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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Suggested Citation:"8 Acid Precipitation." National Research Council. 1981. Atmosphere-Biosphere Interactions: Toward a Better Understanding of the Ecological Consequences of Fossil Fuel Combustion. Washington, DC: The National Academies Press. doi: 10.17226/135.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

8 . AC ID PREC I PITATION The deposition of acid from the atmosphere was recognized in President Carter's second environmental message to the U.S. Congress on August 2, 1979, as "one of the most serious global pollution problems associated with fossil fuel combustion," rivaled only by the buildup of carbon dioxide in the atmosphere. The topic, generally known as "acid rain," has been of much international concern because acids are deposited far from the sources of their precursors. Acid precipitation was the subject of a major international symposium in 1975, when research on many aspects of the topic was in its infancy. Since that time, many studies have been completed, including an enormous amount of work in Scandinavia. Recent symposia (Drablos and Tollan 1980, Hutchinson and Havas 1980, Shriner et al. 1980) treat many aspects of the problem in much more detail than previously, and some long-term studies of effects allow a much more conclusive summary of the problem. We can thus learn much from a case study of acid precipitation about the problems likely to be encountered with other atmospheric pollutants. CAUSES OF ACID PREC IPITATION It is thought that the pH of "pure" rain is controlled by the weak acid, carbonic acid (H2CO3), resulting from atmospheric CO2 in solution. The resulting pH would be near 5.6. When alkaline dust and ocean sea spray are taken into account, the pH of precipitation must be higher than pH 5.6, probably around pH 7.0 (Oden 1976~. But this theory is no longer testable, because global pollution of the atmosphere with sulfur compounds from fossil fuel combustion and the smelting of metalliferous sulfide ores has existed for decades, if not centuries, as shown by analyses of polar icecaps (Figure 8.1~. It seems likely that under unpolluted conditions, small releases of sulfur and nitrogen oxides from volcanic acitivity and microbial metabolism to the biosphere would cause slightly lower natural pH levels than the theoretical level in geologically unbuffered areas, and that wind-carried calcareous dust would cause pH levels higher 140

141 (Ji/4M~ z~Gl) a;~ UO!~onpoJd 0 0 0 00 1 O c~ -_x_ ~ac u, a) ~ Q C' ~n 3 c o X O o __ XX Xxx ~X ~_0 X <) o o o o o ~ X~ - O oX X o o o o o Q o O O O 0N o O O O O r<) c~ (~/6~) al0llns 110s ~as -UoN = (10) 910 -(tOS) o a~ ~ £ o o _ ._ o .= 3 £ ~4 C~ ~ ~ ;> C) ~ ~ ._ o . ~ £ ~ ~) — ~ Cr. C,) C~ Ct ~ s:~ °~ ~ a°~ —o C~ _ Ct ~ ~ C~ a.~ S~ ¢ ao ~° C.) C~ ~ o C 3 ~ ~ ° ~ Ct ._ . ·U, C ~ t4 o4 .= o C) U. C) ° C~ C) Ct ~ o~ a~= o cd cd ~ "s ~ :~ ~ c) - ~ o ct ~ c) o - ~ ~ o ~ ~ c=-s V) ~ ~L r~ . oo s: `3 ~ ~ o ~ - - ~ ~ £ ~ ~ ~ :S

142 than 5.6 in areas where calcareous rocks and soils are plentiful. It is now clear that precipitation is far more acid than theoretically pure rain in regions to which prevailing winds carry oxides of sulfur and nitrogen from heavily industrialized areas (Oden 1968, Dovland and Semb 19801. Direct cause-and-effect linkages between sources of acid and effects on ecosystems will not be possible in the foreseeable future, owing to the remoteness of sources and the complexity of the interaction among emissions from different sources, atmospheric transport, chemical transformations, and specific orographic and geological settings (Table 8.1~. But the increased emission of sulfur and nitrogen compounds from anthropogenic sources is the only plausible explanation for acid deposition. The oxides of these elements~appear to be oxidized further in the atmosphere to form the strong acids, sulfuric acid (H2SO4) and nitric acid (HNO3), which contaminate wet precipitation and atmospheric aerosols (Figure 8.2~. The dry deposition of ammonium sulfate aerosols, which are then converted to sulfuric acid in ecosystems through biological processes, may also contribute substantially to the problem (Oden 1976~. While scientists are in general agreement that industrial emissions of sulfur and nitrogen oxides have caused the contamination of precipitation with strong mineral acids, the timing of the increase and the present rate of increase in acidity are matters of some dispute. Several authors have claimed that the acidity of precipitation has increased rapidly in the past few decades (for example, Cogbill and Likens 1974, Oden and Ahl 1970, Dickson 1975; see Figure 8.3~. This evidence has been disputed by others, who claim that the apparent increase in acidity of precipitation is due to methodological changes (Hansen et al. 1981~. A number of changes in the emissions of acid precursors have taken place over the past few decades, which may influence the acidity of precipitation. While SO2 emissions have not changed greatly for several decades, owing to a switch from coal to other fuels and to increased control of gaseous sulfur emissions, there was unquestionably a great increase in anthropogenic sulfur emissions in the 20th century, causing increased sulfate deposition in remote regions (see Figure 8.1~. Despite the relative constancy of annual SO2 emissions during the past century, three technological changes may cause the emissions to produce acid precipitation more efficiently. First, the height at which gases are injected into the atmosphere has increased nearly threefold (Fig. 8.4), causing SO2 to be transported farther and to remain in the atmosphere longer, increasing the probability of oxidation to sulfuric acid. Second, recent controls of particulate emissions have reduced the amount of alkaline fly ash discharged from smokestacks, and it is conceivable that in the past such material partially neutralized acid emissions. Third, there has been a gradual change from seasonal to year-round emission. Less coal is used for space heating and more coal is used for generating electricity. Also the demand for electricity during the summer months

143 TABLE 8.1 Factors affecting the vulnerability of an ecosystem to acid rain A. Anthropogenic 1. Spatial and temporal patterns of urban/industrial development 2. Kinds and amounts of energy resources in use 3. Controls on atmospheric emissions 4. Degree of agricultural activity (cultivation, liming, fertilization) B. Geologic 1. Nature of bedrock, as regards both basic minerals and acid-soluble toxic metals 2. Patterns of glaciation 3. Depth, texture, mineralogy, and organic content of soil C. Climatic Amount of precipitation Atmospheric humidity, as it affects gas absorption and particle collision 3. Direction and speed of winds and air-mass movements 4. Temperature, especially as it affects the proportions of rain and snow, and rates of chemical reaction in the atmosphere Ratio of precipitation to evaporation, as it affects leaching and the residence time of water in lakes D. Topographic 1. 2. Altitude, as it influences soil depth, precipitation, etc. Order of streams and lakes in the hydrologic network 3. Lake depth and ratio of watershed area to lake area, controlling residence time of water E. Biotic 1. Height, type, and duration of leaf canopy 2. Magnitude of transpiration 3. Sensitivity of critical species, including the microbes mediating biogeochemical cycles F. Natural, episodic 2. 4. 1. Volcanoes, producing locally acid rain Fires in deposits of fossil fuel such as coal or lignite Forest fires, entraining alkaline particulates into the atmosphere Dust storms, entraining alkaline soil particles into the atmosphere

144 IV C) o/ I 2' ._ .= 'a J N O O C/' C/) O ~ o ~ .m o 4— Cot ._ 1~ 1° \ Cal \ O \ c o CD —\ ._ X o o Cat ._ C) Cal Cal o Cal Cal ._ o o ._ en o . ~ ~ \ O ~ O en _ O an \ - ~n~ _~ 4) Go -

145 6-.0 Q 5.8 5.4 pH 5.0 4.6 4.2 5.8 5.4 pH 5.0 4.6 4.2 5.8 5.4 pH 5.0 5.0 4.2 I I 1 1 1955 1960 1965 1970 YEAR (a) — Roba~ksdalen I_. Kise 5.8 5.4 5 . _ Flahult ~ I (Jonkoping) 42 _ A_ 5.8 5.4 5.0 4.6 4 2 5.4 5.0 4.6 - 1 1'.\ ~ 4.6 4.2 1 1 ~ —~ 1 1955 1960 1965 1970 YEAR (b) FIGURE 8.3 The pH levels of precipitation in Scandinavia, 1955-1975. SOURCES: (a) Dickson (1975); (b) Oden and Ahl (1970). /li ~ Pionninge ~ (Halmstad) - T~ ~ .~- Smedby it_ . I I I 1 1955 1960 1965 1970 YEAR

1~76 1 400 - ~ 1000 c o ID o ~3 al 600 - - ._ I 200 , —— ,, Tallest , ~ , , ~ , \ , \. ~ ~ ~ ~ _ l I; \, ~ stack - Average stack height ,........... 1956 1960 1964 1968 1972 1976 FIGURE 8.4 Average stack height and tallest stack reported among power plants burning fossil fuels (bituminous coal, lignite, oil) included in biannual design surveys of new power plants, 1956-1978. SOURCE: Patrick et al. (1981). Reprinted with per- mission from Science 211:446448. Copyright O 1981 by the American Association for the Advance- ment of Science.

