Acid rain occurs when compounds introduced into the atmosphere through burning fossil fuels fall to the Earth as rain. Acid rain has the potential to change the pH of both aquatic and terrestrial ecosystems, affecting organisms' ability to survive. In lakes and ponds, some plants cannot survive acidic conditions, while others grow in abundance. If you have a pond on your property or as part of your garden and live in an area that receives high levels of acid rain, the growth of your pond plants could be affected.
Plants and pH
Changes in pH alter the chemistry of a lake or pond. In this delicate balance, certain elements occur in quantities adequate for plant nutrition but not in excesses that cause toxicity. Level of pH affects the availability of different chemical elements, increasing some to toxic levels while "complexing" others. Complexed nutrients are not available for plants to use and contribute to nutrient deficiencies that limit growth.
As pH rises, aluminum becomes more available, approaching toxic levels. According to the U.S. Environmental Protection Agency, aluminum toxicity is as much to blame for the loss of aquatic organisms in acidic lakes as low pH. In the article "Aluminum Toxicity and Tolerance in Plants" in the 1995 volume of Plant Physiology, Emmanual Delhaize and Peter R. Ryan note that even small increases in aluminum may be toxic to plants, primarily by stunting root growth.
Plants rely on nutrients freed from decomposing organic matter in order to survive. The TIES Project of Ramapo College describes a pH of 5 or 6 as the point where ecology in a body of water changes. One change is the destruction of many decomposing microorganisms. Organic matter ceases to be broken down into molecules that plants can use to meet their nutritional needs, and plants begin to starve. Phosphorus, an essential plant nutrient, also becomes "complexed" and unavailable to plants.
Microscopic plants show different responses to acidification of the water in which they live. According to the Vermont Water Quality Division, acid rain decreases the population of phytoplankton, tiny plants that float suspended in the water, causing acidic lakes to appear deep blue in color. According to Dr. Tom Wolosz of the State University of New York, other algae thrive. As fish and other animals die off, these algae are left without natural predators and, able to tolerate the acidic conditions, grow unchecked. In short, acid rain increases the plants you don't want in your pond--the slimy algae that coats rocks and plants--while threatening the pond plants that you do want. According to the U.S. Geological Survey, nuisance aquatic plants tend to be able to survive higher pH levels.
As the EPA points out, acid rain devastates a lake through a cascade of effects. An action seemingly as small as the reduction of microbial populations creates a ripple effect that disrupts all other species in the lake. As primary producers in the aquatic environment--the organisms able to convert sunlight to chemical energy--plants destroyed by low pH, aluminum toxicity or nutrient deficiency caused by acid rain no longer provide food and habitats for other organisms. Those organisms, in turn, die off. In highly acidic lakes, there may be little or no life at all.