Crisis Looming in Virginia’s Trout Streams

Acid Rain Deposition Destroying Trout Habitat

By Jay Henderson

(This article, which will appear in Virginia Trout Unlimited’s "Coldwater Conservationist," was submitted by Rick Webb)

As many as a third of Virginia’s headwater brook trout streams may be biologically dead by the year 2041 without relief from acid rain, according to a study of the effects of acid deposition recently released by the national office of Trout Unlimited (TU). Using data from the long-term Virginia Trout Stream Sensitivity Study (VTSSS), the report uses computer modeling to estimate the future condition of these streams. The analysis concludes that substantial reductions in acid deposition are needed simply to hold the line against acidification.

The report, ponderously titled "Acid Rain: Current And Projected Status of Coldwater Fish Communities in the Southeastern US in the Context of Continued Acid Deposition," was authored by Art Bulger, Jack Cosby and Rick Webb. Publication was underwritten by TU’s Coldwater Conservation Fund.

Many of Virginia’s brook trout streams are part of acid-sensitive ecosystems in Appalachian forests. Despite the reductions in acid-causing air pollution required by the 1990 Amendments to the Clean Air Act, Virginia’s streams continue to be threatened by acid deposition. Currently only about 50 percent of the 304 Virginia trout streams covered in the study are "not acidic," compared to an estimated 82 percent of pre-industrial, forested watersheds in Virginia. Approximately 6 percent are "chronically acidic" and unable to support brook trout or other fish species.

The report concludes that a 70 percent reduction in acid deposition from 1991 levels will be necessary to retain about 50 percent of Virginia’s brook trout streams in the "not acidic" condition. At lesser reductions in acid deposition, a large number of Virginia’s trout streams – up to 35 percent if no further reductions in deposition levels are accomplished -- will become "chronically acidic" and will no longer support wild trout populations in the year 2041.


The 1990 Amendments to the Clean Air Act provide for significant reductions in emissions of sulfur dioxide and some reductions in emissions of nitrogen oxides, the pollutants that are responsible for acid deposition. Many scientists and resource managers, including EPA, have questioned whether these reductions will be sufficient to protect sensitive ecosystems, while others have questioned whether every region of the country will receive the full benefit of these national cuts.

The research detailed in the report examines a set of sensitive ecosystems (headwater streams in Virginia that hold brook trout) to determine the extent to which acid deposition must be reduced to maintain their current condition. The report uses computer modeling techniques to evaluate the degree of acidification in Virginia’s brook trout streams, together with the capacity of individual streams to neutralize additional acid, in order to predict whether they will continue to support brook trout in the middle term (the year 2011) and long term (the year 2041).

The forested mountain watersheds that provide habitat for brook trout in Virginia are particularly sensitive to the effects of acidic deposition. Among forested mountain watersheds, however, there is a range of sensitivity to acidic deposition directly related to the chemistry of the soil and bedrock of the watershed. Each stream has a greater or lesser ability to neutralize acid precipitation, which can change over time as more acid is deposited on the watershed. The report uses measurement of acid neutralizing capacity ("ANC," expressed in microequivalents per liter, meq/L) to estimate the degree of each stream’s acidification and its remaining buffering capacity.

The authors divide Virginia’s streams into four categories:

1. Chronically Acidic: Acid neutralization capacity values of less than 0 meq/L; these streams are no longer able to neutralize acid deposition and cannot support populations of brook trout or any other fish species.

2. Episodically Acidic: Streams with an acid neutralizing capacity between 0 and 20 meq/L experience regular episodic acidification at levels harmful to brook trout and other aquatic species and host, at best, reduced fish populations.

3. Transitional: Steams whose ANC values fall between 20 and 50 meq/L are extremely sensitive to further acidification; they may or may not host brook trout populations, depending on the frequency and magnitude of acid events and other habitat characteristics. They certainly will host fewer fish species than streams with higher ANC value.

4. Not Acidic: Streams with acid neutralizing capacity greater than 50 meq/L, a level which poses no threat to brook trout (although levels as low as 50 meq/L may still be too acidic for most other fish species). This category includes streams on limestone bedrock, which have the highest ANC values and are at no risk of acidification.


The authors used detailed water chemistry data from 60 Virginia trout stream sites collected each quarter from 1989 to 1992 under the Virginia Trout Stream Sensitivity Study. The well-accepted "MAGIC" (Mode I of Acidification of Groundwater In Catchments) computer model was used to analyze the data to predict future stream chemistry. "MAGIC" can be used to estimate both the past and future acid/base status of streams and likely effects on fish communities; it was the principal model used by the National Acid Precipitation Assessment Program to estimate the potential future damage to lakes and streams in the eastern United States and has also been used to predict brown trout responses to acidification in Norway and Scotland.