147 has grown with the increased use of air conditioning. The high temperatures and humidities in summer may result in more efficient oxidation of SO2 emissions to sulfuric acid. In addition to the technological changes in SO2 emission, there has been an increase in emission of nitrogen oxides for the past few decades (see Table 4.2~. Nitrogen oxides are emitted from a wide variety of sources, with some injected high into the atmosphere while others are ejected and dispersed at ground level through motor vehicle use. In the absence of control technology for nitrogen oxides, their emissions will exceed emission of sulfur oxides by the turn of the century. We stress that emission of nitrogen and sulfur oxides and the consequent acid precipitation are broad regional rather than global problems. When natural and anthropogenic emissions are compared on a regional basis, it is clear that man's acitivities completely overwhelm natural sources of SO2 and NOX, (see Table 4.1), even though the magnitude of anthropogenic emissions of these oxides may seem unimportant when compared with natural emissions on a global scale. The observed recent increases in lake acidity could have resulted either from a rapid increase in acid precipitation in recent time or from long-term, constant acid precipitation over several decades duration. It is difficult to differentiate between these two possible patterns because of the nature of the bicarbonate buffering curve. As a solution of bicarbonate--such as a lake--is titrated by a constant addition of strong acid there is little resulting change in pH until 80 to 90 percent of the bicarbonate has been consumed according to the reaction: H+ + HCO3 -, H2CO3 ~ H2O + CO2 Once the bicarbonate has been converted into carbon dioxide and lost to the atmosphere, the acid (hydrogen ions) accumulates and the pH decreases rapidly (Figure 8.5~. The relatively sudden drop in pH to an acid condition is the same regardless whether the titration (the addition of acid) occurred at a fast or slow rate. The lake will become acidic so long as the rate at which acid is added to the lake exceeds the rate at which geochemical weathering processes replace the bicarbonate. The theory that the acidification observed in poorly buffered fresh waters was due to changing land-use patterns (Rosenqvist 1978a,b) has now been discounted as an explanation for the widespread effects observed, particularly in remote areas. Detailed study over several years of watersheds in Norway, some with changing land-use patterns and some without, has shown that, on the average, both are acidified at equal rates (Drablos and Sevaldrud 1980, Drablos et al. 1980~. Moreoever, studies of lakes in North America in areas where land-use patterns have never changed have also shown substantial increases in hydrogen ion or losses in buffering capacity (Dillon et al. 1978, Watt et al. 1979~. Along with the hydrogen ions supplied by transformation of atmospheric sulfur dioxide and oxides of nitrogen to their soluble,

148 Bir:~rhc~n~t`? Transition Lakes 7.0 pH 6.0- 5.0- 4.0; Acid Lakes \ .............. : .:,:,: .:.:,:,::: _ ::::::::::::::::::::::::::: _ ............... ~ .............. :t: :-::: :-:-:-:-: :-: .,\....................... .-.~-.- - -. ::~:::::::::: :: i. . - -: :-:2:2:, ·:-:-:l2.2.- 2,- ·.,..2,~..-,..:..,2-.. :-::-:-::~::::: . ~ \ -2-.2.- -. :-:-: . . . 100 H ~ added, ~eq/l -100 H CO3—peq/1 - 50 200 FIGURE 8.5 Titration curve for bicarbonate solution at a concen- tration of 100 ,ueq/lj illustrating the acidification process. SOURCE: Henriksen (1980).

149 acid forms, there is a potential for ecosystem acidification by the Vitrification of ammonia, from atmospheric precipitation or from the decomposition of dead organic matter. Two equivalents of hydrogen ion are generated for each equivalent of ammonium ion transformed to NO3 (Reuse 1975a), but one of those equivalents may be consumed upon either uptake or denitrification by components of the biota. If all of the ammonium and nitrate ions are transformed and/or utilized by organisms, the net potential for ecosystem acidification will be measured by the difference of ammonia and nitrate in equivalents. Preliminary data show very high concentrations of ammonia in precipitation over the central United States, possibly as a result of crop fertilization and livestock culture (Figure 8.6~. Oden (1976) has compared the trends in southern Sweden of anthropogenic acidification directly by mineral acids and indirectly by biological processes, and the normal background of biological acidification by nitrogen transformation (Figure 8.71. Mayer (1979) discussed the conditions under which natural acidification may be most significant. Extent of the Problem Large areas of the earth's surface consist of poorly buffered geologic materials. Where such areas occur within several hundred kilometers of sources of atmospheric emissions that are acid precursors, detectable acidification of at least freshwater ecosystems may be expected to occur. In the united States such areas occur largely in the eastern part of the country (Cogbill and Likens 1974~. In Canada, there are roughly 2 million square kilometers of acid-sensitive terrain--most of the eastern half of the country. Similar large geologic areas occur in Scandanivia, Scotland, and the northern part of the Soviet Union. A high proportion of the world's freshwaters occur in such terrain (between 50 and 80 percent depending on estimates); thus, large-scale degradation becomes an important concern. EFFECTS OF AC ID ON THE B IOS PHERE Aquatic Ecosystems Widespread and pronounced effects of acid precipitation have been recorded in poorly buffered aquatic ecosystems. The low chemical capacity of such "soft" waters to buffer against increasing hydrogen-ion supply causes the water to be in precarious pH balance even under natural conditions. Indeed, many natural lakes may become more acidic over hundreds or thousands of years owing to drainage from acid peat bogs. The bog moss, Sphagnum, generates in its cell walls polyuronic acids (Clymo 1964), whose hydrogen ions are exchanged for metal cations from precipitation (Gorham and Cragg 1960~. The hydrogen ions that are released in this way acidify the bog waters.

2 ~ on Goof ~ ~11-/ ~ 11' o %.... a\: ,,(~_ . i'\ it' Is 150 ~ N _~ 3 ~ o o~ ~ IS ~ ~ ~ £ =, =4 ~ ~ ~} - = Z Of =' or o E ~~ All ~ ~ ° .o ^.—V) (D ~ a' =.— Coo ~ o.—~ ~ 0 ~ i ~ O~= `~^ . L_,-~ —,T ~ ~ ,~ p C o J ° ~ ~ o ° r~ ·~ c3 Z ~ _ ? 534 ~S ~ ° ° ~ O ~ . .s $: ~= ~ Z ~3 ¢ Z s: b4 o ~ .= o — ._ ~ Cd _ ~ ca ~ c~ o ._ 3 ~ :~' _ — ~, 3 _ Z .= ~=d=° 5^ ,) s ~ o c, ~ ~ - , U. 04 ·~ ~ Ct r--l 1 l _ _~ \ J ~ ~--~ ~\\ . .\ .\Y ~ ~ ,\ ~ o ~ .! ~ ~ ~ ~fL ! ~ - .;--1 r~~¢ ' ~ ~ ~, ~1 ~~---l-r l )/ -- _ _ 1 J 1- ~ C-__ _-__ ____ I (< ~o I I ~ 1 / I r~ ~' L _ _, 1 1 1 5 , ~ ,_ _ ,, , · I I I ~, ·1 ~ ,,r -

151 800 o 600 Ad 400 a L`J 200 . _ - . _ - __....... _ _ -- ~_.......... A.............. _~ ,~ ~. ,~_ WOW:;;; jig Bio/og/ca/ acidificalion. (NH4 minus NO3 ~ I. I I I 1950 1955 1960 1965 YEaR FIGURE 8.7 Acidification due to mineral acids (computed as excess acids) and to bio- logical processes. The computations are based on data on the southwestern part of Scandinavia from the International Meteorological Institute. SOURCE: Oden (1980). I 1 970 Biological Acidification Induced by Man's Activities Normal Background of Biological Acidif ication l Anth ropogen ic Mineral Acids _ . -

