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Researchers studying clear lakes in Canada have found that climate warming, acid rain and increased ultraviolet sunlight (from depletion of the stratospheric ozone layer) all interact to allow more ultraviolet light to penetrate deeper into lakes, damaging the ecology of the lakes.[1] This is the first study to show that the effects of these three factors are related.

These researchers have been studying a group of lakes in northwestern Ontario, Canada, over a 20-year period from 1971 to 1990. These lakes are in the "boreal forest" belt which encircles the northern hemisphere below the arctic tundra and above the steppes, plains, and hardwood forests. The Canadian and Hudsonian regions of the earth are covered with boreal forests.

Boreal lakes are the most numerous kind of lake on earth. There are perhaps 2 million such lakes in the world. The biggest lakes, Lake Baikal in Russia, and Lake Superior in the U.S., are boreal lakes.

During the 20-year study period, climate near the Ontario lakes warmed by an average of 1.6 degrees Celsius (2.9 degrees Fahrenheit); the cause of this warming is unknown (though of course global warming must be suspected). Whatever the cause, the climate warming has had significant impacts on water quality in the lakes.

During the same 20-year period, ultraviolet-B (UV-B) sunlight striking the surface of the lakes increased about 10%, as a result of thinning of the stratospheric ozone layer.

During the same period, the lakes became more acidic as a result of acid deposition by rain, snow, and fog.


Dramatic changes have been shown to occur in lake ecosystems exposed to the same intensities of UV-B as those found in eastern Canada today. These changes are caused by UV-B's ability to inhibit photosynthesis in floating plants down to depths of several meters. In other words, the photosynthesis of floating plants (phytoplankton) is reduced by increasing UV-B. The deeper the UV-B penetrates into a lake, the more photosynthesis is reduced. Photosynthesis is the process whereby blue-green plants use the energy of sunlight to convert carbon dioxide and water into carbohydrates, thus creating the bottom-most layer of the world's food chains. A meter is 39 inches.

DOC --dissolved organic carbon --is the main factor that prevents the penetration of visible and ultraviolet sunlight in lakes of the boreal and other northern forested regions. Thus the presence of DOC protects deep areas of clear lakes from the harmful effects of ultraviolet sunlight.

The penetration of UV-B in boreal lakes is known to increase as the concentration of dissolved organic carbon (DOC) decreases. In most boreal lakes, DOC provides an effective shield for aquatic organisms against UV-B, restricting penetration of UV-B to a few tenths of a meter.

The increased penetration of UV-B radiation and resulting harm to microorganisms can thus create a positive feedback, accelerating the removal of DOC. (UV-B reduces DOC, which in turn allows UV-B to penetrate deeper into the water, thus further reducing DOC, which in turn allows UV-B to penetrate further, and so on.)

The present study reports that two additional factors, in addition to increased UV-B from ozone depletion, have reduced DOC in boreal lakes during the 20-year study period: climate warming and acidification have caused declines in DOC, allowing increased penetration of UV-B. Thus these three factors have worked together to increase the penetration of UV-B deeper into clear lakes.


Although it is not known whether the observed 20-year warming trend is part of a natural cycle or is caused by increasing emissions of greenhouse gases (chiefly carbon dioxide from combustion of coal and oil), this study provides a useful glimpse of the effects of climate warming on boreal lakes. In addition to the observed increase in air temperature during the 20-year period, precipitation decreased by 25%. Evapotranspiration (total evaporation from earth to atmosphere from lakes, rivers, soil, and plants) increased by 35%, and once-permanent streams became intermittent. As a result of reduced precipitation, fewer organic chemicals were carried into the lakes from the land; the net effect of reduced precipitation has been to reduce DOC and clarify the lakes further.

During the 20 years, DOC in lakes in northwestern Ontario decreased by an average of 15-20%, causing increases of 22-63% in the average depth of UV-B penetration in the lakes. As a result, the percentage of the lakes' volume exposed to any given intensity of UV-B increased by 60% for lake 239 and about 22% for lakes 224 and 240.


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.[2] 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 during the period 1971-1990 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.

The authors say it seems possible that some of the many changes to aquatic communities that have been attributed to lake acidification may have involved harmful UV-B exposure.

However, most species disappear in natural aquatic ecosystems at higher pH values (more 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.


Ultraviolet radiation from the sun is a powerful germicide and disinfectant --which is one reason our grandparents hung the washing out to dry in bright sunlight. The wash not only dried, but it also was sterilized by ultraviolet light from the sun; as a result it smelled "fresh."

Because ultraviolet light is a powerful killer of microorganisms, an ecosystem can be damaged if the available ultraviolet radiation should increase.

Therefore, anything that clarifies the water in an already-clear lake, thus allowing ultraviolet to penetrate deeper than normal beneath the surface, can change the ecosystem in the lake by reducing photosynthesis. Photosynthesis is the process whereby blue-green algae floating in the water use the energy of sunlight to combine carbon dioxide and water into carbohydrates, which are then eaten by small animals (zooplankton). Thus photosynthesis lies at the base of all the planet's food chains.

This study --the first of its kind --shows that clear lakes can be further clarified by:

(a) climate warming, which reduces precipitation and thus diminishes runoff of chemicals from the land into lakes;

(b) acid precipitation, which kills microorganisms;

(c) increased ultraviolet radiation which kills microorganisms.

These three factors work together to reduce dissolved organic carbon, thus clarifying the waters in clear lakes and making a much larger volume of the lake accessible to ultraviolet light.

This is the first study that has related these three phenomena, showing that they enhance each other's effects.

At a time when a great majority of the world's meteorologists have recently said for the first time that they believe global warming is occurring because of human-induced additions of "greenhouse gases" in the atmosphere (see REHW #467), this study provides a preview of some of the effects we might expect from global warming in northern climes.

The study also underscores the point that human-created problems interact in ways that can take decades to recognize, and longer than that to understand. For this reason, it only makes sense for humans to make changes to their environment slowly, which has not been the case for the last 200 years. Therefore, an important question is: how can we slow the rate of technological change in our society? Such a suggestion borders on heresy and sedition. Nevertheless, the environmental policy that we examined last week --the precautionary polluter pays principle (see REHW #510) --might have the desired effect. Slowing the rate of technological change might allow us time to reflect a bit more carefully about the subtle, interconnected ways in which we are damaging the ecosystems upon which our lives depend.

--Peter Montague

(National Writers Union, UAW Local 1981/AFL-CIO)

[1] David W. Schindler, P. Jefferson Curtis, Brian R. Parker, and Michael P. Stainton, "Consequences of climate warming and lake acidification for UV-B penetration of North American boreal lakes," NATURE Vol. 239 (February 22, 1996), pgs. 705-708.

[2] D.W. Schindler, "Effects of Acid Rain on Freshwater Ecosystems," SCIENCE Vol. 239 (January 8, 1988), pgs. 149-157.

Descriptor terms: global warming; acid precipitation; ozone depletion; ultraviolet light; ultraviolet radiation; lakes; water pollution; dissolved organic carbon; doc; synergism; canada; studies;


Environmental Research Foundation provides this electronic version of RACHEL'S ENVIRONMENT & HEALTH WEEKLY free of charge even though it costs our organization considerable time and money to produce it. We would like to continue to provide this service free. You could help by making a tax-deductible contribution (anything you can afford, whether $5.00 or $500.00). Please send your contribution to: Environmental Research Foundation, P.O. Box 5036, Annapolis, MD 21403-7036.

--Peter Montague, Editor

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