Acid Rain Effect On Lakes and Aquatic Ecosystems

One of the direct effects of acid rain is on lakes and its aquatic ecosystems. There are several routes through which acidic chemicals can enter the lakes. Some chemical substances exist as dry particles in the air while others enter the lakes as wet particles such as rain, snow, sleet, hail, dew or fog. In addition, lakes can almost be thought of as the "sinks" of the earth, where rain that falls on land is drained through the sewage systems eventually make their way into the lakes. Acid rain that falls onto the earth washes off the nutrients out of the soil and carries toxic metals that have been released from the soil into the lakes. Another harmful way in which acids can enter the lakes is spring acid shock. When snow melts in spring rapidly due to a sudden temperature change, the acids and chemicals in the snow are released into the soils. The melted snow then runs off to streams and rivers, and gradually make their way into the lakes. The introduction of these acids and chemicals into the lakes causes a sudden drastic change in the pH of the lakes - hence the term "spring acid shock". The aquatic ecosystem has no time to adjust to the sudden change. In addition, springtime is an especially vulnerable time for many aquatic species since this is the time for reproduction for amphibians, fish and insects. Many of these species lay their eggs in the water to hatch. The sudden pH change is dangerous because the acids can cause serious deformities in their young or even annihilate the whole species since the young of many of such species spend a significant part of their life cycle in the water.

Subsequently, sulphuric acid in water can affect the fish in the lakes in two ways: directly and indirectly. Sulphuric acid (H₂SO₄) directly interferes with the fish's ability to take in oxygen, salt and nutrients needed to stay alive. For freshwater fish, maintaining osmoregulation is key in their survival. Osmoregulation is the process of maintaining the delicate balance of salts and minerals in their tissues. Acid molecules in the water cause mucus to form in their gills and this prevents the fish to absorb oxygen as well. If the buildup of mucus increases, the fish would suffocate. In addition, a low pH will throw off the balance of salts in the fish tissue. Salts levels such as the calcium (Ca⁺²) levels of some fish cannot be maintained due to pH change. This results in poor reproduction - their eggs produced would be damaged; they are either too brittle or too weak. Decreased Ca⁺² levels also result in weak spines and deformities. 

For example, crayfish need Ca⁺² to maintain a healthy exoskeleton; low Ca⁺² levels would mean a weak exoskeleton. Another type of salt N⁺ also influences the well-being of the fish. As nitrogen-containing fertilizers are washed off into the lakes, the nitrogen stimulates the growth of algae, which logically would mean an increase in oxygen production, thus benefitting the fish. However, because of increased deaths in the fish population due to acid rain, the decomposition process uses up a lot of the oxygen, which leaves less for the surviving fish to take in.

Indirectly, sulphuric acid releases heavy metals present in soils to be dissociated and released. For example, aluminium (Al⁺²) is harmless as part of a compound, but because acid rain causes Al⁺² to be released into the soils and gradually into the lakes, it becomes lethal to the health of the fish in the lakes. Al⁺² burns the gills of the fish and accumulates in their organs, causing much damage. So, although many fish may be able to tolerate a pH of approximately 5.9, this acid level is high enough to release Al⁺² from the soils to kill the fish. This effect is further augmented by spring acid shock. The effect of acid rain can be dynamically illustrated in a study done on Lake 223 which started in 1976. Scientists monitored the pH and aquatic ecosystem of Lake 223. They observed that as the pH of the lake decrease over the years, a number of crustaceans died out because of problems in reproduction due to the acidity of the lake caused by acid precipitation. At a pH of 5.6, algae growth in the lake was hindered and some types of small died out. Eventually, it was followed by larger fish dying out with the same problem in reproduciton; there were more adult fish in the lake than there were young fish. Finally, in 1983, the lake reached a pH of 5 and the surviving fish in the lake were thin and deformed and unable to reproduce. This case study obviously illustrates the significant effect of acid rain on lakes and its aquatic ecosystem.

The following is a chart which summarizes the effect of the pH level of the lake on its lifeforms.

pH LEVEL	EFFECTS
<6 font="">	*Basic forms of food die off. Eg. Mayflies and stoneflies are important food sources for fish. They can't survive at this pH level.
<5 .5="" font="">	*Fish cannot reproduce. *Young have difficulty staying alive. *More deformed adult fish due to lack of nutrients. *Fish die of suffocation.
<5 .0="" font="">	*Fish population die off.
<4 .0="" font="">	*Very different lifeforms, if any, from before.

The "safe" level of mercury in food has been set at about 0.05 parts per million. Indians and Eskimos in parts of Canada and the United States eat fish and seal meat with mercury levels as high as 15.7 and even 32.7 parts per million.

Fish, being one of the primary members of the food chain, is food for many other lifefoms, including hunans. Because toxic materials such as mercury are deposited in the fish due to acid rain, it is dangerous for humans to consume the fish. Like the domino effect, fewer fish can be sold as food, fishermen lose their hobby and people selling fishing supplies are affected. Amphibians are also affected; like the fish, they cannot reproduce in an acidic environment. The amphibian embyos have membranes that are too tough because of the acids, such that they are unable to break through at the proper time. So, they continue to grow, only to have deformed spines. They are then killed by a fungus that has been allowed to grow on their membranes. Hence, in essence, the effects of acid rain on lakes and its aquatic ecosystem are numerous and overwhelmingly magnified as we move down the food web.

In just ten years, from 1961 to 1971, Lumsden Lake in the beautiful Killarney region of Ontario, Canada, went to a pH reading of 6.8 to 4.4. That's an increase in acidity of more than 200 times. Most lakes with dropping pH values are at higher elevations. These lakes are usually small and located in watersheds where the rock and soil have a low neutralizing capacity.