152 Bogs may also generate sulfuric acid through the oxidation of organic sulfur compounds during dry periods (Gorham 1967~. By these means bog waters may exhibit pH values below 4 even where the precipitation has a pH of 5.0 or more. Lakes and streams naturally acidified in this way are always distinctly tea-colored, and can readily can be distinguished from the clear-water lakes now undergoing acidification because of man's activities. There are clear-water lakes in areas such as Japan, Indonesia' and Germany that are strongly acidic (pH down to 0.8~. The strong mineral acidity is generated by volcanic activity or pyrite oxidation (Hutchinson 1957~. Such naturally acidic lakes are the exception rather than the rule. The 10- to 40-fold enhanced hydrogen-ion content of current precipitation, however, has increased the number of clear-water, acidified lakes and the rate of acidification to the point where substantial changes are notable within a decade or decades. Chemical Changes Routine surveys in the past generally have used measurement techniques too insensitive to detect changes in lakes susceptible to acidification. It follows from the previous discussion on bicarbonate buffering that the measurement of alkalinity using modern methods is a far more sensitive method than is pH determination for measuring the degree of lake acidification. Where sufficiently sensitive techniques have been used in lakes subjected to acid precipitation, spectacular losses of alkalinity have been recorded. For example, Dillon et al. (1978) found that two lakes in south central Ontario, a region where the average pH of precipitation is 3.95 to 4.38, had lost over 50 percent of their alkalinity in a period of 5 to 10 years. Seasonal data are necessary in such studies to separate long-term trends from annual variation, because alkalinity in soft water systems is also greatly affected by seasonal cycles of plant production. More typically, long-term measures of alkalinity are not available and the effects of acid precipitation must be interpreted through changes in pH. Watt et al. (1979), Thompson et al. (1980), the Ontario Ministry of Environment (1978), and Harvey (1980) have shown moderate to substantial decreases in pH over periods of a few years to two decades. In the latter two studies, rates of pH change {once again in south central Ontario) were often 0.1 pH units per year or more, indicating the total exhaustion of bicarbonate buffering in the lakes. Major damage to lakes may occur well before such dramatic reduction in pH takes place. For example, in an experimental acidification of a small lake, Schindler et al. (1980) found that 70% of the lake's alkalinity had been depleted before pH values decreased detectably below normal. Some studies show a decline in the pH of lakes over the past two decades, although early surveys were done calorimetrically and then later on by electrode so that the magnitude of observed changes may not be entirely accurate. Examples are those of Schofield (1976) for the Adirondack area of the United States and Wright et al. (1980) for Norway.

153 Another chemical index of acidification is found in the ionic ratios of lakes. When the contribution of sea salts has been removed by indexing other ions to the chloride ion, most lake-water chemistry is dominated by calcium or calcium and magnesium, and the anions are dominated by bicarbonate--a reflection of the fact that the weathering products of rocks and soils which control the ionic composition of lakes are similar throughout most of the northern hemisphere (Rodhe 1949). As sulfuric acid is supplied by precipitation, the bicarbonate is consumed according to the above equation, being replaced in the ionic balance by the sulfate ion from precipitation. As a result, as lakes are acidified, there is a shift from a solution dominated by Ca++ and HCO3 to one dominated by Ca++ and SO-4. The relative increase in SO-4 and H+ or the relative decrease in HCO3 with respect to Ca++ or Ca++ plus Mg++ has been the basis for a number of models to predict the degree of acidification. For example, Henriksen (1979) uses a plot of pH versus Ca++ (Figure 8.8) while Almer et al. (1978) employ a plot of HCO3 (or alkalinity) versus Ca++ plus Mg++ (Figure 8.9~. While the ratios may be affected to some degree by the amount of calcium and magnesium weathered from terrestrial catchments, there is little doubt that the index is a reliable indicator of damage due to acidification where noncalcareous bedrock and soils predominate. The possible effect of increased cation leaching on the magnitude of the ratio is addressed by Henriksen (19801. Scandinavian and North American studies appear to agree on one point: acidification of sensitive waters is detectable within one to two decades where pH values of precipitation are less than 4.6--a 10-fold increase in acidity over the theoretical "pure" rain pH value of 5.6 (Henriksen 1979, 1980; Watt et al. 1979; Thompson et al. 1980~. In areas where precipitation has pH values of 4.0 to 4.3, degradation is much more rapid (Dillon et al. 1978, Keller et al. 1980~. Changes in the pH or alkalinity of water bodies of small areas of Belgium, the Netherlands, and Denmark also have been recorded (Vangenechten and Vanderborght 1980, Van Dam et al. 1980, Rebsdorf 1980). Metal concentrations also tend to increase in acidified lakes. While this may be due in part to the co-deposition of hydrogen ion and trace metals emitted from anthropogenic sources (discussed in Chapter 4), ion exchange from acidified soils and lake sediments are more important sources in many cases. Most notable is the correlation between hydrogen ion and aluminum in lakes from a number of areas (for example, Fig. 8.10~. Aluminum is abundant in both soils and sediments. In terrestrial catchments, it exchanges for hydrogen ion, and when the pH of groundwater or surface flow is 5 or less, high concentrations of the element may be carried to lakes and streams (Johnson 1979, Cronan and Schofield 1979, Johannessen 1980, Hermann and Baron 19801. The chemistry of aluminum in lake water is a complex function of pH, sulfate, and nitrate concentrations. It is reviewed by Almer et al. (1978~. In general, at pH values above 5.5, aluminum forms

154 4.0~ ~ 4.5-` 5.0- :: 5.5- 6.0 6.5- 7.0- 7.5 \ ~ ELA,,~,sudbu~r,',~Y~;' (:,Sierra :: Nevada . \ \ O ~ ~ 3 4 , , I 50 100 150 200 [Ca] -~Hubbard Brook ~ '~ Adirondack Mtns pH 4.6 a' 5 (ma 1-') t 250 (,uEq1-') O average for 170 lakes in the Sierra Nevada, California x average for 109 lakes in the Experimental Lakes area, north- western Ontario average for 216 lakes in the Adirondack Mountains, New York average for 11 years at Hubbard Brook, New Hampshire (Schof ield, personal commu nication) average for 178 lakes In the vicinity of Sudbury, Ontario FIGURE 8.8 Average pH levels and calcium concentrations in regions of North America. Waters in areas receiving acid precipi- tation lie well above the solid curve, whereas those in other areas lie below. Inset shows locations of these areas. Areas east of the isoline receive precipitation more acidic than pH 4.6. SOURCE: Henriksen (1979). Reprinted with permission from Nature 278: 542-545. Copyright ~ 1979 Macmillan Journals Limited.

155 Alk. meq/l 1.0 o.5 o Alk meq /l 0. 5 - p , - 0 // 0/~ 1: 1 // /~ ~ less polluted area /sy 0.5 / / / / / / / / / / l////4 / ,0/ O W~~~ ' 1.0 1:1 / / / / 1.5 Non marine Ca+Mg meq /l ,a' 0 highly polluted area l.o Non marine Ca+Mg meq / I Go 0.5 FIGURE 8.9 Alkalinities and contents of calcium and magnesium of nonmarine origin in lakes in two regions in Sweden with different sulfur loads. SOURCE: Almer et al. (1978).

156 700 600 500 400 . 1 . · 300 200 100 . . · · ·— . ..: ·~ ~ I. 4.0 5.0 6.0 7.0 pH FIGURE 8.10 The pH values and aluminum contents in lakes on the Swedish west coast, 1976. SOURCE: Dickson (1980). .

