Salt in the Wound: Trophic Mismatches Emerge Between Macroinvertebrate Secondary Production and Basal Food Resources Along a Salinity Gradient in Central Appalachian Headwaters

Abstract

Mountaintop removal mining practices in central Appalachia have directly altered headwater stream biotic communities through elevated ions (e.g. Cl-, Ca2+, SO42-) that increase salinity, as measured by specific conductance (SC). It has been long understood that benthic macroinvertebrate communities, particularly Ephemeroptera and scraper feeding groups, decline in abundance and biomass at elevated SC in streams, though compensation by certain salt-tolerant taxa that are often detritivores maintains community biomass. The physiological and trophic mechanisms behind salt-induced community shifts remain debated and largely untested in field experiments. I investigated how functional feeding groups (FFGs) respond to freshwater salinization by measuring macroinvertebrate secondary production across nine streams in the southwestern Virginia and West Virginia portion of the central Appalachian coalfield, with mean annual SC ranges from 16-1,185 μS/cm. Overall, secondary production along the salinity gradient was not expected to change. However, I predicted that scraper feeding groups would increase production at intermediate SC levels (250-500 μS/cm) from enriched food resources and an osmo- and iono-physiological optimum before declining from salt-induced stress in streams with higher SC levels. Additionally, I expected that some shredding and detritivore taxa would increase continuously with increasing SC due to heightened physiological functioning at high SC and less affected food resources than scrapers. Monthly macroinvertebrate sampling was done from September 2023 to August 2024. I found that secondary production of sensitive scraping FFGs declined by 99.9 % along the salinity gradient, though there was no indication of resource limitation for this FFG as there was a 137.5% increase by tolerant Coleopteran scrapers. Instead, shredder, collector-filterer, and predator production increased in parallel with chlorophyll-a along the mining-induced SC gradient. These results suggest a trophic mismatch between basal food resource availability and the expected secondary production of certain taxa (e.g., Ephemeroptera, scrapers). This mismatch is likely driven by ion-related and osmoregulatory stress acting as a constraint for sensitive organisms and a subsidy for others, which may be compounded by the competitive release of food resources for tolerant taxa. Unpacking the functionality of community energetics through secondary production revealed physiological constraints to assimilating available food resources by certain taxonomic and FFGs in mining-influenced headwater streams because of direct SC stress. Further understanding the interaction between physiological stress and food resource availability is relevant as vulnerable taxa, alongside their ability to transform food into fuel, are being lost in many river networks.