Acid deposition, commonly known as acid rain, is a broad term describing any precipitation—including rain, snow, fog, or dry particles—that contains elevated levels of nitric and sulfuric acids. These acids form when atmospheric emissions of sulfur dioxide (\(\text{SO}_2\)) and nitrogen oxides (\(\text{NO}_x\)) react with water, oxygen, and other chemicals in the atmosphere. While natural sources contribute a small amount, the majority of these compounds come from the burning of fossil fuels for electricity generation and transportation. These chemical changes ultimately translate into profound biological harm when they fall back to earth, altering the habitats and survival of animal populations across various ecosystems.
How Acid Rain Alters Aquatic Environments
Aquatic environments, such as lakes and streams, are the most immediately and severely impacted by acid rain, primarily because the water itself becomes acidified. Normal rainwater is slightly acidic, with a pH of about 5.6, but acid rain can have a pH as low as 4.3. The ability of a water body to neutralize this acid—known as its buffering capacity—determines how quickly the pH drops and how rapidly the ecosystem declines.
The most damaging consequence of this acidification is the mobilization of aluminum (\(\text{Al}\)) from the surrounding soil. As the acidity of the water increases, hydrogen ions leach aluminum from previously stable soil compounds, releasing it into the water as a toxic free-ion (\(\text{Al}^{3+}\)). This dissolved aluminum is highly toxic to fish and other aquatic life.
In fish, elevated aluminum concentrations cause a lethal physiological response by clogging the gills. The aluminum precipitates onto the gill surface, leading to a buildup of mucus that impairs the fish’s ability to regulate salts and water, ultimately causing asphyxiation. Even at sublethal levels, the combination of low pH and aluminum causes chronic stress, resulting in reduced body weight and smaller size, making fish less competitive for food and habitat.
Aquatic invertebrates, which form the base of the food web, are also highly sensitive to these changes. Acid-sensitive species, such as certain insect larvae and snails, are lost as the pH declines, leading to a reduction in biodiversity and a major disruption in the food chain. This loss of acid-intolerant prey cascades upward, reducing the food supply for larger predators like birds and mammals that rely on a healthy aquatic ecosystem.
Impact on Terrestrial Food Webs
Acid deposition profoundly affects terrestrial food webs by chemically altering the composition of the soil, which then transfers toxicity up the trophic levels. The acidic input accelerates the leaching of essential nutrient cations, such as calcium (\(\text{Ca}\)), from the soil’s structure. The loss of these nutrients compromises the health of vegetation and the animals that feed on it.
Simultaneously, the lower soil pH acts as a chemical trigger, mobilizing toxic heavy metals that were previously bound to soil particles. Metals such as mercury (\(\text{Hg}\)) and cadmium (\(\text{Cd}\)) are released into the soil water, where they become bioavailable to plants and terrestrial invertebrates. These soil-dwelling organisms, including earthworms, take up the toxic metals.
This process of contamination leads to biomagnification as the metals move up the food chain. Birds and mammals that consume contaminated terrestrial invertebrates or fish accumulate these toxic substances in their tissues. For instance, methylmercury is a neurotoxin that can reach high concentrations in predators like otters, mink, and certain bird species that rely on aquatic food sources.
Chronic Effects on Reproduction and Development
Beyond immediate mortality, acid rain causes chronic, long-term effects that severely limit the ability of animal populations to recover and sustain themselves. Reproductive failure is a major driver of population decline in acidified environments. For fish, the eggs of many species fail to hatch when the water pH drops below 5.
Amphibians are particularly vulnerable because their permeable skin and dependence on water for reproduction expose them directly to acidic conditions. Low pH and elevated aluminum levels can cause high embryo mortality and developmental deformities in tadpoles and larvae. These deformities include spinal curvature, tail kinks, and stunted gills, which reduce the likelihood of survival to adulthood.
The leaching of calcium from the soil also creates a physiological barrier to successful reproduction in birds. Birds that rely on insects and snails—invertebrates that need calcium to form their exoskeletons—as their primary food source suffer from a calcium deficiency. This lack of calcium results in the production of thin, fragile eggshells, leading to reproductive failure when the shells break during incubation. These chronic issues prevent populations from successfully replacing themselves, even if the ambient acidity is only moderately elevated.