157 complexes that will tend to precipitate from solution. Below pH 5, several weak acid species predominate. Toxicity is high at low pH, as discussed later. Aluminum has a pronounced effect on other chemical cycles. For example, it is known to precipitate the humates that cause the dissolved color of lakes, leading to increased transparency. The increased light penetration causes higher phytoplankton production deep in the water column (Almer et al. 1978, Schindler 1980b), and the thermocline may deepen due to greater penetration of solar energy. Dickson (1980) has shown that increased aluminum concentrations precipitate phosphorus from lake water, particularly in the pH range 4.5 to 6.0 (Figure 8.111. Because phosphorus is the key element controlling the productivity of many lakes, this secondary effect of acidification may be responsible for the apparent "oligotrophication" which has been observed (Grahn et al. 19741. The co-precipitation with aluminum appears to more than counteract the increased deposition of phosphorus from polluted precipitation (Dickson 1980~. Hultberg and Wenblad (1980) found high concentrations of aluminum and manganese in acidified ground water. Their study provides an excellent example of how a problem may develop quickly as acid deposition continues. High sulfates reaching an aquifer from surface deposition of acids caused no acidification for many years, because a high water table in the area allowed microbial reduction of the sulfate to sulfide, which was then precipitated as pyrite by combination with dissolved iron. After two dry years, however, during which the groundwater table lowered, the accumulation of sulfide from several years was Deoxidized rapidly to H2SO4, causing highly acid groundwater, which leached high concentrations of several cations from surrounding soils including the above two metals. Manganese, zinc, nickel, lead, and cadmium also appear to be washed into lakes and streams from acidifying terrestrial systems (Figure 8.12, Dickson 1980; Hutchinson et al. 1978; Havas 1980; Troutman and Peters 1980) and mobilized from lake sediments (Schindler et al. 1980b). Schindler and his colleagues found concentrations of zinc known to be toxic to aquatic animals to be supplied from sediments into overlying water at pH 5.1, while Havas (1980) found a similar situation for both zinc and nickel. At low pH, methylated forms of mercury appear to occur in the monomethyl form (Figure 8.13), leading to its more rapid accumulation in fish. High mercury concentrations in fish from acidified waters have been reported from the United States, Norway, and Sweden. Jackson et al. (1980) found that adsorption of the isotope mercury-203 to organic materials in sediments at pH 5.1 was much lower than at near-neutral pH. Ionic mercury also appears to be more efficiently scavenged from the atmosphere than elemental mercury by aerosols, clouds, and rain with low pH levels. Thus acid precipitation, coupled with the enhanced emission of mercury to the atmosphere described in earlier chapters may cause serious problems. While hydrogen-ion activity affects the speciation of a host of other trace metals, there is as yet no evidence for their increased mobilization from lake sediments at low pH. Schindler et al. (1980b)

158 Refound phosphorus ,ug/l 100 80 60 40 20 _(lOO,ug/l add.) ~ ~ (100,ug/1 add.) ~~~~Cty \b `` O-~O~~9~ ~ "c 'a_, at, p .¢ ., A_ Humic lake water (0.2 mg Al/l and pH 6.2) a_ S:'~ Clear lake water (0.5 mg Al/l and pH 4.1 ) ~ -—~ . , ~ pH 8 9 3 4 5 6 7 FIGURE 8.11 Phosphorus recovered from the supernatant after experi- mental additions of orthophosphate to lake water samples. Fifty and 100 ,ug of orthophosphate were added to one-liter samples from Lake Horsikan (clear lake water with an initial pH of 4.1 and containing 0.5 mg aluminum per liter), and 100,ug of orthophosphate was added to one-liter samples of humic lake water (with an initial pH of 6.2 and containing 0.2 mg aluminum per liter that is mainly complexed with dissolved humic sum stances). The pH of the water was adjusted to different values, and the water samples were stored for 5 days before analysis. SOURCE: Dickson (1980).

159 0.3 0.2 0.1 0.05 <0.05 30 ~ 20 N 10 -1 \ 200 - ~ ~ 100 _ - ~ ~ . ' 1 ~ 1- 5.0 6.0 7.0 pH '\ \- · ~ - .\ ·\ _ \. - · ·-1.— 7.0 5.0 6.0 pH ~ l ~ _ \ ~ . I ~ 7.0 8.0 5.0 6.0 pH . 5.0 6.0 7.0 8.0 pH FIGURE 8.12 Metals in 16 lakes on the Swedish west coast with similar metal deposi- tion but with different pH, December 1978. SOURCE: Dickson (1980).

160 CH3 HO 70-~ C) LU ~ 60— o IL Or ~ 50— C) Ct UJ ~ _ J ~ 40— LU ~ 30— _~Q (CH3)2 Hg \ A 1 1 — 5 6 7 8 9 10 pH FIGURE 8.13 Formation of mono- and dimethyl mercury in organic sediments at dif- ferent pH levels during 2 weeks, with a total mercury concentration of 100 ppm in sum strafe. SOURCE: Tomlinson et al. (1980).

161 found that detectable copper, cadmium, cobalt, and lead were not released by sediments at pH values down to 5.1, while zinc, manganese, aluminum, and iron were mobilized from sediments at this pH. In general, low pH favors transformation of metals into more toxic ionic forms. While theory predicts that acidification should negatively affect microbial denitrification, ammonification, and Vitrification, as observed in terrestrial ecosystems, no evidence on these effects is currently available. Silica has been thought to be unaffected over a pH range of 2 to 8 (Birkeland 1974, Driscoll 1980~. Concentrations in Swedish lakes, however, are lower at lower pH levels (Figure 8.14~. This fact is currently inexplicable, particularly in view of the observed decrease in diatoms as pH decreases. Biological Changes As a result of acidification a number of organisms have been reduced or eliminated over significant parts of their ranges. A considerable change in algal species results from acidification and the changes associated with it. Some species of diatoms and chrysophyceans are eliminated in both plankton and periphyton communities, and are replaced by chlorophytes or cyanophytes (Schindler 1980a, Muller 1980, Lazarek 1980~. The total number of phytoplankton species is known to be lower in acid lakes (Raddum et al. 1980~. In lakes where cyanophytes dominate instead of chrysophytes, acidification appears to cause a shift toward chlorophyte domination (Crisman et al. 1980, Yan and Stokes 1978~. In some cases, the genus Mougeotia produces a filamentous mat on littoral sediments (Schindler 1980a, Almer et al. 1978) as a lake becomes more acid. In acid lakes in Ontario, the algae Zygnema and Zygogonium form benthic mats. Although lakes appear to become clearer as they are acidified, leading some authors to fear that they are becoming less productive (Grahn et al. 1974), it is still not known whether the increased transparency is due to loss of productivity or a change in the color of dissolved humic material. The phenomenon does not appear to be a direct result of acidification (Schindler 1980b), but may be due to the precipitation of both phosphorus and humates by aluminum leached from acid-stressed terrestrial materials, as illustrated in Figure 8.11. Effects are severe on animals as well as plants. Studies of distribution show that numerous lacustrine molluscs and crustaceans are not found even at weakly acid pH values of 5.8 to 6.0 (Figure 8.15; J. ~kland 1980, K. Okland 1980, Okland and Okland 1980~. Experimental, whole-lake acidification studies have revealed similar tolerances for benthic Crustacea (Schindler 1980a). The crayfish Orconectes virilis rapidly loses its ability to recalcify after moulting as the pH drops from 6.0 to 5.5 (Malley 1980) and is probably thus rendered more vulnerable to predation and protozoan infection. In Scandinavia, the once-common crayfish Astacus astacus has become rare in lakes where the pH is below 6. The phyllopod Lepidurus

162 1.50 1.00 0.50 _ @ @ 1 it 7.0 4.0 4.5 5.0 55 pH 6.0 6.5 FIGURE 8.14 Concentration of silicon in relation to pH level in 20 Swedish west coast leek August 1978. SOURCE: Dickson (1980t

163 15 oh LL aid 1 0 In LL o LL 1 l i No Species ./ Present / All Species Present (snails, mussels, crustaceans) O . . .: ~ - ' 4.0 5.0 6.0 7.0 pH FIGURE 8.15 The pH tolerance limit for 17 widespread species of molluscs and crustaceans. SOURCE: Okland and Okland (1980).

