EPISODIC ACIDIFICATION AND CHANGES IN FISH DIVERSITY IN PENNSYLVANIA HEADWATER STREAMS

Current water chemistry and fish communities in 70 Pennsylvania stream s were compared with historical records to determine whether fish spec ies richness had declined and, if so, the possible role of acidificati on. First-, second-, and third-order streams were selected, and stream sites sampled during the 1961-1971 survey were resampled during May a nd June 1994 in the Appalachian Plateaus province and during June 1995 in the Valley and Ridge province. Streamflow was measured and a habit at assessment was completed at each site. Dominant bedrock types influ encing the stream sampling site were determined for the Appalachian Pl ateaus streams. Episodic water chemistry was collected for 39 of the 5 0 Appalachian Plateaus streams and 14 of the 20 Valley and Ridge strea ms during the winter and spring of 1996. Thirty-eight (76%) streams of the Appalachian Plateaus province and 13 (65%) streams in the Valley and Ridge province had a loss of fish species since the 1961-1971 samp ling period. Habitat scores were not related to losses of fish species . Of the 53 streams sampled during runoff episodes 22 (42%) increased in total dissolved aluminum by more than 50 mu g/L, and 31 (58%) strea ms decreased in pH by 0.5 units or more. Minnows (Cyprinidae) and dart ers (Percidae) are sensitive to acidity and were the species most ofte n lost. Streams draining watersheds of the Appalachian Plateaus provin ce dominated by Pottsville bedrock had more acidic water quality durin g base flow and storm Bow sampling periods than streams dominated by P ocono bedrock. The results of this study indicate that many Pennsylvan ia streams have undergone an alarming reduction in fish diversity duri ng the past 25-34 years. In many of these streams the loss in fish div ersity may be attributed to episodic acidification.