. In the Sudbury area of eastern Canada a combination of smelter closures and SO2 controls have reduced emissions to about one-third of their value in the early 1970’s. This has been accompanied by rapid increases in alkalinity, and decreases in the concentrations of SO4Іˉ and aluminium and toxic metals (Schindler, 1998).

It is not clear whether lakes will be able to recover completely. If recovery is possible it may take many years. Schindler (1998) ends one part of his paper by saying; ‘Although it is now clear that reducing emissions of SO2 will allow the rapid recovery of lakes, it is unlikely that original pH values will be reached for many years. Unassisted biological recovery of all original species also appears to be unlikely… It therefore seems prudent to prevent as much ecological damage as we can’.

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