164 arcticus is not found in water below pH 6.1 (Almer et al. 1978~. Other planktonic crustaceans such as Daphnia species and the fairy shrimp, Branchinecta paludosa are also very sensitive to small declines in pH below 5.5. Daphnia magna and D. middendorfiana are susceptible to fungal infections at low pH (Haves 1980~. The effects of heavy metals are negligible at such high pH values, and thus the direct toxicity of hydrogen ion is implicated. Fish have been more extensively studied than other aquatic organisms in acidified lakes. Fromm (1980) believes that impairment of reproduction occurs at any pH below 6.5. Kennedy (1981) found high incidences of embryonic abnormality and high embryonic mortality rates of lake trout at pH 5.8. The reproductive failure appears to be due to disruption of calcium metabolism and deposition of protein in the oocyte (Fromm 1980~. Other stages of the life cycle usually appear to be less sensitive. Adult fishes of most species survive at pH values of 5.0 to 5.5. Changes in ionic balance appear to result from exposure to acid. Effects seem to be mitigated by increased calcium concentrations (McDonald et al. 1980~. Blood pH also decreases, resulting in less capacity to carry oxygen. The fathead minnow, Pimephales promelas, disappeared from an experimentally acidified lake at pH 5.8 (Schindler 1980a). Spawning and egg production are known to be affected at such pH values (Spry et al. 1981; Figure 8.16~. At slightly lower pH values, other fish populations begin to decline. At pH values less than 5.0, most of the species of value to sport or commercial fisheries have disappeared (Figure 8.17~. In Scandinavia, stocks of the sensitive roach, Rutilus rutilus, were destroyed by acidification of some lakes as early as the 1920s. ThiS cyprinid and the minnow Phoxinus phoxinus require pH values greater than 5.5 for successful reproduction (Almer et al. 1978~. Salmonid species react similarly to their North American counterparts (Figure 8.17). The combination of aluminum and hydrogen ion is highly toxic to fish at concentrations where neither is toxic alone. The fish kills observed at spring melt in Scandinavia and the Adirondacks appear to be due to a combination of high acidity from melting snow plus high levels of aluminum leached from terrestrial soils (Cronan and Schofield 1979, Driscoll 1980, Baker and Schofield 1980, Grahn 1980, Muniz and Leivestad 1980~. The effect appears to be due to a clogging of the gill by irritation-induced mucus discharges, causing severe respiratory stress. The loss of plasma sodium and chloride also occurs (Muniz and Leivestad 1980, Rosseland 1980~. Studies of the effects of acidification on aquatic microbial communities are scarce. In some cases, decreased decomposition of organic matter has resulted from acidification (Traaen 1980), while in others none has been observed (Schindler 1980a). It is possible that these differences are due to differences in trace metal concentrations. Sulfate reducers are stimulated by the increased sulfate that accompanies sulfuric acid inputs, and under appropriate environmental conditions they may partially counteract the effects of acidification (Schindler et al. 1980a).

165 LD a) ~ ~ ~ ~ ~ cn 0 0 0 ~ car o~ 1_ -C -o -O ~ t13 Q (0 ~ 11] —' <0 41) ~ ~v) ~ ~ o~ ~ ~ ·_ ~ ~ it_ -~ ~ ~ ~ ~ q' ~ ~ ~ =` C, 'A C 2=, ', ~ ~ ~ ~ ~ ~ C =, C Z ~0 Q o hi+ ~ ~~ a) ° a) ° 4 ~ ° a' ,~ -' s ~ ~ ~ ° 3 -' ~ s ~ ~~ cc z ~ m m ~ ~ ~ ~ 4- 4~ WO -I ~ . _ a' _ ~ (,, — a) .4J 4 - ~ ' . — Q .m CO - O IQ cn ~ a' 0 ~ c) ~ . _ 0 .... 4J `0 I ~ Q 0 ~ 0 ·- 4 - C5, ~O 0~ (15 LD ~ ._ ~ O 0 ~ ._ — J q' r~ o 11 o ~5 a . _ c~ o LO C~ UJ · ~ o u~ oo . - c~ c, v: . . o c~ o o ._ c~ cd c~ ct v: c) .~ c~ - c' c~ o :^ - ce cd

166 SPECIES YELLOW PERCH PUMPKINSEED ROCK BPiSS WHIT E SUCK ER NORTHERN PIKE LAKE HERRING BLUEG ILL LAKE WHITEFISH SMALLMOUTH BASS LARGEMOUTH BASS LAKE TROUT BROWN BULLHEAD GOLDEN SHINER IOWA DARTER JOHNNY DARTER COMMON SHINER BLUNTNOSE MINNOW pH NUMBER of LAKES 4.0 4.5 5.0 5 5 6.0 6S 7.0 11 19 13 8 10 0 OCCURRENCE _ 40 37 29 25 20 23 6 6 19 7 9 7 10 20 1 1 6 9 populations appear unaffected populations showing stress, or are very small lowest pH recorded for the species FIGURE 8.17 Occurrence of fish species in six or more La Cloche lakes, in relation to pH. SOURCE: Harvey (1980).

167 Some indirect effects of acidification on community structure have also been demonstrated. Many insect larvae survive well in the sediments of acidified lakes, especially chironomids. As predatory fishes disappear, more acid-resistant carnivorous invertebrates increase in numbers to fill the vacant trophic niche, including corixids, Chaoborus larvae, and dytiscid beetles (Henrik son et al. 1980~. After the disappearance of Pimephales as mentioned above, the previously rare pearl dace, Semotilus margarita, increased rapidly to fill the vacant ecological niche. Terrestrial Ecosystems Vegetation Adverse effects of acid precipitation on forests have not been proven. It is difficult to assess the effect of acid precipitation on forest yield against a background of yield differences caused by annual climatic variation. In addition, over much of the area subjected to acid precipitation, the soils are naturally acidic podsols with a vegetation adapted to these acidic conditions. Indeed, some experimental studies of Scandinavian forested areas show the additional sulfur and nitrogen supplied by acid precipitation to have a slight fertilizing effect in the short term, particularly in mature podzolic soils, which are capable of adsorbing large amounts of sulfate {Abrahamsen 1980, Tamm and WiRlander 1980, Tveite 1980~. Long-term effects are uncertain, but the initial stimulation caused by adding nitrogen to a nitrogen-deficient forest may give way to deficiencies of other elements, such as Ca, K, Mg, P that are more rapidly leached away. As magnesium and other elements are mobilized at low pH and leached from the soil, long-term permanent damage to the ecosystem may result, causing chronic deficiencies of the elements in plants. Also increased mobilization of ions could result in toxic concentrations of aluminum and manganese. Seedling establishment may be affected. There is a paucity of studies of understory vegetation, although Horntvedt et al. (1980) mention reduced moss cover under forests artificially subjected to precipitation of pH 3, and Evans and Curry (1979) found that gametophyte fertilization in bracken fern was highly sensitive to precipitation of pH 3.4. Effects on crops as a result of direct foliar damage have been reported, though effects are species specific and depend on environmental and physiological conditions. Some positive effects of acid precipitation on soybean yield have been observed (Irving and Miller 1980~. However, Evans et al. (1980) found that simulated rain at pH 4 and below the caused number of seed pods per plant to be reduced and thus the seed mass of soybeans produced was significantly decreased. In a comprehensive review of the subject, Jacobson (1980) concluded that there was experimental evidence for damage to agricultural crops by strongly acid precipitation. Rains with a pH below 3.0 have been noted in many experiments to cause foliar lesions (Tables 8.2 and 8.31. Differences among studies may reflect differences in soil type or treatment technique.

168 TABLE 8.2 Results of Recent Experiments on Effects of Simulated Acidic Precipitation on Crops Grown Under Greenhouse Conditions Laboratory Crop Argonne National Soybean Laboratory Boyce Thompson Lettuce Institute Brookhaven National Laboratory Cornell University Pinto bean Soybean 11 of 13a 13 of 13a 3 of lla 5 of 14a Tomato, Effect Foliar symptoms No effect on growth pH 3.0 3.0 Increased growth 3.0 and 3.2 Increased and decreased (dependent on nutrient content sulfate and nitrate concentrations) Reduced growth 2.5, 2.7, 2.9, 3.1 Reduced yield 2.5, 2.7 Reduced growth 2.5, 2.7, 2.9, 3.1 Reduced yield 2.5 Increased yield 3.1 Foliar symptoms 4.0 Foliar symptoms 3.0 Reduced growth (ht.) 3.0 Reduced growth (wt.) 3.0 Reduced yield and cabbage, quality 3.0, 4.0 and pepper Reduced growth and yield 3.0 Corvallis 5 of 35a Foliar symptoms 4.0 Environmental 28 of 35a Foliar symptoms 3.5 Research 31 of 35a Foliar symptoms 3.0 Laboratory 5 of 28a Decreased yield 3.0, 3.5, 4.0 6 of 28a Increased yield 3.0, 3.5, 4.0 Oak Ridge National Red kidney Foliar symptoms 3.2 Laboratory bean Reduced growth 3.2, 4.0 aNumber of species exhibiting effect out of total number exposed. SOURCE: Jacobson (1980).

