Microbial and macroinvertebrate communities, but not leaf decomposition, change along a mining-induced salinity gradient

Natural levels of salinity in aquatic ecosystems drive biodiversity patterns across broad spatial scales; however, less is known about changes in biotic communities and the ecosystem functions they support along anthropogenic salinisation gradients. Resource extraction often causes salinisation of freshwater ecosystems which may extirpate salinity-sensitive macroinvertebrates and microbes and thus reduce rates of organic matter decomposition. We quantified bacterial (16S), fungal (ITS) and macroinvertebrate taxonomic richness, composition, and beta-diversity across the resulting gradient of specific conductance (annual mean: 25-1,383 mu S/cm) in 24 headwater streams in the eastern U.S.A. variously influenced by surface coal mining but selected to minimise habitat and water-quality differences other than salinity. Furthermore, we measured rates of organic matter decomposition in submersed leaf packs (Quercus alba) across these same sites. Bacterial and reach-wide macroinvertebrate richness was reduced along the mining-induced salinity gradient whereas fungal or leaf-pack macroinvertebrate richness remained similar. Community composition of microbes and macroinvertebrates changed along the salinity gradient, with mining-influenced sites becoming increasingly dissimilar to reference sites as salinity increased. Beta-diversity was driven by taxonomic replacement rather than nestedness for microbial and macroinvertebrate communities. Organic matter decomposition rates in mining-influenced streams were not reduced even at mean specific conductance levels 10x higher than reference sites. We attribute maintenance of decomposition rates to salinity tolerance of both fungi and macroinvertebrate shredders found in leaf packs. Our study informs theory linking anthropogenic alteration of biotic communities to rates of ecosystem functions by providing evidence that taxonomic replacement and stressor-tolerance of specific functional groups along a mining-induced salinity gradient can maintain certain ecosystem functions, at least within the studied salinity range. We therefore advise against generalising biodiversity-ecosystem function relationships in other salinised systems, but caution that organic matter decomposition may be altered at salinity levels above those observed in our study and that other ecosystem functions may be affected by loss of salinity-sensitive taxa.