The role of dissolved organic carbon in Great Smoky Mountains National Park streams impacted by long-term acid deposition.

Following reductions in acid deposition in the Great Smoky Mountains National Park (GRSM) since the 2000s, many streams remain acidic and the role of organic acids (OA ^- ) remains unknown due to limited OA ^- data. This study investigated dissolved organic carbon (DOC) concentrations as a surrogate for OA ^- across GRSM and its relationships with watershed characteristics, seasons, flow, and stream chemistry. Baseflow water samples were collected from seven watersheds for 2 years and stormflow samples from three watersheds for 1 year. During baseflow, DOC concentrations ranged from < 0.04 to 2.29 mg L ^-1 with watershed medians between 0.61 and 1.00 mg L ^-1 . Stormflow DOC concentrations ranged from 1.36 to 5.66 mg L ^-1 . During the summer, median DOC concentrations were about twice that of the other three seasons. Stream DOC concentrations decreased with increasing elevation during baseflow but increased with increasing elevation during stormflow. Considering high elevations historically received greater acid deposition, this gradient between baseflow and stormflow suggests higher elevation streams are more impacted by OA ^- . Based on an OA ^- /DOC acidity model, it was estimated that during baseflow OA ^- was a minor contributor to stream acidity, in the order of 5.3 μeq L ^-1 , however stormflow OA ^- was estimated at 52.5 μeq L ^-1 , contributing to nearly half of stream acidity. Baseflow DOC was significantly correlated with pH and Ca ²⁺ , suggesting stream acidification/recovery is governed by base cations and Ca ²⁺ availability. Furthermore, this study provides essential data for future research to evaluate stream DOC trends during acidification recovery and changes in biogeochemical processes.
(© 2024. The Author(s), under exclusive licence to Springer Nature Switzerland AG.)