169 TABLE 8.3 Results of Recent Experiments on Effects of Simulated Acidic Precipitation on Field~rown Crops Laboratory Crop Effect pH Argonne National Laboratory (rain and simulated rain) Boyce Thompson Institute (simulated rain only) "Beeson" and ``Williams" Brookhaven Soybean, ``Amsoy" National Laboratory (rain and simulated rain) Cornell University (rain and simulated rain) North Carolina State University (rain and simulated rain) Tomato, pepper, snapbean, cucumber Soybean, "Davis" Soybean "Wells" No effect on seed mass Increase in seed size No foliar symptoms or 3.1 effects on growth Soybean, Decreased growth, yield, and seed quality (germination) Increased yield No foliar symptoms Decreased yield and quality (protein content), foliar symptoms No effect on growth 3.0 or yield, reduced quality 2.8 (high ambient ozone) 2.8, 3.4 (low ozone) 2.8, 3.4, 4.0 (low ozone) 2.3, 2.7 Slight foliar injury 2.8 No effect on growth 2.8, 3.2, 4.0 or yield SOURCE: After Jacobson (1980).

170 Plants differ greatly in their responses to pH (Russell 1973), some being adapted to calcareous soils (calcicoles) and others to acid soils (calcifuges). Still other plants tolerate a wide range of pH, and indeed species vary in their degree of tolerance in different parts of their range (Snaydon 1962~. Many acid-tolerant species, moreover, are found in nature on acid soils because they can compete most successfully there; in the absence of competition they may grow as well or better on neutral soils. Likewise, some calcicoles can grow on acid soils in the absence of competition (Weaver and Clements 1938~. For more than 50 years it has been generally accepted that the elevated aluminum and sometimes manganese concentrations in acid soils are key factors in determining survival of many species (Rorison 1980~. The effects of acidification upon biota are often extremely difficult to establish, owing to the diverse ways in which pH affects the availability of nutrients and toxicants, which may sometimes counterbalance one another. Even for a single nutrient element the situation may be highly complex, as shown in the hypothetical model suggested by Tamm (1976) for the effects of acidification upon the nitrogen cycle in a forest ecosystem (Figure 8.18~. Abrahamsen and Dollard (1979) have recently reviewed the complex effects of acid rain on the nitrogen cycle. The potential effects of acid rain on foliar susceptibility of fungal attack and of roots to infection by fungal pathogens is an area of concern, but one lacking a detailed examination (Shriner and Cowling 1980~. Most forest species require a symbiotic association with specific soil fungi (mycorrhiza) for adequate mineral uptake. This delicate association--particularly the infection stage--may be affected. Soils Acid rain has particularly severe effects upon the cycles of metallic toxicants and nutrients, because of the influence of hydrogen ions upon cation exchange and weathering and the consequences of mobilizing various elements. An excellent example of such effects is provided by Gjessing et al. (1976), who observed that in small, undisturbed granitic watersheds, the sum of Ca + Mg + Al output was closely related to input of hydrogen ions. Metal output (O. as kg equivalents km~2 yr~l) is correlated very highly (r = 0.99) with net retention and neutralization of hydrogen ions (R), the regression being given by 0 = 14.9 + 0.86 R. With high input of acid rain, metal output only slightly exceeded the retention of hydrogen ions. However, at low acid-rain input, the metal output is substantially in excess of the retention of hydrogen ions, because where rain is not strongly acid, natural inputs of hydrogen ions from such processes as decomposition and root respiration are of relatively greater importance than anthropogenic inputs ~n mobilizing metals. Chelation may also be significant where the effects of acid rain are not predominant (see references cited by Gorham et al. 19791.

~ - ~ ' \ ~ - Qua - ~ i of ~ o ht \ hi_ it,, fib - ~ C'0 / \ ~$ 171 ~5~ i;~ a- \ he\ J O ~ b fib god\ I. ,~ _t Co O C'3 ~ Co ·$ c c C' C' ._ - o o ~ -- ~— ~ z o Z ~ N0 o ~r O J I ~ Z Z 0 cn o L <: ~ C~ Q cn =) o ~ o o z o I ~: z ~L 111 C~ E~ . . - o C~ C~ q) ~o .s C) C) ~o o s~ .= a~ o ~: o ._ ._ ._ C) a~ ._ C~ e~ o C) a~ 4) a, o - - C) ._ a, o oo oo ~ -

172 The soils most susceptible to rapid acidification are well-drained brown forest soils (alfisols) that are sandy and noncalcareous but not already strongly acid (Wiklander 1973, 1974, 1979~. Such coarse soils are moderately to highly saturated by "basic" cations such as Ca++, Mg++, K+, and Na+, which are leached away as they are replaced by "acid" cations such as hydrogen ions, aluminum ions, and hydroxy-aluminum ions. As the exchange complex becomes dominated increasingly by the "acid" cations, the pH of the soil declines. Fine-textured mineral soils, because of their high clay content, have a much greater cation-exchange capacity, which is usually strongly saturated by "basic" cations, and such soils are much better buffered against acidification. Strongly acid podzol (spodosol) soil horizons--whether organic, with a high cation-exchange capacity, or sandy, with low cation-exchange capacity--are only slightly susceptible to further acidification. Their exchange sites are already dominated by the "acid" cations, and thus added hydrogen ions are more likely to pass through the system in the percolating waters. Nevertheless, because such soils are already impoverished, even slight losses owing to increased acidification may be critical for soil fertility. The influence of acid rain upon the impoverished lateritic soils of the tropics could also be very significant but does not seem to have been examined. An interesting problem is the synergistic influence of neutral salts upon cation exchange under acid conditions. Wiklander (1975, 1979, cf. Abrahamsen et al. 1979) has made the important point that adding divalent neutral sulfates--often abundant in acid rain--to acid leaching solutions may retard significantly the replacement and loss of basic cations from already acid soils and thus favor the acidification of receiving waters. Monovalent chlorides have a lesser effect. Addition of strong acids to precipitation will cause not only ion exchange on the surfaces of soil particles but also alteration of the particles themselves by weathering, either directly or by increasing the hydrogen-ion saturation of organic and inorganic soil colloids, which act as weathering agents (Loughnan 1969~. Carbonates are weathered very readily indeed, but unless they are present only in small amounts (cf. Salisbury 1922, 1925) effects upon the soil are likely to be extremely slight. Aluminosilicate minerals are more slowly dissolved by acid rain, but according to Norton (1976, cf. Johnson 1979) the solubility of aluminum compounds such as gibbsite, amorphous aluminum hydroxide, and kaolinite increases rapidly below a pH of about 505. In this connection, lake waters generally show an order-of-magnitude rise in dissolved aluminum as pH falls from 5.5 to around 4 (Wright et al. 1976~. The solubility of hydrous oxides of iron, produced by the weathering of iron-bearing minerals, is affected by a lowering of pH to a somewhat lesser degree than oxides of aluminum, according to Black (1967, see also Birkeland 1974~. The dissolution of silica is essentially unaffected over the pH range 2 to 8 but rises rapidly above pH 8 (Birkeland 1974~.

