Evaluating the role of flow and specific conductivity on stream metabolism across a mining-induced salinity gradient in the Appalachian Coalfield

Abstract

Freshwater salinization is a notable concern for headwater streams in Appalachia with the history of resource extraction in the region. Mountaintop removal/valley fill mining techniques in Appalachia result in the burial of headwater streams and mobilization of ions, specifically SO42-, Ca, and Mg into waters, raising specific conductivity (SpC). Organismal responses to salinization have been well documented in freshwater ecosystems, but there are few measurements assessing how salinity effects on organisms influence whole-ecosystem processes, specifically stream metabolism. Understanding how gross primary production (GPP) and ecosystem respiration (ER) respond to salinity, stream flow, and their association is needed to characterize the consequences of salinization on stream processes. To assess the role of salinization and discharge on metabolism, we recorded high-frequency SpC, discharge, and dissolved oxygen data in three headwater streams in the Appalachian (U.S.) coalfields, where mining has resulted in widespread headwater stream salinization. Sites included a reference stream (REF) with SpC ranging from 0.2 - 57.69 μS/cm, a mid-salinity site (MID; 2.03- 594.7 μS/cm), and a high-salinity site (HI; 84.6-1920 μS/cm) with similar flow regimes, helping to characterize these covarying and potentially interacting drivers of metabolism. Across all sites, SpC decreased with increased discharge and with significant breakpoints. This study did not find a clear relationship between SpC and ecosystem metabolism. All streams, regardless of SpC levels, were heterotrophic (|ER|>GPP). Our high salinity site had the highest GPP and ER, suggesting salt may subsidize ecosystem metabolism, though differences in canopy cover complicate this. While we saw no consistent effect of SpC on metabolism, deviation in patterns at mining-impacted sites from expected discharge-metabolism patterns at our reference sites suggests some impact of salinity. This study reinforces the need for research across diverse ecosystems and salinization sources to characterize the role of freshwater salinization on ecosystem metabolism, as mediated by discharge.