332
ferences in watershed geology. Lakes in southern
Wisconsin are underlain by sedimentary materials
containing substantial amounts of carbonates, while
those in the north lie upon glacial till originating
from the Laurentian Shield region of Canada,
which is predominated by minerals that are resistant
to weathering (Martin 1965).
Even in regions with resistant geologic materials,
hydrology influences lake chemistry by determining the amount of time that water remains in contact
with minerals before it reaches a lake or stream. A
higher proportion of groundwater inputs in a lake's
hydrologic budget will produce chemical conditions that provide greater resistance to acidification
(Anderson and Bowser 1986). Just for a single lake,
shifts in hydrology driven by climatic effects such
as drought can substantially change acid-base
chemistry and pH levels over a short time period
(Webster et al. 1990). Hydrologic regimes with the
highest proportions of inputs as precipitation generate situations with the highest potential for
acidification.
Acid within a lake or stream, or being transported to a water body, can be neutralized by chemical and biological processes. Some minerals such
as calcium carbonate can neutralize acid in water.
Such direct chemical effects are measured as acidneutralizing capacity, once termed alkalinity
(Wetzel and Likens 1991). Biological processes can
also neutralize the effects of acid. Much nitric acid
can be neutralized by its uptake as a plant nutrient.
This effect can be substantial in many aquatic habitats. Sulfuric acid can be neutralized by the reduction of sulfate to less oxidized forms by bacterial
processes in anaerobic environments (Cook et al.
1986). Nitric acid can likewise be neutralized in
anaerobic environments. The overall net effects of
acid additions to lakes can, therefore, be much less
than would be expected on the basis of hydrogen
ions being added. Biological alkalinity generation
can be substantial and must be factored into any
predictions of the effects of acid loading within a
region (Schindler 1986).
Ecological Consequences
of Acidification
The effects of acidification on ecological processes
are substantial and they can influence a wide range
of lake features. Acid effects have been inferred
Thomas M. Frost and Janet M. Fischer
from comparative surveys that have examined biological and chemical characteristics across series of
lakes that exhibit different pH conditions (e.g.,
Schindler et al. 1989). Long-term patterns of acidification have also been inferred from paleolimnological studies (Smol et al. 1984). The clearest demonstrations of acidification effects have been
through direct pH manipUlations conducted at a variety of different scales. We emphasize these experimental studies here to help illustrate the value
of such research approaches. We summarize
smaller-scale experiments that have been used to
assess acid effects and then describe experiments
conducted at the scale of a whole ecosystem. We
outline the range of the factors that must be considered in conducting and interpreting acidification
experiments.
Smaller-Scale Experiments
to Evaluate the Effects
of Acidification
Smaller-scale experiments have been a powerful
tool for studying the effects of acidification on
aquatic communities (e.g., Havens 1992; Locke
and Sprules 1993; Fischer 1997). While these studies lack the realism of acidification experiments
conducted at the whole-lake scale, they have several advantages including (1) opportunities to compare several different acidified treatments (e.g.,
different levels of pH), (2) replication of experimental treatments, and (3) reduced costs of
implementation.
In situ mesocosm experiments have been used to
study the responses of lower trophic levels (bacteria, phytoplankton, and zooplankton) to acidification. Fishes have generally been excluded from
these experiments because it is difficult to recreate
natural densities and behaviors in mesocosms.
Mesocosms are constructed of clear polyethylene
(>4 mil) tubes that are suspended from floating
rafts made from wood or PVC pipe. Mesocosm volumes range from less than 60 liters (Havens 1992)
to 16,000 liters (Locke and Sprules 1993); however,
most mesocosms are greater than 2000 liters. Some
mesocosms are open to sediments (Arvola et al.
1986, Barmuta et al. 1990), while others are sealed
at the bottom (Locke and Sprules 1993, Fischer
ferences in watershed geology. Lakes in southern
Wisconsin are underlain by sedimentary materials
containing substantial amounts of carbonates, while
those in the north lie upon glacial till originating
from the Laurentian Shield region of Canada,
which is predominated by minerals that are resistant
to weathering (Martin 1965).
Even in regions with resistant geologic materials,
hydrology influences lake chemistry by determining the amount of time that water remains in contact
with minerals before it reaches a lake or stream. A
higher proportion of groundwater inputs in a lake's
hydrologic budget will produce chemical conditions that provide greater resistance to acidification
(Anderson and Bowser 1986). Just for a single lake,
shifts in hydrology driven by climatic effects such
as drought can substantially change acid-base
chemistry and pH levels over a short time period
(Webster et al. 1990). Hydrologic regimes with the
highest proportions of inputs as precipitation generate situations with the highest potential for
acidification.
Acid within a lake or stream, or being transported to a water body, can be neutralized by chemical and biological processes. Some minerals such
as calcium carbonate can neutralize acid in water.
Such direct chemical effects are measured as acidneutralizing capacity, once termed alkalinity
(Wetzel and Likens 1991). Biological processes can
also neutralize the effects of acid. Much nitric acid
can be neutralized by its uptake as a plant nutrient.
This effect can be substantial in many aquatic habitats. Sulfuric acid can be neutralized by the reduction of sulfate to less oxidized forms by bacterial
processes in anaerobic environments (Cook et al.
1986). Nitric acid can likewise be neutralized in
anaerobic environments. The overall net effects of
acid additions to lakes can, therefore, be much less
than would be expected on the basis of hydrogen
ions being added. Biological alkalinity generation
can be substantial and must be factored into any
predictions of the effects of acid loading within a
region (Schindler 1986).
Ecological Consequences
of Acidification
The effects of acidification on ecological processes
are substantial and they can influence a wide range
of lake features. Acid effects have been inferred
Thomas M. Frost and Janet M. Fischer
from comparative surveys that have examined biological and chemical characteristics across series of
lakes that exhibit different pH conditions (e.g.,
Schindler et al. 1989). Long-term patterns of acidification have also been inferred from paleolimnological studies (Smol et al. 1984). The clearest demonstrations of acidification effects have been
through direct pH manipUlations conducted at a variety of different scales. We emphasize these experimental studies here to help illustrate the value
of such research approaches. We summarize
smaller-scale experiments that have been used to
assess acid effects and then describe experiments
conducted at the scale of a whole ecosystem. We
outline the range of the factors that must be considered in conducting and interpreting acidification
experiments.
Smaller-Scale Experiments
to Evaluate the Effects
of Acidification
Smaller-scale experiments have been a powerful
tool for studying the effects of acidification on
aquatic communities (e.g., Havens 1992; Locke
and Sprules 1993; Fischer 1997). While these studies lack the realism of acidification experiments
conducted at the whole-lake scale, they have several advantages including (1) opportunities to compare several different acidified treatments (e.g.,
different levels of pH), (2) replication of experimental treatments, and (3) reduced costs of
implementation.
In situ mesocosm experiments have been used to
study the responses of lower trophic levels (bacteria, phytoplankton, and zooplankton) to acidification. Fishes have generally been excluded from
these experiments because it is difficult to recreate
natural densities and behaviors in mesocosms.
Mesocosms are constructed of clear polyethylene
(>4 mil) tubes that are suspended from floating
rafts made from wood or PVC pipe. Mesocosm volumes range from less than 60 liters (Havens 1992)
to 16,000 liters (Locke and Sprules 1993); however,
most mesocosms are greater than 2000 liters. Some
mesocosms are open to sediments (Arvola et al.
1986, Barmuta et al. 1990), while others are sealed
at the bottom (Locke and Sprules 1993, Fischer