Acidic Deposition and the Status of Virginia’s Wild Trout Resource:  Revisited


Wild Trout VII, Yellowstone National Park, October 2000

J.R. Webb, F.A. Deviney, Jr., B.J. Cosby, A.J .Bulger, J.N. Galloway

Department of Environmental Sciences
University of Virginia
Charlottesville, Virginia 22903

 

Discussion

Although large, near-census sampling surveys can provide comprehensive information for stream populations at particular points in time, comparison of results between surveys can be misleading because of short-term variations. Short-term variations have less impact on interpretation of data obtained through more frequent periodic sampling.

Differences in results between the 1987 survey and the 2000 resurvey may be related to flow conditions. As indicated by the Palmer Drought Severity Index, conditions were generally much wetter in 1987. Consistent with observations of episodic acidification during storm-flow conditions, this difference may explain much of the sulfate decrease and ANC increase between the two points in time. It may also explain the differences between the survey-resurvey and the quarterly sampling results.

Trend analyses conducted with the quarterly sampling data are more realistic because the effects of transient variations from flow extremes are reduced by including 48 points in time. The trend analyses indicate that few long-term sampling sites are recovering from acidification; most are continuing to acidify. As indicated by changes in site distribution within the defined ANC categories for brook trout responses, the continuing acidification is biologically significant.

How well the long-term sample sites represent the regional population of brook trout streams has not been quantitatively evaluated. However, we initially selected the sites to represent the percentage of the regional population of brook trout streams that is associated with relatively undisturbed and non-carbonate landscape. Therefore we believe that our findings for the long-term sample sites can be applied to approximately 70% of the state’s brook trout streams.

The meta-analysis of trend behavior among the sites classified by watershed lithology further supports the reliability of regional inferences. The regional decline in ANC during the critical winter season may have adverse impacts on brook trout reproduction.

Conclusions
1. Changes in stream water concentrations between the 1987 and 2000 surveys suggest some recovery from acidification, but the results may be strongly influenced by differences in stream flow levels.
2. More reliable results are provided by quarterly sampling of representative long-term monitoring streams during the 1988-1999 period. Trend analyses for these streams indicate acidification is continuing for most Virginia brook trout streams.
3. Estimates of change in the distribution of long-term monitoring streams among ANC categories for brook trout response indicates that acidification during the 1988-1999 period may have caused biological harm.