The 60 streams were chosen from 344 brook trout streams included in a 1987 water quality survey of the state's undisturbed brook trout streams. Although no one is certain of the total number of brook trout streams in the state, estimates by the Virginia Department of Game and Inland Fisheries indicate that the 344 streams represent approximately 80 percent of the historical total. Because the goal was to evaluate the effects of deposition on relatively undisturbed streams, watersheds with significant development or other disturbances that would alter stream chemistry were not included in the 1987 survey. Based on geological data, the subset of 60 was chosen to be representative (in terms of acid sensitivity) of all the mountain brook trout streams in Virginia. The results of the analysis were then used to predict the future status of the general population of streams in the state.

About 10 percent of Virginia trout streams have carbonate (limestone) bedrock in their watersheds, and therefore they are not sensitive to acidification; these streams were not included in this analysis. About 20 percent have substantial direct human disturbance in the watershed (housing, agriculture, etc.); streams in this category were also excluded because their stream chemistry may be affected by anthropogenic factors other than atmospheric deposition. Of the 344 streams sampled in 1987, 40 fell into one of these two causes for exclusion and the results from the 60 streams in this analysis can therefore be applied directly to 304 streams. This represents approximately 70 percent of all the historical brook trout streams in the state and approximately 90 percent of the present population of brook trout streams without significant direct human disturbance.

The landscape/bedrock geology categories for the 60 streams in this analysis were Blue Ridge basaltic (4 streams), Blue Ridge granitic (18 streams), Blue Ridge siliciclastic (16 streams). and Valley and Ridge siliciclastic (22 streams). Basaltic, granitic, and siliciclastic (such as sandstone or shale) bedrock types represent a series of increasing acid sensitivity, with basaltic being the least sensitive and siliciclastic the most sensitive.


The MAGIC model was first used to evaluate the historical vs. current (as of 1991) condition of these streams. The results indicated that about 82 percent of pre-industrial forested watersheds in Virginia would be in the "not acidic" category, and the remaining 18 percent could be classified as "transitional." The assessment shows that currently only 50 percent of the population of streams on non-limestone bedrock are "not acidic." An estimated 20 percent of the streams have ANC values between 20 and 50 meq/L and fall into the "transitional" category. Another 24 percent experience regular episodic acidification at levels harmful to brook trout and other aquatic species. The remaining 6 percent of streams are "chronically acidic" and cannot host brook trout or any other fish species.

Although nitrate deposition can be an important factor in acid precipitation, recent research has demonstrated that acidification of Virginia’s headwater streams is currently being driven by sulfate deposition. The TU assessment considered three scenarios of future sulfate deposition: Scenario 1 -- Constant deposition at 1991 levels; Scenario 2 -- 40 percent reduction from 1991 levels; Scenario 3 -- 70 percent reduction from 1991 levels.

For the 40 and 70 percent scenarios, it was assumed that sulfate deposition would be reduced linearly for 20 years beginning in 1991 (through year 2011) and that levels would remain constant for the following 30 years (through 2041). The year 2011 point represents the responses of the streams at the completion of the deposition reductions and might be considered the direct effect of the assumed reduction. The later year is included to examine any delayed effects that might occur as a result of continued deposition at the reduced levels and gives a more accurate picture of the ultimate condition of these streams.

Scenario 1: No Reduction. Although it is likely that Virginia watersheds will see some level of sulfate reductions under the Clean Air Act, it is worthwhile to look at what would happen if the current deposition levels were allowed to continue. At constant deposition rates, the number of chronically acidic streams would jump from 6 percent currently to 35 percent by the year 2041. This represents approximately 88 streams (29 percent of 304) that would become effectively dead ecologically. The number of streams in the "not acidic" category would drop from 50 percent currently to 42 percent in the year 2041.

Scenario 2: 40 percent Reduction. According to this study’s results, reducing sulfate deposition by 40 percent relative to 1991 levels by 2011 would still result in too much acid for some streams in the currently "not acidic" category, resulting in a decrease in the percentage of streams suitable for healthy populations of brook trout from 50 percent currently to about 45 percent in the year 2041. The "chronically acidic" streams would increase from 6 percent currently, to about 22 percent in 2041.

Scenario 3: 70 percent Reduction. It appears that a 70 percent reduction in acid deposition is necessary to retain about 50 percent of the streams in the "not acidic" category. Nevertheless, even a 70 percent reduction will allow the number of "chronically acidic" streams, which are not suitable for brook trout, to increase from about 6 percent of the population currently, to about 11 percent in 2041 (compared to 0 percent in 1851).

Jay Henderson is the editor of "The Coldwater Conservationist." _