Abstract
Shenandoah National Park receives more atmospheric sulfate loading than any other USA national park. pH has been gradually declining in low-ANC streams for more than 10 years. We have completed four 1-to-3 month-long field bioassays in three streams differing in acid neutralizing capacity (ANC), using a total of 18,000 hatchery brook trout eyed-eggs through fry. In three of the four bioassays, embryos/fry showed poorer survivorship in the low-ANC stream, compared to the high-ANC stream. Substantial mortalities occurred under different hydrological conditions, including steady rain plus significant rain events (fall 1992), low rainfall followed by a significant rain event (spring 1993), and steady light rain and snowmelt with no large rain events (spring 1994). In a fourth bioassay (fall 1993), poor survivorship occurred in all three streams due to drought conditions. Trout placed in the intermediate-ANC stream showed variable survivorship, in two bioassays as high as in the high-ANC stream, and in one bioassay as poor as in the low-ANC stream. Baseflow ANC in the intermediate-ANC stream is 40–100 ueq/L, and pH never falls below 6.0. However, during episodes, pH in this stream sometimes fluctuates rapidly in the range of 6.0 to 7.0, and this fluctuation itself may be a source of physiological stress.
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