22
Assessing the Effects of Acidification
on Aquatic Ecosystems:
Insights from Lake Experiments
Thomas M. Frost and Janet M. Fischer
Introduction
A large number of aquatic ecosystems worldwide
are acidic. In some, acidity has been generated by
natural causes such as geologic processes in volcanic regions, chemical actions of mosses in sphagnum bogs, and plant decay products in brown-water
habitats (Hutchinson 1957). In many other cases,
however, acid conditions have been caused by human activities, which frequently have substantial
consequences for ecosystems. For example, some
mining processes generate acidified lakes or
streams that usually occur in fairly restricted areas
(e.g., McKnight and Bencala 1988; Schrenk et al.
1998). Much more widespread are situations in
which acidification has been generated by atmospheric deposition contaminated with sulfuric or nitric acid that has been produced by fossil fuel burning. Such acid deposition is a relatively recently
recognized phenomenon that has had major environmental consequences, particularly in regions
downwind of industrial or highly populated areas
(Charles 1991).
The long-range transport of acid contamination
creates environmental problems that are complex
both scientifically and politically. As with other
widespread environmental problems, ecologists
have been called upon to evaluate the full range of
effects caused by acidification. Experimental studies have played a major role in assessing the effects
of this ecological stress. Investigations of problems
such as acidification have also provided an opportunity to improve the understanding of basic ecosystem features, in large part through experimental
investigations. In this chapter we describe the wide
330
range of methods that have been developed to evaluate the effects of acidification on aquatic ecosystems, emphasizing experimental approaches in
lakes. We review hydrologic, geologic, chemical,
and biological aspects of acidification for aquatic
ecosystems and illustrate some of the complexities
of experimental work on the phenomenon of
acidification.
The range of ecosystems effected by acidification is quite broad. We focus our chapter on lakes,
even though stream and terrestrial habitats have
certainly been influenced by acid effects in some
regions and they have received a fair degree of experimental investigation. We have emphasized
lakes in the interest of maintaining a reasonable degree of focus in this chapter. Many of the general
features of lake responses to acidification are important for other ecosystems as well. Investigators
with interests in other habitats should also consult
Crosby et al. (1991), Elwood et al. (1991) and other
related reports for information on acidification effects on streams (Jenkins et al. 1995) and related
reports for terrestrial studies. It is also important to
consider that the understanding of acidification's
effects on ecosystems has not been derived solely
from experimental work. Tremendous insights have
been derived from comparative analyses across a
range of diverse ecosystems (e.g., Confer et al.
1983; Dillon et al. 1987) and from paleolimnological investigations of historic conditions (Smol et
al. 1984). As in other assessments of stress effects
on ecosystems, overall evaluations must be based
on a variety of different research approaches (e.g.,
Breitburg et al. 1999).
Assessing the Effects of Acidification
on Aquatic Ecosystems:
Insights from Lake Experiments
Thomas M. Frost and Janet M. Fischer
Introduction
A large number of aquatic ecosystems worldwide
are acidic. In some, acidity has been generated by
natural causes such as geologic processes in volcanic regions, chemical actions of mosses in sphagnum bogs, and plant decay products in brown-water
habitats (Hutchinson 1957). In many other cases,
however, acid conditions have been caused by human activities, which frequently have substantial
consequences for ecosystems. For example, some
mining processes generate acidified lakes or
streams that usually occur in fairly restricted areas
(e.g., McKnight and Bencala 1988; Schrenk et al.
1998). Much more widespread are situations in
which acidification has been generated by atmospheric deposition contaminated with sulfuric or nitric acid that has been produced by fossil fuel burning. Such acid deposition is a relatively recently
recognized phenomenon that has had major environmental consequences, particularly in regions
downwind of industrial or highly populated areas
(Charles 1991).
The long-range transport of acid contamination
creates environmental problems that are complex
both scientifically and politically. As with other
widespread environmental problems, ecologists
have been called upon to evaluate the full range of
effects caused by acidification. Experimental studies have played a major role in assessing the effects
of this ecological stress. Investigations of problems
such as acidification have also provided an opportunity to improve the understanding of basic ecosystem features, in large part through experimental
investigations. In this chapter we describe the wide
330
range of methods that have been developed to evaluate the effects of acidification on aquatic ecosystems, emphasizing experimental approaches in
lakes. We review hydrologic, geologic, chemical,
and biological aspects of acidification for aquatic
ecosystems and illustrate some of the complexities
of experimental work on the phenomenon of
acidification.
The range of ecosystems effected by acidification is quite broad. We focus our chapter on lakes,
even though stream and terrestrial habitats have
certainly been influenced by acid effects in some
regions and they have received a fair degree of experimental investigation. We have emphasized
lakes in the interest of maintaining a reasonable degree of focus in this chapter. Many of the general
features of lake responses to acidification are important for other ecosystems as well. Investigators
with interests in other habitats should also consult
Crosby et al. (1991), Elwood et al. (1991) and other
related reports for information on acidification effects on streams (Jenkins et al. 1995) and related
reports for terrestrial studies. It is also important to
consider that the understanding of acidification's
effects on ecosystems has not been derived solely
from experimental work. Tremendous insights have
been derived from comparative analyses across a
range of diverse ecosystems (e.g., Confer et al.
1983; Dillon et al. 1987) and from paleolimnological investigations of historic conditions (Smol et
al. 1984). As in other assessments of stress effects
on ecosystems, overall evaluations must be based
on a variety of different research approaches (e.g.,
Breitburg et al. 1999).