173 Increased leaching of potassium, calcium, aluminum, and magnesium due to ion exchange reactions with hydrogen ions is one of the most commonly observed effects of acid precipitation (Figure 8.19; also see review by Abrahamsen 1980~. These increased rates of leaching appear to outstrip compensatory increases in weathering, reducing the exchangeable pool of the above cations. Sulfate is generally adsorbed in soil, largely in the B horizon (Farrell et al. 1980, Singh 1980, Figure 8.20~. Whitby and Hutchinson (1974) found that soils acidified to pH values below 4.0 by smelter fumigations in the Sudbury, Ontario area released sufficient aluminum into the soil solution to severely inhibit the establishment and elongation of seedling roots. Ulrich et al. (1980) have suggested that elevated aluminum concentrations caused the crown dieback in beech (Fagus sylvatica) and the failure of seedlings in the Solling project research forest in Germany. The direct role of aluminum in this response as opposed to the effect of severe droughts which occurred during the same period has not been established. It is not clear at present whether the effects of acid precipitation on soil will result in long-term degradation of terrestrial ecosystems. At least one controlled 5-year study has revealed a significant depletion of cation exchange capacity in soils subjected to loading of 13.3 k. eq. H+ per ha of artificial acid rain (Farrell et al. 1980~. Troedsson (1980) found declining quantities of Ca++, Mg++, and Kit in Swedish soils over a 10-year period. Both theoretical and experimental studies suggest that such declines may be widespread in vulnerable soils after several decades to a few centuries of acid precipitation (Oden 1968, Reuss 1975b, Maimer 1976, Norton 1976, Tamm 1977, McFee 1978, Abrahamsen and Stuanes 1980~. Differences in effects of different levels of acidification upon trace metals in soils are even less well understood. The only detailed study is that of Tyler (1978), in which purely organic mor humus layers (Romell 1932, 1935, Lutz and Chandler 1946) from Swedish spruce forests were leached in the laboratory by simulated rain acidified to 5 different pH levels. Two sets of humus layers were examined, one far from and the other near to a brass mill. The latter exhibited very strong contamination by copper (695 x the control set, far from the pollution source) and zinc (124 x), with lesser enrichment of cadmium (24 x), lead (7.6 x), chromium (3.7 x), manganese (3.1 x), nickel (2.2 x), and vanadium (1.3 x). Simulated rains at pH values of 4.2, 3.4, 3.2, 3.0, and 2.8 were applied over 125 days in amounts totaling 625 ml/g of humus. The s~mulated rain at pH 4.2, a value often reached in Swedish precipitation, released (over 125 days) substantial percentages of several trace metals in the control humus layers, notably manganese (44~), nickel (44%), cadmium (33%), and zinc (25~. Percentage release was much less for the other metals, copper (12~), vanadium (11~), chromium (8.9~), and lead (2.2~. With the notable exception of vanadium release (40~), percentage releases were generally less in the humus layers of the polluted site, as follows: manganese (4.1~), nickel (11%), cadmium (9.3~), zinc (18%), copper (1.7%), chromium

174 pH 2 1400 - 800- 700 600 N - ~ 500 a) 400 300 - 200- 100 - O- -100 ~ pH4 oH 3 control, not watered Ca Mg K I: K,Mg,Ca ~ _ ~ . us SOURCE: Stuane s ~ l 98 0) FIGURE 8.19 Loss of nutrients by weathering and mineralization. Artificial rain of varying acidity was applied in a field experiment over a 5-year period. The "rain" amounted to 50 mm per month for the 5 frost-free months of the year. SOURCE: Stuanes (1980).

175 35So2- S adsorbed 3 0 25 50 75 100 125 1 1 1 1 ' 1 E ~ P Bsl Bs2 Bc . C1 C2 C3 23 30 50 72 Q 95 a, . _ o cat . \\\\\\\\\\\\\\\\\\\\\\\\~ ~~\~\~: \\\\ I ~r- 8 E 25 Bs 76 2C 134 142 - 0 25 50 75 100 0 25 n ~ ~~ 6 Or 7 Ah Bw 20W 4 Ah 63 2C 90 0 25 50 75 ~ I ~ ~~ P. \\\\\\\\\\\ 4 - . ~ 50 0 15°07 A2BhC2r FIGURE 8.20 Distribution of adsorbed 3sSO4 sulfur in soil profiles. PI and Ps are iron-podzols; P4 iS a semipodzol (all typic udipsamments); and P6 and P7 are brown- earths (umbric dystrochrept and aquic haploboroll, respectively). SOURCE: Singh (1980). 25 50 ' 1 1 P7

176 (1.5~), and lead (0.24%~. This difference is most likely due to the much lesser acidification of the polluted humus, which yielded a percolate with a pH of 6.1 at the end of the experiment, in contrast to a pH of 4.5 in the percolate from the unpolluted humus. The effects of acidification at 5 different pH levels in this experiment may be examined by comparing releases at each of the more acid pH values to that at pH 4.2, taking the release at this pH as unity. Figure 8.21 demonstrates that once again no simple rule can be given. For instance, increasing acidification has much less effect upon lead than upon zinc at all pH values above 2.8, but at pH 2.8 the release of lead is strongly accentuated and exceeds that of zinc in both polluted and unpolluted soils. In the case of vanadium, acidification reduces the amount released, except from the unpolluted humus at the lowest pH, 2.8, from which the release is doubled. In the polluted humus, increasing acidification below pH 4.2 has about the same effect upon zinc and nickel, whereas in the unpolluted humus, acidification has a much greater effect upon zinc. Elucidation of the influence of acid precipitation upon the release of polyvalent metals from mineral soils will be greatly complicated by the fact that organic acids and polyphenols produced by organisms are also of much importance in the mobilization of oxides of aluminum, iron, and manganese from soils, and breakdown of these oxides will release the many trace metals adsorbed by them (several references in Russell 1973~. Presumably acid rain--with its strong mineral acids--will have some effect upon the weathering action of the organic acids and polyphenols, the latter being known to reduce iron more strongly in acid than in neutral conditions (Russell 1973~. Acidification can also reduce the stability of the fulvic acid components of humic acids in soils and their metal complexes, and thus metal availability should be greater at lower pH (Schnitzer 1980~. Little is known about such interactions, however, and they should be given greater attention. A number of soil microbial processes appear to be affected by acidification. Reduced soil pH may cause a reduction in nitrogen fixation (Alexander 1980), although mineralization of organic nitrogen may increase (Nyborg and Hoyt 1978~. Increased acidity also causes Vitrification to decrease; vitrification usually ceases entirely at about pH 4.0. There may be some degree of acclimation in acid environments (Walker and Wickramasinghe 1979~. Denitrification, the main means by which nitrogen is released to the atmosphere from the biosphere, is also affected by acidification (Figure 8.22~. Decreased rates, and a change in end product from molecular nitrogen to nitrous oxide, are observed in anoxic environments at pH values less than 6.0. At lower pH levels an appreciable yield of nitric oxide is observed, which is phytotoxic (Wijler and Delwiche 19541. Only the mechanisms involved in nitrogen fixation have been studied directly. Legumes, which depend on Rhizobium for nitrogen fixation, are particularly sensitive to acidification. Alexander (1980) suggests that this may be due to the high concentrations of A1, Mn, or Fe in acid soils. Another possibility is limitation by

177 U NPOLLUTED SOIL POLLUTED SOIL 4 3 2 o LLJ 1 - \ Pb _ ~ \ ~ _ O ~: o 4 z o 3 cr 2 3 _ 2 _ 1 _ O _ C U ~_ ' ' ' ' , ~ 2.8 3.0 3.2 3.4 4.2 , I I l , ~ 2.8 3.0 3. 2 3.4 4.2 I ' ' ' ,' ' 2.8 3.0 3.2 3.4 4.2 4C 30 20 10 o Cul I Pb 1~1 I T I I ~ 2.8 3.0 3.2 3.4 4.2 pH OF LEACHING SOLUTION FIGURE 8.21 Effect of acidity upon the leaching of heavy metals from spruce humus layers, normalized to pH 4.2. Left-hand graphs represent unpolluted humus layers; right- hand graph represents humus layers polluted from a brass mill.

I78 00 50 40 30 20 10 o V= 12.Sx-47.2 r= 0.~ , i./ 3 4 @ e 6 7 pH FIGURE 8.22 CoIIelation Bitten denitdOcation Ivies and sod pH. SOURCE: ~UDeI ~ at. (1980t

