Lake Acidification Causes
Acidification caused by acid rain precipitation has been, and remains, a major environmental issue because of its life-threatening effects on biota, its global spread, and the prolonged recovery period associated with it. International cooperation to reduce the precursors of acid precipitation has provided a textbook example of how society can address a complex environmental pollution problem with support from science. A key step in that success was the achievement of a broad scientific consensus that acid precipitation was a serious threat to ecosystems in sensitive regions. That consensus was built during two decades of scientific research starting with the first United Nations conference on the environment in 1972 and continuing to 1990 with the conclusion of major research programs in Europe and in the United States. But is this the only cause? A new study of the role of dissolved organic carbon, which comes from living organisms and can also make lakes acidic, suggests that power station emissions may have played less of a role than previously thought. Martin Erlandsson of the University of Reading, United Kingdom, and his colleagues wondered whether it was possible to distinguish the historical effects of organic acids and power station emissions by assessing findings during the 20 years since lake acidification started to decrease in Sweden. They describe their results in the August issue of BioScience.
Although there are few measurements of the amount of dissolved organic carbon in Swedish lakes before the 1980s, the amount of dissolved organic carbon in them has continued to increase despite the stabilization of power station emissions around 1990. The reason is unknown, but the increase supports the idea that as power station emissions increased during the 20th century they may have partly suppressed organic acidity in lakes that was present in pre-industrial times—at higher levels than when it was assessed in 1990.
Erlandsson and colleagues estimated the pre-industrial acidity of 66 lakes under different assumptions about the amount of dissolved organic carbon in them, and found that the assumptions had a large effect on estimates of how much the lakes had been affected by power station emissions. Studies of sediments in some of the lakes seem to bear out the idea that preindustrial organic carbon levels were at least as high as they are today—and considerably higher than they were in 1990. That in turn means the power station emissions did not contribute as much to lake acidification as was thought when liming programs were instigated.
The DOC concentrations in freshwaters vary over both historical and recent time scales. At present, there is no sign that the decadal trends of increasing DOC are abating in Sweden, even though sulfate concentrations in rain and surface waters are stabilizing. The results demonstrated in this study are not intended to imply that the question of what preindustrial DOC concentrations really were has been resolved, but rather to exemplify the significance of using different DOC reference levels.
Application of study technique to four lakes in southern Sweden indicates 15%—50% higher organic matter concentrations around 1850 relative to those in 2009. For two of these lakes, the reconstructed preindustrial DOC is actually sufficiently high for DOC suppression to completely compensate for acid deposition with no net change in the pH relative to preindustrial conditions. For the other two lakes, however, much higher DOC levels than what the paleolimnological reconstructions suggest are required for full pH compensation.
What this study shows is that acidification of a lake is a complex matter which is not yet clearly understood. Multiple cause may exists.
For further information: http://www.aibs.org/bioscience-press-releases/resources/current-press-release.pdf