22. Assessing the Effects of Acidification on Aquatic Ecosystems: Insights from Lake Experiments
331
The Chemistry of Acidification
The acid level of a water sample is defined as its
hydrogen ion activity. This is the concentration of
active hydrogen ions, which is usually reported as
pH, the logarithm (base 10) of the reciprocal of the
concentration of free hydrogen ions (Wetzel 1983).
Hydrogen ion activity is controlled by the dissociation of water molecules and their interactions with
the diversity of other ions present in aqueous solutions. Natural waters typically exhibit pH levels
between 4 and 10. Anthropogenic acidification has
occurred when pH levels have been decreased due
to inputs of sulfuric and/or nitric acid. Such inputs
have the most adverse effects in waters with low
acid-neutralizing capacity. These dilute waters have
relatively little resistance to the changes caused by
acid inputs. Waters that have been influenced by
acid deposition usually exhibit a pH level of less
than 6.5 prior to acid effects, which is then reduced
sometimes to levels of 5.0 or lower.
It is important to keep in mind that all waters
with pH values less than 7.0 can not be considered
as acidic due to human activities, even in situations
where organic acids do not control water chemistry.
Carbonic acid exerts a major influence on natural
waters. It is formed by the dissolution of carbon
dioxide and its interactions with other ions, particularly calcium and magnesium (Stumm and Morgan 1981). Even pure distilled water exhibits a pH
level of close to 5.6 when it comes into equilibrium
with atmospheric carbon dioxide levels. Many
freshwater ecosystems are substantially oversaturated with carbon dioxide (Cole et al. 1994), which
generates naturally low pH conditions in some situations (Kratz et al. 1987). Actually, uncontaminated precipitation can exhibit pH values as low as
5.0 because it contains trace amounts of weak and
strong acids (Schindler 1988). At the same time,
precipitation with pH values down to 4.0 or lower
has been observed in regions with high industrial
contamination worldwide and these acid levels are
clearly generated by human processes (Schindler
1988). As a consequence, there are numerous
aquatic ecosystems with pH values below 5.0 that
have been acidified through human activities. We
have provided only a brief synopsis of acidification
chemistry here and we suggest that researchers interested in the details of this chemistry consult
Munson and Gherini (1991) for a much more thorough review.
What Controls the Anthropogenic
Acidification of Aquatic
Ecosystems?
Whether the pH of a particular habitat is lowered
by acidic deposition depends upon the amount and
form of acid loading within that region and the
chemical and biological resistance to acid inputs
that it receives.
A number of programs have been established to
categorize the rates of acid loading that occur in
regions worldwide. Within the United States, deposition is reported by the National Atmospheric
Deposition Program (Husar et al. 1991), which reports specific rates for a network of sites throughout
the country. When evaluating acid loading, it is important to consider the deposition of both sulfuric
and nitric acids rather than just the pH of deposition
because these acids have very different effects because of their specific chemistries and their processing by microorganisms (see discussion below).
It is also important to consider that deposition can
occur in both wet and dry forms. Dry deposition is
particularly difficult to incorporate as a portion of
input in a region's overall budget. The overall loading of acid within a region is estimated as rates of
hydrogen ion, nitric acid, or sulfuric acid deposition. This quantity can be difficult to estimate but
it is one key factor in evaluating acidification in a
region.
The processes that influence acid deposition once
it has fallen within a region are also complex. Controls on the ultimate fate of acids that have been
loaded within a region are influenced by geology
and hydrology, as well as by microbial processes
that take place within it. Regions that are most sensitive to acid deposition's effects are underlain by
geologic materials that are resistant to weathering
and that are poor in carbonate minerals. Areas high
in carbonate minerals are unlikely to be influenced
by even high rates of acid deposition. For example,
in the United States, although lakes in southern
Wisconsin receive around five times more acid deposition than lakes in northern Wisconsin, these
lakes are less likely to become acidified due to dif-
331
The Chemistry of Acidification
The acid level of a water sample is defined as its
hydrogen ion activity. This is the concentration of
active hydrogen ions, which is usually reported as
pH, the logarithm (base 10) of the reciprocal of the
concentration of free hydrogen ions (Wetzel 1983).
Hydrogen ion activity is controlled by the dissociation of water molecules and their interactions with
the diversity of other ions present in aqueous solutions. Natural waters typically exhibit pH levels
between 4 and 10. Anthropogenic acidification has
occurred when pH levels have been decreased due
to inputs of sulfuric and/or nitric acid. Such inputs
have the most adverse effects in waters with low
acid-neutralizing capacity. These dilute waters have
relatively little resistance to the changes caused by
acid inputs. Waters that have been influenced by
acid deposition usually exhibit a pH level of less
than 6.5 prior to acid effects, which is then reduced
sometimes to levels of 5.0 or lower.
It is important to keep in mind that all waters
with pH values less than 7.0 can not be considered
as acidic due to human activities, even in situations
where organic acids do not control water chemistry.
Carbonic acid exerts a major influence on natural
waters. It is formed by the dissolution of carbon
dioxide and its interactions with other ions, particularly calcium and magnesium (Stumm and Morgan 1981). Even pure distilled water exhibits a pH
level of close to 5.6 when it comes into equilibrium
with atmospheric carbon dioxide levels. Many
freshwater ecosystems are substantially oversaturated with carbon dioxide (Cole et al. 1994), which
generates naturally low pH conditions in some situations (Kratz et al. 1987). Actually, uncontaminated precipitation can exhibit pH values as low as
5.0 because it contains trace amounts of weak and
strong acids (Schindler 1988). At the same time,
precipitation with pH values down to 4.0 or lower
has been observed in regions with high industrial
contamination worldwide and these acid levels are
clearly generated by human processes (Schindler
1988). As a consequence, there are numerous
aquatic ecosystems with pH values below 5.0 that
have been acidified through human activities. We
have provided only a brief synopsis of acidification
chemistry here and we suggest that researchers interested in the details of this chemistry consult
Munson and Gherini (1991) for a much more thorough review.
What Controls the Anthropogenic
Acidification of Aquatic
Ecosystems?
Whether the pH of a particular habitat is lowered
by acidic deposition depends upon the amount and
form of acid loading within that region and the
chemical and biological resistance to acid inputs
that it receives.
A number of programs have been established to
categorize the rates of acid loading that occur in
regions worldwide. Within the United States, deposition is reported by the National Atmospheric
Deposition Program (Husar et al. 1991), which reports specific rates for a network of sites throughout
the country. When evaluating acid loading, it is important to consider the deposition of both sulfuric
and nitric acids rather than just the pH of deposition
because these acids have very different effects because of their specific chemistries and their processing by microorganisms (see discussion below).
It is also important to consider that deposition can
occur in both wet and dry forms. Dry deposition is
particularly difficult to incorporate as a portion of
input in a region's overall budget. The overall loading of acid within a region is estimated as rates of
hydrogen ion, nitric acid, or sulfuric acid deposition. This quantity can be difficult to estimate but
it is one key factor in evaluating acidification in a
region.
The processes that influence acid deposition once
it has fallen within a region are also complex. Controls on the ultimate fate of acids that have been
loaded within a region are influenced by geology
and hydrology, as well as by microbial processes
that take place within it. Regions that are most sensitive to acid deposition's effects are underlain by
geologic materials that are resistant to weathering
and that are poor in carbonate minerals. Areas high
in carbonate minerals are unlikely to be influenced
by even high rates of acid deposition. For example,
in the United States, although lakes in southern
Wisconsin receive around five times more acid deposition than lakes in northern Wisconsin, these
lakes are less likely to become acidified due to dif-