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 Is rain getting more or less acidic?
One measure of the acidity of acid rain is the pH. The pH of rain depends
on two things: the presence of acid-forming substances such as sulphates, and
the availability of acid-neutralizing substances such as calcium and magnesium
salts. Clean rain has a pH value of about 5.6. By comparison, vinegar has a
pH of 3.
Although the acidity of acid rain has declined since 1980, rain is still
acidic in eastern Canada. For example, the average pH of rain in Ontario's
Muskoka-Haliburton area is about 4.5 - about 40 times more acidic than normal.
Reductions in the acidity of acid rain are due to reductions in emissions
of SO2.
How does acid rain
affect lakes, rivers and streams?
Lakes that have been acidified cannot support the same variety of life as healthy
lakes. As a lake becomes more acidic, crayfish and clam populations are the first to
disappear, then various types of fish. Many types of plankton-minute organisms that
form the basis of the lake's food chain-are also affected. As fish stocks dwindle, so
do populations of loons and other water birds that feed on them. The lakes, however, do
not become totally dead. Some life forms actually benefit from the increased acidity.
Lake-bottom plants and mosses, for instance, thrive in acid lakes. So do blackfly larvae.
Not all lakes that are exposed to acid rain become acidified. In areas where there is
plenty of limestone rock, lakes are better able to neutralize acid. In areas where rock is
mostly granite, the lakes cannot neutralize acid. Unfortunately, much of eastern
Canada-where most of the acid rain falls-has a lot of granite rock and therefore
a very low capacity for neutralizing acids.
What happens to the
fish, frogs, birds and bugs that live there?
There are many ways the acidification of lakes, rivers and streams harm fish. Mass fish
mortalities occur (during the spring snow melt) when highly acidic pollutants-that
have built up in the snow over the winter-begin to drain into common waterways. Such
happenings have been well documented for salmon and trout in Norway.
More often, fish gradually disappear from these waterways as their environment slowly
becomes intolerable. Some kinds of fish such as smallmouth bass, walleye, brook trout and
salmon, are more sensitive to acidity than others and tend to disappear first.
Even those species that appear to be surviving may be suffering from acid stress in a
number of different ways. One of the first signs of acid stress is the failure of females
to spawn. Sometimes, even if the female is successful in spawning the hatchlings or fry
are unable to survive in the highly acidic waters. This explains why some acidic lakes
only have older fish in them. A good catch of adult fish in such a lake could mislead an
angler into thinking that all is well.
Other effects of acidified lakes on fish include: decreased growth, inability
to regulate their own body chemistry, reduced egg deposition, deformities
in young fish and increased susceptibility to naturally occurring diseases.
Here are the effects of an acidified ecosystem on the natural environment:
| As water pH approaches |
Effects |
| 6.0 |
- crustaceans, insects, and some plankton species begin to disappear.
|
| 5.0 |
- major changes in the makeup of the plankton community occur.
- less desirable species of mosses and plankton may begin to invade.
- the progressive loss of some fish populations is likely, with the more highly valued
species being generally the least tolerant of acidity.
|
| Less than 5.0 |
- the water is largely devoid of fish.
- the bottom is covered with undecayed material.
- the nearshore areas may be dominated by mosses.
- terrestrial animals, dependent on aquatic ecosystems, are affected. Waterfowl, for
example, depend on aquatic organisms for nourishment and nutrients. As these food sources
are reduced or eliminated, the quality of habitat declines and the reproductive success of
birds is affected.
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Are the lakes
recovering?
Some acidified lakes are recovering, but many more are not. Of 202 lakes
that have been studied since the early 1980s, 33% have reduced levels of acidity
while 56% have shown no change and 11% have actually become more acidic. The
greatest improvements have been seen in the Sudbury area, where local emissions
of acid-causing pollutants have declined by 90% in the last three decades.
Here, fish populations have rebounded and fish-eating birds, such as loons,
have increased. However, no substantial wildlife recovery has been seen beyond
the Sudbury area. The least improvement has been seen in Atlantic Canada, even
though lakes in this region were never as highly acidified as those in some
parts of Ontario and Quebec. Since 1990, scientists have confirmed that maintaining
lake pH at 6.0 or more is the most appropriate criterion for calculating critical
loads. This pH level encourages healthy aquatic systems in lakes, rivers and
streams.
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