Purpose and Study Design
8
associated with a variety of ANC benchmarks, the most common of which are
ANC equal to 0, 20, and 50 microequivalents per liter (μeq/L). ANC of 0 μeq/L
or less is of significance because waters at or below this level have no capacity
to neutralize acid inputs; they are acidic by definition. Lakes and streams with
ANC chronically below 0 μeq/L are often fishless or contain few species of fish.
The relatively acid-tolerant species, brook trout (Salvelinus fontinalis), has been
shown to be sensitive to episodic acidification * when chronic stream ANC is
below about 20 μeq/L. A general benchmark for sensitivity of other types of
aquatic biota is often established at ANC equal to 50 μeq/L (Driscoll et al. 2001).
Some species may be affected at higher ANC values, even at levels of 100 μeq/L
or above. However, model hindcast simulations suggest that many lakes and
streams did not have ANC that high prior to the Industrial Revolution and initiation of acidic deposition. Generally, surface waters with ANC of 50 μeq/L or
less are considered prone to episodic acidification in some regions (DeWalle
et al. 1987, Eshleman 1988), especially where seasonal snowpack accumulations are substantial. Such low-ANC waters may also be susceptible to future
chronic acidification at current or increased rates of acidic deposition.
Common reference values for pH are 5.0, 5.5, and 6.0. Such values are often
used to evaluate the possible extent of adverse effects on fish and other aquatic
organisms. Threshold pH levels for adverse biological effects have been summarized for a variety of aquatic organisms (Haines and Baker 1986, Baker et al. 1990).
The effects of low pH are specific to the organism and region under consideration
and depend on the concentrations of other chemical constituents in the water,
notably Al i and Ca 2+ . Lakes or streams having pH below about 5.0 generally also
have ANC below 0 and often do not support fish. Depending on the region, waters
having pH above about 6.5 and ANC above about 50 to 100 μeq/L support large,
but variable, numbers of species. Populations of salmonid fish are generally not
found at pH levels less than 5.0, and smallmouth bass (Micropterus dolomieu)
populations are usually not found at pH values less than 5.2 to 5.5 (Haines and
Baker 1986). A number of synoptic surveys † indicated loss of species diversity
and absence of many other fish species in the pH range 5.0 to 5.5 (Haines and
Baker 1986). Levels of pH less than 6.0 to 6.5 have been associated with adverse
effects on populations of dace, minnows, and shiners (family Cyprinidae), and
bioassays suggest that given sufficient Al i concentrations, pH less than 6.5 can
lead to increased egg and larval mortality in blueback herring (Alosa aestivalis)
and striped bass (Morone saxatilis; Hall 1987, Klauda et al. 1987).
* Episodic acidification refers to the temporary (typically hours to days) decrease in lake, or especially stream, ANC that occurs in response to hydrologic events such as rainfall or snowmelt.
† These are surveys to represent water quality across a group of lakes or streams within a particular region.
8
associated with a variety of ANC benchmarks, the most common of which are
ANC equal to 0, 20, and 50 microequivalents per liter (μeq/L). ANC of 0 μeq/L
or less is of significance because waters at or below this level have no capacity
to neutralize acid inputs; they are acidic by definition. Lakes and streams with
ANC chronically below 0 μeq/L are often fishless or contain few species of fish.
The relatively acid-tolerant species, brook trout (Salvelinus fontinalis), has been
shown to be sensitive to episodic acidification * when chronic stream ANC is
below about 20 μeq/L. A general benchmark for sensitivity of other types of
aquatic biota is often established at ANC equal to 50 μeq/L (Driscoll et al. 2001).
Some species may be affected at higher ANC values, even at levels of 100 μeq/L
or above. However, model hindcast simulations suggest that many lakes and
streams did not have ANC that high prior to the Industrial Revolution and initiation of acidic deposition. Generally, surface waters with ANC of 50 μeq/L or
less are considered prone to episodic acidification in some regions (DeWalle
et al. 1987, Eshleman 1988), especially where seasonal snowpack accumulations are substantial. Such low-ANC waters may also be susceptible to future
chronic acidification at current or increased rates of acidic deposition.
Common reference values for pH are 5.0, 5.5, and 6.0. Such values are often
used to evaluate the possible extent of adverse effects on fish and other aquatic
organisms. Threshold pH levels for adverse biological effects have been summarized for a variety of aquatic organisms (Haines and Baker 1986, Baker et al. 1990).
The effects of low pH are specific to the organism and region under consideration
and depend on the concentrations of other chemical constituents in the water,
notably Al i and Ca 2+ . Lakes or streams having pH below about 5.0 generally also
have ANC below 0 and often do not support fish. Depending on the region, waters
having pH above about 6.5 and ANC above about 50 to 100 μeq/L support large,
but variable, numbers of species. Populations of salmonid fish are generally not
found at pH levels less than 5.0, and smallmouth bass (Micropterus dolomieu)
populations are usually not found at pH values less than 5.2 to 5.5 (Haines and
Baker 1986). A number of synoptic surveys † indicated loss of species diversity
and absence of many other fish species in the pH range 5.0 to 5.5 (Haines and
Baker 1986). Levels of pH less than 6.0 to 6.5 have been associated with adverse
effects on populations of dace, minnows, and shiners (family Cyprinidae), and
bioassays suggest that given sufficient Al i concentrations, pH less than 6.5 can
lead to increased egg and larval mortality in blueback herring (Alosa aestivalis)
and striped bass (Morone saxatilis; Hall 1987, Klauda et al. 1987).
* Episodic acidification refers to the temporary (typically hours to days) decrease in lake, or especially stream, ANC that occurs in response to hydrologic events such as rainfall or snowmelt.
† These are surveys to represent water quality across a group of lakes or streams within a particular region.