| 词条 | Draft:The Effects of Acid Rain on Ecosystems |
| 释义 |
Acid rain is a form of rain (or any other form of precipitation) that is acidic, meaning it has an elevated concentration of hydrogen ions and therefore a pH value less than the natural rainwater level of 5.6. This occurs as a result of atmospheric gasses, such as sulfur dioxide and different nitrogen oxides that react with water molecules to form strong acids. This effect in on top if the acidity introduced by the anthropogenic CO2which in water forms a weak carbonic acid [1]. The acid rain has adverse effects on various ecological communities such as terrestrial and aquatic ecosystems, along with their many components. This is mostly due to the altering of the pH in the environment by this acidic precipitation[2]. Aquatic EcosystemsThe Effect of Acid Rain on Freshwater EcosystemsAcid rain greatly affects freshwater ecosystems such as lakes, streams, and rivers. Freshwater systems do not have the volume and buffering capacity of the oceans and therefore are much more vulnerable to acidification. The alkalinity in lakes has been lowered due to the supply of sulfate and other acids deposited in areas such as the northeastern United States, Canada, and even Scandinavia [3]. This can present a serious problem due to the fact that the alkaline compound serves as buffers in the water, having the ability to neutralize some of the supplied acids present. However, the continued supply of SO42− in the acid rain, causes an imbalance in the acid-base ratio of the water and exhausts the ability of these alkaline materials to successfully buffer the solution[1]. The increased acidity affects various organisms in the aquatic community. Most of the adult fish can tolerate a pH value of around 5.5, therefore living in the bodies of water with lower pH values ( ie..more acidic) may cause severe negative effects. An abundance of sulfuric acid can directly interfere with a fish's ability to take in oxygen, nutrients, and salts. Fish need to maintain osmoregulation, which is a process that maintains the ratio of minerals and salts in their tissue. Acid molecules can also cause a mucus-like substance to accumulate around the gills and prevent the absorption of oxygen leading to possible death[5]. Specifically, Atlantic salmon runs in Nova Scotia, Canada have been directly impacted as a result of acid rain. These salmon have become extinct in up to fourteen rivers, severely impacted twenty rivers, and lightly impacted sixteen[6]. The survival rate of fish eggs will also decrease as a result of acid precipitation. The waters lower pH value enables the eggs ability to hatch or will cause deformations. Therefore, if fish cannot produce offspring it will severally affect the population of this species in this ecosystem[7]. Similarly, juvenile fish and many other organisms on the lower end of the aquatic food web have shown to be intolerant of this much lower pH value [8]. As the members on the lower trophic level such as plankton are unable to withstand the more acidic pH levels the effect may propagate up the food web, causing the decline in the food base for the higher level consumers in the food web. The Effect of Acid Rain on Marine EcosystemsThe marine ecosystems on earth are also being affected by acid rain. The acidification of the entire global oceans is already underway as a result of the increased levels of carbon dioxide in the atmosphere, which then makes its way into the oceans as a result of precipitation like rain and snow. This increase of the ocean acidity weakens the buffering capacity of the ocean waters. This effect is then compounded by the non- CO2 of acidity from land either in the form of waters from the acidified rivers entering the coastal ocean or the acidity deposition of the polluted air downwind from major industrial centers. This means that in the water adjacent to the continent the acidification is particularly intense as these two sources of acidity are combined. As acidification adds hydrogen ions to the water it lowers the pH and decreases the concentration of carbonate ions, shifting the reaction below to the left:[9]. Many impacts of the ocean acidification are caused by the decrease in the concentration of the carbonic acid ions. These ions are critical components for marine calcifying organisms, such as corals and shellfish, that normally use them to build and maintain their calcium-carbonate-based outer shells or exoskeletons, the drop of CO3^2-.Shifts the reaction below to the left:[10]. By lowering the degree of saturation, it makes CaCO3 more soluble, and therefore requires more energy to deposit and maintain calcareous shells, thus diverting the resources from the survival growth and reproduction in these species[11]. As in the freshwater, there could be indirect impacts for higher trophic levels, if their food source is affected. Among those may be the primary producers like Coccolithophora, small algae producing plates made of calcite[12]. If as a result of acidification they suffer, so will the organisms relying on them for nutrients. Terrestrial EcosystemsIn terrestrial ecosystems, soil productivity and forest growth are the factors that are the most affected by acid rain. The Effect of Acid Rain on SoilWhen acid rain enters the soil, the soil becomes acidic. This is a problem for soils that are already naturally acidic since they become even more acidic, causing plants that are acid sensitive to weaken and become more susceptible to disease and early death. This, in turn, affects animals that normally rely on these plants as a food source and alters the types of plants that grow in the environment. Soils that are rich in limestone, or have a bedrock composed of limestone or marble, are virtually unaffected by this process[13]. These soils are more alkaline and therefore produce a higher pH when in contact with water. This buffers the additional acid which results in a more neutral pH with minimal effect on soil productivity[14]. Acid rain lowers the pH of the soil, and when the pH drops below 5, typically insoluble aluminum is able to dissolve into the water and become toxic to plants. This mobilization of aluminum ions causes the stunting of root growth and the prevention of roots from taking up calcium and nutrients that are necessary for the plant to thrive. A combination of the lowered pH and mobilized aluminum can also reduce the population of soil microorganisms. These bacteria help with the breakdown and decaying of leaves and release nutrients such as calcium, magnesium, nitrate and more and therefore these processes are inhibited[15]. The Affects of Acid Rain on Forest EnvironmentsAcid indirectly affects tree growth. The acid typically weakens a tree by damaging its roots and causes necrosis to occur in its leaves, which doesn’t allow them to effectively undergo processes necessary for gaining nutrients. The acidic water also dissolves many of the nutrients in the soil before the tree even has a chance to absorb them[16]. Forests of higher elevation often are the most affected by acid precipitation. This is because they come in contact with the acidic clouds and fog from which the rain originates. These clouds are even more acidic than the rain itself and severely damage the trees leaves[17]. This prevents the leaves from undergoing photosynthesis, which is how the plant creates food for itself. References[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] |
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