GLOBAL PROBLEMS ARE RELATED
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ACID RAIN, CLIMATE WARMING, OZONE DEPLETION ARE RELATED
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.
EFFECTS OF INCREASED UV-B LIGHT
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.
EFFECTS OF CLIMATE WARMING
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.
EFFECTS OF ACID RAIN, SNOW AND FOG
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.
SUMMARY
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;
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--Peter Montague, Editor
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