Sensitivity of Blacknose Dace (Rhinichthys Atratulus) to Moderate Acidification Events in Shenandoah National Park, U.S.A.

Abstract

Shenandoah National Park (SNP) receives more acid deposition than any other national park in the United States of America. As part of an effort to assess the impact of acidification upon fish in the park, in situ sub-lethal stress bioassays (measured by monitoring hematocrit) were conducted with acid sensitive blacknose dace Rhinichthys atratulus. Study streams experienced moderate acidification events that resulted in reduced pH (the largest reductionbeing 6.18 to 5.37) and increased total monomeric aluminum (TMA) concentration (the largest being 15 to 39 μg L-1). Although some of these acidification events were within the pH and TMA range expected to result in blacknose dace stress, none was detected by monitoring hematocrit. In an acid-sensitive stream, mean baseflow (pre-event) hematocrit ± SD, was 33.5±4.5%, and hematocrit during the largest event was 31.5±4.9%, A moderate acidification event also occurred in a less acid sensitive stream; pH dropped from 7.18 to 6.38 but TMA remained below 10 μg L-1. Mean hematocrit during baseflowin this stream was 32.3±2.8%, and hematocrit during event flow was 34.1±5.0%. At the time of this bioassay, acidificationevents that would result in acute toxicity for most species of fish did not occur, however the conditions documented in this investigation do not represent the most acute acidification events that have been observed in SNP. It should be noted that a chronic sub-lethal stress, measured by condition indices, has been observed among blacknose dace in SNP streams.