Acid Rain and its Effects
Acid rain primarily refers to precipitation that has a pH of 5.2 or below. Acid rain is caused by the mixing of nitrogen oxides and sulphur dioxide produced as a result of the combustion of fossil fuels with that of rainfall. Terms such as acid deposition or acid precipitation are also used to describe acid rain.
The effects of acid rain are manifold. From corroding surfaces of structures that are exposed to air pollution and deteriorating monuments and buildings made of marble or limestone to impacting biodiversity, acid rain is a major cause of concern in the present-day world.
Acid rain causes trees to weaken, thus increasing their vulnerability to stress factors such as cold temperature, pests and drought conditions. Owing to acid rain, the soil, especially in acid-sensitive areas, is depleted of essential plant nutrients and minerals such as magnesium and calcium. These are instead replaced by aluminium (usually in dissolved toxic form) bound to rock and soil particles. Also in such landscapes, the deposition of acid due to acid rain leads to a reduction of the pH of surface water resulting in biodiversity loss.
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Effects of Acid Rain on Lakes and Rivers
Among its many outcomes, the effect of acid rain on lakes and rivers is very prominent. Aid rain, particularly caused by the emissions of nitrogen oxides or NOX has a major effect on the environment. The acid rain effect on lakes and rivers is a prime example of this. A number of coastal marine ecosystems, lakes and estuaries receive an abundance of nitrogen from acid rain combined with terrestrial runoff. This over-enrichment causes overgrowth of algae and plants in the water bodies, a process defined as eutrophication. As and when these plants and algae die, their decomposition causes a depletion in the oxygen supply that is necessary for aquatic life to survive in water bodies. As such, the effects of acid rain on lakes and rivers leads to a major environmental problem across the world.
Acid rain is the term commonly used in place of the scientific term ‘acid deposition’. In addition to acidic rain, acid deposition also includes other forms such as fog, hail, sleet, snow, etc. The dry deposition of gases and acidic particles is also included and it leads to landscapes getting affected even during the dry periods. Acid rain is thus capable of affecting animals residing within such landscapes even when there is a lack of precipitation.
Effects of Acid Rain on Aquatic Life
So far we have seen the effects of acid rain on rivers and lakes and the resulting eutrophication, and how it adversely affects the life of aquatic animals. In addition to this, there are several effects of acid rain on aquatic life. As early as the 1960s and 1970s, parts of eastern North America and western Europe witnessed a decline in the health of many aquatic animals - crayfish, fish and clam mostly residing in rivers and lakes situated in the more remote locations.
The increased amounts of acid deposition in acid-sensitive areas as such causes the lakes and rivers and other water bodies to become more acidic than before. In general, areas predisposed to acidification owing to the low buffering and acid-neutralising capacity of the soils are termed acid-sensitive. The high concentration of aluminium released from soils in acid-sensitive areas usually enters the lakes, streams and rivers. The leached aluminium in combination with the increased acidity of the water bodies can impair the respiratory systems of fishes by damaging their gills.
Studies have shown that the species diversity of fishes is greatly affected by acid rain. It has been shown that there is often a sharp decline in the number of fish species when the pH of water bodies drops to 4.0-4.5 from 6.0-7.0. In addition to fishes, other aquatic animals are also negatively affected by acid rain creating ripples throughout the entire food chain and aquatic ecosystems.
Effects of Acid Rain on Animals
Acid rain can also affect animals to a severe extent. Acid rain that is primarily caused by sulphur dioxide can lead to the deposition of sulphur. This accumulated sulphur hastens the conversion of mercury in its elemental form to methyl mercury, its deadliest form. This type of conversion is caused by acid rain occurring in soils saturated with water and wetlands. The low-oxygen environments of such areas are the optimum conditions required for methylmercury formation by bacteria.