179 molybdenum, an essential element for nitrogen fixation in legumes, because Mo solubility decreases at low pH. Francis et al. (1980) found that increased acidity decreased rates of decomposition, ammonification, Vitrification, denitrification, and N2 fixation in soil. Microbial degradation of pesticides was also reduced at low pH. Lohm (1980) found that the biomass of fungal mycelium increased in artifically acidified forest soils, replacing bacteria, which were reduced in number under acid conditions. Springtails also increased. The number of Enchytraeidae (oligochaete worms), Collembola (spring/ails), and Acari (mites) also decreased under acid conditions. The overall result was a net decrease in decomposition. Haagvar's (1980) study of invertebrates under acid conditions produced analogous results. It appears that the detrimental effects of acidification on soils may not be due to--or at least, wholly attributable to--acidification per _ but may be caused by the altered concentrations of nutrients and heavy metals in acidified soils. Direct toxicity of hydrogen ions in soils was found to be negligible over the pH range 4 to 8 (Ar non and Johnson 1942, Arnon et al. 1942~; around pH 3 roots may be injured and above pH 8 phosphate absorption may be inhibited. In general, pH affects plant growth--and thereby the cycles of many (especially biophile) elements--by influencing the concentrations of different ions in the soil solution (Russell 1973, Nyborg 1978, Hutchinson and Collins 1978~. Some of the elements affected are major plant nutrients (e.g., calcium, potassium, phosphate, nitrate), and others are toxicants (e.g., aluminum, lead), while several trace elements (e.g., manganese, copper, zinc) may be nutrients at low concentrations and toxicants at high concentrations (Bowen 1966), as for example around metal smelters (Stokes et al. 1973, Hutchinson and Whitby 19771. The biogeochemistry of an ecosystem varies systematically as both vegetation and soil change in the course of succession, and acidification (whether normal or anthropogenic) plays a marked role in this process (Gorham et al. 1979~. For a time, the leaching effect of acid precipitation upon a soil with appreciable reserves of calcium may enrich the stream and lake waters draining such soils without acidifying them (Gordon and Gorham 1963, Gorham 1978a), because the acids will be neutralized in percolating through the upland soils and liberating basic cations. However, if water flow occurs mainly as surface runoff, for example over frozen ground in spring, or downward through old root channels, animal burrows, and along rock faces (Tamm and Troeds son 1957~--then acidification of the receiving water may take place even if the upland soil possesses quite substantial buffering capacity. As time goes on and upland soils become more acid, they also become less susceptible to further acidification by acid rain, because exchange sites within the soil are already highly saturated by hydrogen ions. In this situation, substantial amounts of acid will percolate to the receiving waters of streams and lakes, which consequently undergo a pronounced decline in pH. Acidification may be regarded as a normal tendency of ecosystem succession on base-poor substrata, because the biota produce acids of

180 various kinds metabolically (carbonic acid and various organic acids through the oxidation of organic carbon compounds; nitric acid by Vitrifying bacteria; sulfuric acid by bacterial oxidation of organic sulfur compounds; and hydrogen ions attached to the surfaces of roots, Sphagnum mosses, fungal hyphae, and bacteria, cf. Wiklander 1979~. But natural acidification is accelerated appreciably by acid rain, which often brings about a change within decades in vulnerable aquatic ecosystems, once their buffering capacity is exhausted. Acid rain may also cause a slower change--perhaps over centuries--in upland ecosystems, where soil minerals and ion-exchange complexes provide a greater degree of buffering, even in the most vulnerable watersheds. If acidification should lead to substantial reduction in soil base saturation, recovery following removal of the acid loading could well take decades to centuries. Both natural and anthropogenic acidification processes in terrestrial soils deserve increased study, so that we may assess their relative importance in different ecosystems. Wetlands The disappearance of several species of the bog moss Sphagnum from the vast blanket bogs of the southern Pennines in the British Isles is the major vegetational change that has resulted from atmospheric pollution in that country since the Industrial Revolution (Talks 1964~. In this connection, Gorham (1958b) has shown that pools in bogs with an intact Sphagnum cover ranged in pH from 4.5 in remote areas of Britain, where the acidity is chiefly biogenic, to as low as 3.9 in the northern Pennines closer to urban/industrial centers. In these bog waters the correlation between HE and non-marine SO4 ions was highly significant (r = 0.985~. Near Sheffield in the southern Pennines the pH of bog pools was only 3.25, and the concentration of SO4 reached 46 mg/1. Although it is impossible to ascribe with certainty the disappearance of the bog moss from the southern Pennines to acid precipitation, recent experiments with the same Sphagnum species indicate that either acid rain or SO2 fumigation alone have detrimental effects consistent with the observed disappearance (Ferguson, Lee, and Bell 1978~. In contrast, Hemond (1980) has calculated that in a Massachusetts peatland dominated by Sphagnum the reduction of sulfate and biological utilization of nitrate totally buffer the effect of acid deposition upon interstitial waters there. Vast areas of sphagnum bog occur in North America, including portions of northern Minnesota and the lowlands of Hudson and James bays where rain is now quite acidic. Many of these peatland ecosystems have very low buffering capacities, which suggests that they may be easily unbalanced by acid precipitation. Although data

181 are sparse, the abundance of peatlands in northern latitudes around the globe suggests that they may be significant reservoirs in the global cycles of many elements. ASSESSMENT OF ECOLOGICAL EFFECTS For improvements in our understanding of the effects of acid deposition we must concentrate on two major research areas. First, we need long-term monitoring (Botkin 1978) of sensitive organisms or communities (e.g., the "neuston" of freshwater surfaces, cf. Gorham 1976, 1978a,b) in especially vulnerable ecosystems, so that we can receive early warning of ecosystems at hazard. Such monitoring presupposes the identification of a series of indicator organisms and communities (Thomas 1972) or indices to community structure (Cairns 1974) and the development of an adequate scheme for rating ecosystem vulnerability. The Calcite Saturation Index developed by Conroy et al. (1974, see also Kramer 1976) provides a useful guide to the vulnerability of lakes, but because it uses only the concentrations of calcium, bicarbonate, and hydrogen ions in the water, it does not take into account the full range of factors involved (cf. Table 8.11. Second, we need experimental ecosystem-scale studies to examine linkages among and processes within uplands, wetlands, streams, and lakes (Likens and Bormann 1974, Schindler 1980b) and the mechanisms by which acid deposition alters ecosystem function. Some of these studies could be conducted on watersheds exposed to ambient levels of acid rain, while other studies, in relatively unpolluted areas, could subject whole ecosystems to experimental acidification. Perhaps the recently proposed national network of experimental ecological reserves (TIE 1977) could provide suitable sites for such studies, as could several of the experimental watersheds (Table 22 in Likens et al. 1977) set up for other purposes. One of these, the Hubbard Brook Experimental Forest, has already become an important site for research on acid rain (Likens et al. 1977~. AMELIORATION Of the options presently available only the control of emissions of sulfur and nitrogen oxides can significantly reduce the rate of deterioration of sensitive freshwater ecosystems. It is desirable to have precipitation with pH values no lower than 4.6 to 4.7 throughout such areas, the value at which rates of degradation are detectable by current survey methods, as mentioned above. In the most seriously affected areas (average precipitation pH of 4.1 to 4.2), this would mean a reduction of 50 percent in deposited hydrogen ions. Control of SO2 from new electrical generating plants alone would be insufficient to accomplish this, and thus restrictions on older plants must be considered. Furthermore, there are no proposed restrictions on the emission of nitrogen oxides, and the amounts of these substances emitted are expected to continue to increase (see Figure 4.6).

182 The alkalinity of waters endangered by acidification can be enhanced by a number of means, most notably by "liming"--adding calcium carbonate or oxide--and by adding phosphorus to stimulate biological fixation of nitrate and CO2. All of these techniques are expensive ($50 and more per hectare of water surface), and treatments must be repeated every few years. Due to high costs and logistic difficulties, lime cannot be applied to the vast areas that are currently endangered by acidification. In the areas most susceptible to acid deposition, it will therefore be impossible to maintain the alkalinity and pH of more than a few selected bodies of water. Furthermore, pH and alkalinity cannot be artificially maintained without increasing the ionic concentration of the receiving water, the consequences of which have not been investigated. The fate of dissolved toxic metals after liming is also poorly known. Addition of nutrients to increase alkalinity has been investigated at a few sites in Ontario, both by the Ontario Ministry of Environment and Canadian Department of Fisheries and Oceans, but results are not available yet. Many of the objections to liming will also apply to nutrient additions. SUMMARY Acid deposition, due to the further oxidation of sulfur and nitrogen oxides released to the atmosphere by anthropogenic sources, is causing widespread damage to aquatic ecosystems, including loss of bicarbonate, increased acidity, and higher concentrations of toxic metals. As a result, several important species of fish and invertebrates have been eliminated over substantial parts of their natural ranges. Effects on terrestrial ecosystems are less pronounced. Increased leaching of both nutrients and toxic elements is evident in poorly buffered soils sensitive to acidification. There is some evidence for damage to crop plants, and many soil microbial processes are negatively affected at low pH. Trees appear to be slightly stimulated by acid precipitation, although this effect is expected to be short-lived, because of increased leaching of cationic nutrients and the buildup of toxic co wentrations of metals in soil water. Better long-term studies of deposition processes and of effects on ecosystems are required to illuminate the complex ecological effects of acid precipitation and associated nutrients and toxicants. The control of emissions of sulfur and nitrogen oxides from fossil fuels is necessary to halt the acidification of sensitive aquatic ecosystems.

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