Episodic Stream Acidification in the Great Smoky Mountains National Park: An Investigation into the Mechanisms of Acidification and Impacts on Native Brook Trout

Abstract

In 2006, 67-km of 12 streams in the Great Smoky Mountains National Park (GRSM) were listed on the 303d list as impaired due to low pH from atmospheric deposition and unknown sources, requiring a TMDL to be developed. The GRSM receives some of the highest rates of atmospheric acid deposition in the U.S. in the form of sulfur and nitrogen oxides, which can cause stream pH to drop below 5.0 (minimum of 4.0) for 2-days or longer. Acids enter poorly buffered streams through wet deposition and from naturally occurring organic acids and accumulated dry deposition flushed from watersheds, temporarily reducing pH and ANC in streams. Stream acidification has been shown to have damaging effects on the health of aquatic ecosystems and biota, and is suspected to be a primary cause of the extirpation of native brook trout ( Salvelinus fontinalis ) in six headwater streams in the GRSM. To develop appropriate TMDLs, it is imperative to understand the environmental processes associated with stream acidification, determine system responses to atmospheric deposition, and evaluate impacts to biota. The objectives of the current research are to 1) characterize the chemical constituents in stream water during baseflow and stormflow in three forested watersheds in the GRSM, 2) identify potential mechanisms responsible for episodic acidification, and 3) evaluate physiological distress in native brook trout during episodes of stream acidification. Conductivity, pH, turbidity, stage height and temperature were monitored continuously (15-minute intervals) at three study sites using multi-parameter data sondes. Baseflow grab samples and precipitation samples were collected; automatic water samplers captured stormflow samples. Stormflow, baseflow, and precipitation samples were analyzed for pH, ANC, trace metals, and major cations and anions. To provide evidence that native brook trout are impacted by stream acidification, in situ bioassay experiments were conducted. Changes in native brook trout physiology were determined during two acid runoff episodes. Brook trout were put in cages at the three sites and fish were sampled before and after stream acidification events. To assess physiological stress in brook trout as a response to acid conditions, whole-body sodium concentrations of individual fish were evaluated. ANC and pH depressions were observed during all stormflows at the three study sites. Sulfate, nitrate, and organic acid concentrations increased during runoff episodes. Base cation concentrations generally increased during stormflow at two stream sites, but diluted occasionally at the third site. The relative changes in ion concentrations were used to determine which ions (acids) were most responsible for ANC depression. ANC contribution analysis indicates acid deposition may be the primary cause of episodic stream acidification, but it appears organic acids and cation dilution may also contribute. Results of the in situ bioassay demonstrate that stream acidification can negatively affect native southern brook trout physiology in the GRSM under actual field conditions. Trout lose the ability to regulate critical blood ions, as exemplified by a loss of whole-body sodium, when stream pH dropped below 5.1.