Acid deposition, from human-caused emissions of oxides of sulphur and nitrogen, is probably the greatest threat to small boreal lakes in Canada and Eurasia. Acidifying sulphur oxide emissions have been reduced by over 50% in Canada. Legislation has been passed to compel similar reductions in the United States by early in the twenty-first century. However, these measures are estimated to have reduced the potential effect of acid precipitation on Canadian lakes by only about half.
The rapid decline in DOC caused by acidification caused much greater increases in UV-B penetration than climate warming alone. In the most acidified lake, 302S, the depth of 1% UV-B penetration increased from about 0.3 meters to over 2.8 meters --nearly a tenfold increase. As a result, the proportion of the lake's volume exposed to greater than 1% of UV-B increased over eightfold, from 6% to nearly 50%.Overall, the study's authors estimate about 140,000 of the nearly 700,000 lakes (i.e., about 20 percent) in eastern Canada may have DOC concentrations low enough for UV-B penetration to be of concern. The highest concern, they say, must be for clear, shallow lakes, streams and ponds, where even modest declines in DOC may eliminate the small regions that are deep enough to provide refuges from damaging UV- B radiation.
However, most species disappear in natural aquatic ecosystems at higher pH values (less acidic conditions) than predicted by laboratory tests, thus suggesting that, in ecosystems, additional stresses enhance the effects of acidification. It is possible that one such stress is the increased exposure to UV-B caused by DOC decreases in acidified lakes. These results indicate that in aquatic systems, climate warming and/or acidification can increase the exposure of organisms to UV-B much more than changes in UV-B caused by depletion of the stratospheric ozone layer. The authors say that, in clear oligotrophic lakes [(.e., clear, healthy lakes that lack excessive nutrients for plants, and have plenty of dissolved oxygen), the decreases in DOC caused by climate warming, drought and acidification should be of much more concern than depletion of stratospheric ozone, so far as UV-B exposure is concerned.