Acid Formation in the Atmosphere
First, let us review some basic chemistry as it applies to acid precipitation.
Carbonic acid forms naturally in the atmosphere due to the reaction of water (H2O) and carbon dioxide (CO2),
H2O + CO2 -> H2CO3
while the burning of coal and other organics adds sulfur dioxide (SO2) and Nitrous oxides (NOx) to the atmosphere where they react to form sulfuric acid and nitric acid,
2SO2 + H2O + O2 -> 2H2SO4
4NO2 + 2H2O + O2 -> 4HNO3
All of these acids will be buffered by reacting with rocks, minerals, etc. on the earth's surface. The most important (and fastest) buffering comes from the reaction with (weathering of) calcite in the form of limestone, dolomite or marble.
H2CO3 + CaCO3 -> 2HCO3- + Ca+2
When this reaction occurs, the acid is neutralized and the calcite dissolved. While the reaction with calcite is very fast (the standard test for calcite in introductory geology labs is to put very dilute acid on a sample to see if it bubbles (reacts)), the reaction with other rocks is very slow, so most of the acid is not affected. This is why ponds in the Adirondacks became acidified (non-calcite rock in those areas), while Lake Champlain (abundant calcitic bedrock) did not.
The degree of acidification is the pH of the water, which is defined as the negative logarithm of the concentration of hydrogen ion (H+), or
pH = -log [H+].
(This to a certain degree comes from the old definition of an acid as a proton donor. A hydrogen ion is little more than a proton, so think of it as the amount of free protons floating around).
A pH of 7 is considered neutral, while a pH less than 7 is considered acidic. For example, wine has a pH of about 3.5 and your stomach digestive fluids have a pH of about 1.9.
We should also be aware that increased acidity does not have to be constant, but instead can be episodic. High surface water discharge events (storms, snowmelts) can increase the pH of streams and ponds to dangerous levels for short times.
Effects of Acidity on Plants and Animals
As a first example of the effects of acid rain, we can examine a case which is not obvious - effects on non-aquatic, tree nesting birds. This study was carried out in the Netherlands. It was observed that the proportion of birds laying defective eggs rose from roughly 10% in 1983-84 to 40% by 1987-88. The defective eggs had thin and highly porous egg shells, which resulted in eggs failing to hatch because of shell breakage and desiccation. As a result, there was also a high proportion of empty nests and clutch desertion. It was also observed that these effects were limited to areas of acid rain.
Since the birds did not appear to be directly affected by the acidity, the food chain was examined (these birds are positioned at the upper part of the local food chain). The difference between areas of normal soil pH (buffered by high calcium content due to limestone and marble outcrops and bedrock) and those with acidic soil appeared to be the presence of snails. The snails depend on the soil as their calcium source as they secrete their shells. With much of the CaCO3 leached out of the soil by the acid precipitation, the snails could not survive in the area. The birds did not, at first, appear to be affected, because they continued to eat spiders and insects which, while supplying a sufficiently nutritious diet for the birds, where a poor source of calcium.
To test the hypothesis that the lack of calcium was the cause of the bird's laying defective eggs, ecologists "salted" the area with chicken egg shell fragments. The birds began to eat the chicken egg shells, and those that did laid normal eggs.
In this case, acid precipitation had affects that passed on up the food chain.
AFFECTS ON AQUATIC SYSTEMS
Mollusks - snails and clams.
- these invertebrates are highly sensitive to acidification because of their shells which are either calcite or aragonite (both forms a CaCO3) which they must take from the water.
- in Norway, no snails are found in lakes with a pH of less than 5.
- of 20 species of fingernail clams, only 6 were found in lakes with pH of less than 5.
- crustaceans are not found in water with a pH less than 5.
- crayfish are also uncommon in water where the pH is less than 5. This is an important consideration because crayfish are an important food source for many species of fish.
- many insects also become rare in waters with a pH less than 5.
- as you may know, many species of amphibians are declining. To what extent acid rain is contributing to this decline is not exactly known. However, one problem is that in places like northeastern North America amphibians breed in temporary pools which are fed by acidified spring meltwater. In general, eggs and juveniles are more sensitive to the affects of acidity.
Zooplankton in lakes
- changes in diversity among zooplankton have been noted in studies carried out in lakes in Ontario, Canada. These studies found that in lakes where the pH was greater than 5 the zooplankton communities exhibited diversities of 9 - 16 species with 3 - 4 being dominant. In lakes where the pH was less than 5, diversity had dropped to 1 - 7 species, with only 1 or 2 dominants.
- many acidified lakes exhibit a large increase in the abundance of periphytic algae (those that coat rocks, plants and other submerged objects). This increase has been attributed to the loss of heterotrophic activity in the lake (i.e., the loss of both microbial and invertebrate herbivores in the lake).
- as a result of acidification, fish communities have suffered significant changes in community composition attributed to high mortality, reproductive failure, reduced growth rate, skeletal deformities, and increased uptake of heavy metals.
- effects on embryos and juveniles:
- Atlantic salmon fry have been observed to die when water with pH <>
- in fish embryos, death appears to be due to corrosion of epidermal cells by the acid. Acidity also interferes with respiration and osmoregulation. In all fish at a pH of 4 to 5 the normal ion and acid/base balance is disturbed. Na+ uptake is inhibited in low pH waters with low salinity. Small fish are especially affected in this way because due to their greater ratio of body and gill surface area to overall body weight, the detrimental ion flux proceeds faster.
- in all fish low pH water causes extensive gill damage. Gill laminae erode, gill filaments swell, and edemas develop between the outer gill lamellar cells and the remaining tissue.
- at pH <3>
Reproductive failure has been suggested as the main reason for fish extinction due to acidity. In Ontario, Canada it was observed that in acidified lakes female fish did not release ova during mating season. When examined, the fish were found to have abnormally low serum calcium levels which appears to have disrupted their normal reproductive physiology.
Growth may increase or decrease depending on resistance of a species to acidity. For resistant species, growth can increase due to the loss of competing non-resistant species. On the other hand, growth can decrease due to increase in metabolic rate caused by sublethal acid stress. In this case the organism's rate of oxygen consumption goes up because the excess CO2 in the water increases the blood CO2 level which decreases the oxygen carrying capacity of the hemoglobin.
This occurs in some fish as a response to the lowered blood pH caused by increase in CO2 described above. Bones decalcify in response to a buildup of H2CO3 in the blood as the body attempts to maintain its normal serum osmotic concentration (i.e., the body attempts to return to a normal blood pH level).