ACID DEPOSITION ON NATURAL ECOSYSTEMS
(Aquatic, Terrestrial Ecosystems and Bird Population)

Grace Anggrainy


The recognition in the 1960s of any changes in the chemistry of precipitation brought up the causes and consequences of acid rain in massive attention1. The major role of sulfur dioxide and nitrogen dioxide emissions as causal factors was recognized, as was the long distance regional transport of airborne pollutants. Acid precipitation accelerates exchange processes at all surfaces and causes leaching and mobilization of metal ions, especially Ca, Mg, Al, Mn, Fe, and Zn, from soils and bedrock. Both aquatic and terrestrial ecosystems have been affected by acidification, but the most obvious effects to date are on freshwater systems, especially poorly buffered ones, and on nutrient-poor terrestrial systems with soils of low base exchange.

Part of the obvious parallels in effects between European ecosystems and those in USA is that in both regions large areas are underlain by hard, shield granitic bedrock, which is resistant to dissolution and yields nutrient-poor lakes with low base status. In the 10,000 to 12,000 years since the retreat of the glaciers, acidic soils have developed under the cold, wet, short-growing seasons of the boreal forest. These soils are shallow, but support forests of acid-tolerant spruce, pine, fir, oak, maple, and beech1.

The collection of rainfall at an expanding area of recording stations in Europe led to the recognition that the chemistry of the rain was changing, especially in its increasing sulfate and hydrogen ion content. The pH decreased and the area receiving acidic rain expanded. Inorganic acids contributed to the acidity of the rain, with sulfuric and nitric acids accounting 95% of this acidity, and sulfuric contributing 2.5 to 3 times as much as nitric. The source of the increased rainfall acidity was industrial emissions of sulfur dioxide and sulfate aerosol, carried from their source to be deposited as acid rain, often several hundred kilometers from source.

For some time, the focus of concern was on lakes and rivers, and also on the fish in these acidifying systems. Lakes, which were naturally acidic, were found and cited as evidence that acidity was natural and not harmful. Areas deficient in sulfur were located and used to show that ecosystems need more sulfur, so that acid deposition might be a good thing for them.

The effects of acid precipition in aquatic systems are well documented for lakes, streams, and rivers. Acidification is associated initially with changes in species composition and lower species diversity and, as the acidification becomes more extreme, with lower biomass. Acid-sensitive species of fish, zooplankton, insect larvae, mollusc, crustaceans and phytoplankton are replaced of displaced by acid-tolerant forms. Also a shift in the decomposers from bacteria to fungi, which can reduce decomposition dates and interfere with nutrient cycling in environments already nutrient poor. The first evidence of the biological effects of acidification was reported by fisheries biologists. Fish populations, especially those of brown trout and Atlantic salmon, were declining rapidly at the turn of the century. Fisheries biologist suspected that the extremely acidic runoff waters were responsible for the declining populations. Not only were the number and age-structure within a fish species altered, but overall species diversity was also in decline. The number of fish species present in a lake is positively correlated to lake pH. Healthy lakes with pH typically above 6.0 have between 7 to 17 species of fish. Below pH 5.5 many lakes are devoid of fish, and those that still support some species generally have higher concentrations of dissolved calcium in the water which hydrogen ion toxicity 1.

The effects on terrestrial ecosystems are not nearly so clear cut. At least one major reason for this is that soils, relative to lake waters, are well buffered. Acidic additions would normally be neutralized. Also, many terrestrial plant species grow routinely on soils of pH 3.8-5.5 and are adapted to the chemistry of the soil solution2. Much of the rain falling on a forest is initially intercepted by foliage, which at its surface also causes chemical exchanges.

In considering soil acidification, we need to account for the exchange processes at the root surface in the soil which means that the growing forest exchanges hydrogen ions for essential nutrients, causing a normal soil acidification over time. When trees die and are decomposed on the forest floor, the acidification process is reversed. A scientist named Mayer reported that some soils have acidified above and beyond the natural process of acidification and that this accelerated rate of acidification was in fact taking place, with sulfur and nitrogen inputs from acid deposition being the most probable cause.

A large number of researchers attempt to dissect out primary and secondary causes of forest decline, food-chain dependents (on the forest) may be getting into trouble2. The heterogeneity and complex structure of forest ecosystems makes it difficult to discern direct cause-effect linkages between acid deposition and the health of the forest ecosystem.. In recent study, severe reproductive impairment of several bird species was being related to low calcium concentration in their diet. Some of the highest rates of forest decline attributed to acid precipitation. This is driven by exceptionally high levels of atmospheric nitrogen deposition. Highest proportions of inferior eggshells were found among territorial birds (i.e. birds staying longest in one location) nesting in areas of poor soil. Bird nesting in areas with rich clay or loam soils, as well as non territorial birds which have relatively short periods of local residence. The poor-quality eggshells were related to insufficient deposition of calcium. In areas where acid rain mobilizes aluminum in the soil and results in a Ca to Al ratio of less than 1, uptake of calcium may be impaired2. Calcium concentrations in caterpillars, an important source of food for hole-nesting birds, reflected the calcium concentrations in tree leaves. The lower concentration of calcium may have been due to elevated aluminum concentrations in the water that gave a higher Al/Ca ratio, since aluminum is known to interfere with calcium metabolism and to interfere with eggshells formation.

Eggshell thinning, commonly linked to chlorinated organics in fish-eating birds of prey, is showing up among insect-eating species around acidic lakes in Europe, and may be the first indication that the problem of forest decline extends through the terrestrial food chain, just as the loss of fish from acidified lakes extends throughout the aquatic food chain.

In conclusion, it seems a great deal has been learned since the 1960s about acid rain, the causes and effects. Major moves are now well underway to reduce acid inputs to the atmosphere and to slow or reverse the acidification of the industrialized world.

References