Methyl mercury is a neurological toxin and concentrates in organisms as it progresses through the food chain. This phenomenon is termed bioaccumulation. While methyl mercury is present in smaller amounts in zooplanktons and phytoplanktons, the animals at the higher tiers of the food chain consume large numbers of such lower-tier food chain organisms. The methyl mercury accumulates in the fat cells of animals or other predators including humans that consume them. Eventually, the concentration of methyl mercury in animals increases to amounts where they could prove to be harmful.
Several government health advisories thus recommend lesser consumption of fish from marine and freshwaters due to the bioaccumulation of methylmercury in the fish tissues.
FAQs on Acid Rain Effects on Lakes and Rivers
1. What is the primary cause of acid rain that affects lakes and rivers?
The primary cause is air pollution from human activities, mainly the burning of fossil fuels in power plants and vehicles. This process releases large amounts of sulphur dioxide (SO₂) and nitrogen oxides (NOx) into the atmosphere. These gases react with atmospheric water, oxygen, and other chemicals to form sulphuric and nitric acids. These acids then fall to the earth as acid rain, snow, or fog, and are carried into lakes and rivers through runoff.
2. How does acid rain chemically alter the water in lakes and rivers?
Acid rain chemically alters aquatic environments by lowering their pH, a process known as acidification. While normal rain is slightly acidic (pH ~5.6), acid rain is significantly more so. This increased acidity depletes the natural buffering capacity of the water, which is its ability to neutralize acids. As the pH drops, the fundamental chemical balance of the lake is disrupted, which can also cause toxic metals like aluminium to leach from the surrounding soil into the water.
3. What are the direct effects of increased acidity on aquatic life?
Increased acidity has severe and often fatal effects on aquatic organisms. The key impacts include:
- Reproductive Failure: Many fish eggs cannot hatch in water with a pH below 5.0. This prevents fish populations from sustaining themselves and can lead to their local extinction.
- Physiological Damage: Low pH levels can damage the gills of fish, disrupting their ability to take in oxygen and maintain salt balance.
- Food Web Collapse: Acid-sensitive organisms like crayfish, snails, and certain types of plankton are often the first to die. Their disappearance removes a critical food source for fish and other predators, causing the entire aquatic food web to collapse.
4. How does acid rain release toxic metals like aluminium into water bodies, and why is this so dangerous?
When acid rain seeps into the soil surrounding a lake or river, its acidity breaks down soil minerals, releasing naturally-bound toxic metals like aluminium into the water. This is dangerous because soluble aluminium is highly toxic to aquatic life. It can clog the gills of fish, leading to suffocation, and interfere with the growth and development of young fish. This secondary effect of acid rain is a major reason for the sharp decline in fish populations in acidified lakes.
5. Why are some lakes more sensitive to the effects of acid rain than others?
A lake's sensitivity depends entirely on its buffering capacity, which is determined by the geology of its surrounding watershed. Lakes in areas with limestone or other carbonate-rich rocks have a high buffering capacity because these alkaline minerals neutralize the acid. In contrast, lakes in regions with thin soil and hard, crystalline bedrock like granite have a very low buffering capacity. They cannot neutralize the incoming acid and are therefore highly vulnerable to rapid and severe acidification.
6. What is meant by the 'buffering capacity' of a lake in the context of acid rain?
The buffering capacity of a lake is its natural, inherent ability to resist changes in pH when acidic compounds are introduced. It is a measure of how much acid the water can absorb and neutralize without its pH dropping significantly. This ability is provided by alkaline substances, such as bicarbonates and carbonates, which leach from rocks and soil in the lake's catchment area. A lake with high buffering capacity can withstand more acid rain before its ecosystem is harmed.
7. Can lakes and rivers recover from the effects of acid rain?
Yes, recovery is possible, but it is a very slow and long-term process. The most critical factor for recovery is a significant reduction in the emissions of sulphur dioxide and nitrogen oxides. Once the source of acid rain is controlled, natural processes can gradually restore the water's chemical balance. In some cases, a process called liming (adding powdered limestone to the water) can be used to manually raise the pH, but this is an expensive, temporary fix that does not fully restore the original biodiversity.
