Acid Rain and Algal Growth

Acid Rain or atmospheric deposition that contains higher than normal concentrations of nitric and sulfuric acid has an impact on both the terrestrial and aquatic ecosystems. Acid Rain is produced primarily from human made emissions from burning fossil fuels. There is a limited natural addition from decaying vegetation and volcanoes. Many lakes within the Northeastern United States are impacted by Acid Rain. This impact can range from minimal, with only a slight change in the pH of the water, to the entire lake almost devoid of life.

Acid Rain deposits nitrates that can increase nitrogen loading within forests surrounding bodies of freshwater. This can lead to nitrogen saturation and the removal of calcium and magnesium from soils, thereby impacting the aquatic ecosystem. The excessive nitrogen within the forests will make its way into streams and lakes feeding excessive algal growth. Acid Rain will decrease the available carbon within an aquatic ecosystem and make the body of water more acidic. This shifts the species composition of algae to only acidophilic forms (acid tolerant). Acid Rain will not cause phytoplankton blooms, as both phytoplankton and zooplankton populations will decrease. The added nitrogen will increase near-shore or littoral algal abundance. In essence you can have a lake that has an extremely productive near-shore area that experiences excessive algal growth. That alone can alter the carbonate balance of the body of water and cause hypoxic conditions and fish kills, then add to that a lifeless water column and deep water area. With the stripping of forest soils of calcium and magnesium and the subsequent addition of the calcium and magnesium to a body of water, it can lead to the introduction and full colonization of invasive species. This can be compounded by the addition of road salt application with the watershed, as road salts have the same saturation/ leaching impact on calcium and magnesium within soils.

The major concerns with Acid Rain are fish populations. A pH below 6.5 units will start to have an impact to native trout populations. Below a pH of 5 units, juvenile fish species and aquatic insects are impacted and fish eggs will not hatch. Below a pH of 4.5 units, the water generally becomes unsuitable for most aquatic organisms.