Acidification in Swedish lakes down to nature?
A British-Swedish team of scientists led by the University of Reading in the United Kingdom has discovered that emissions from power stations may have contributed less to the acidification of lakes in the Nordic state of Sweden than previously believed. Presented in the journal BioScience, the study highlights the historical effects of organic acids and power station emissions, focusing on the decrease in acidification of Swedish lakes in the last two decades.
Researchers and policymakers in the 1970s and 1980s expressed concerns about how the acidic emissions of sulphur dioxide and nitrogen oxides, generated by power plants, impacted the environment. These worries led to the development and implementation of controls. Specifically in Europe, the concerns focused on how the myriad lakes in Sweden were affected by these emissions. Some experts had even found a number of cases to be noticeably acidic. Officials in Stockholm then instituted a programme to counter the acidification of the lakes by adding thousands of tonnes of lime.
Diverse theories about what triggered acidification, such as climate change and land use, have various implications for defining the preindustrial levels for dissolved organic carbon (DOC), which are required for the assessment of acidification and other aspects of water quality. In this latest study, the researchers postulated that DOC, which is generated by living organisms and can trigger acidification of lakes, puts into perspective the role that power plant emissions have on lake acidification.
Lead author Dr Martin Erlandsson, who is currently a research fellow at the Department of Geography and Environmental Sciences at the University of Reading, along with his colleagues, assessed findings during the 20 years since lake acidification began to decrease in Sweden.
The amount of dissolved organic carbon in Swedish lakes has increased despite the stabilisation of power plant emissions in the 1990s. While there is no conclusive evidence for what triggered the increase, studies of sediments in a number of lakes suggest that preindustrial organic carbon levels were as high as today's levels and higher than levels reached 20 years ago. The led the researchers to believe that power plant emissions did not significantly influence the levels of lake acidification.
In their study, the scientists classified the acidification of 66 lakes with long-term observations, representative of some 12 700 acid-sensitive lakes in nemoral and boreal Sweden. The breakdown shows that 47% of these lakes are classified as significantly acidified (assuming that preindustrial DOC levels were the same as those seen in 1990). However, if the higher DOC levels seen in 2009 define preindustrial conditions, around 24% of the lakes are acidified.
Assessing the sediments in some of the lakes add weight to the theory that preindustrial organic carbon levels were at least as high as current levels, and potentially higher than the levels of 1990. So while past studies postulated that power plant emissions contributed significantly to the acidification of lakes, the observations from this study indicated the opposite.
'This emphasises the need to establish reference levels for DOC and casts new light on the classic controversy about natural versus anthropogenic acidification,' the authors write in the paper.
They also note that the findings of this study highlight the importance of fuelling our understanding of the amounts of dissolved organic carbon in lakes in preindustrial times, because they have a significant influence on the degree of acidification that can be blamed on the emissions of power plants.