Sulphate in freshwater ecosystems: A review of sources, biogeochemical cycles, ecotoxicological effects and bioremediation

Abstract Sulphate (SO42-) concentrations in freshwaters have increased globally over the last decades even though a strong reduction in atmospheric sulphur (S) deposition has occurred across large parts of North America and Europe. However, the extent and effects of increased SO42- concentrations in freshwater and terrestrial ecosystems remain poorly understood regarding many aspects of ecosystem structure and functioning. Here, we review the sources of SO42- pollution, environmental impacts on freshwater ecosystems and bioremediation opportunites and we identify key knowledge gaps and future research needs. Natural sources of dissolved SO42- in freshwater ecosystems include mineral weathering, volcanic activity, decomposition and combustion of organic matter, oxidation of sulphides, and sea spray aerosols. Acid mine drainage, fertiliser leaching from agricultural soils, wetland drainage, agricultural and industrial wastewater runoff as well as sea level changes are the main direct and indirect sources of the anthropogenic SO42- input to waterbodies. Increasing SO42- concentrations in freshwater systems influence the biogeochemical processes of carbon, nitrogen and phosphorus. Similarly, iron availability can be critical in determining the adverse effects of SO42- on environmental receptors. The literature reviewed clearly demonstrates that SO42- pollution may have toxic effects on aquatic plants and animal organisms, including, among others, fishes, invertebrates and amphibians, and it may also have negative implications for human health. Bioremediation systems provide opportunities to mitigate the impacts of SO42-, but removal efficiencies range widely from 0% to 70% across treatment systems such as constructed wetlands, permeable reactive barriers and bioreactors. We conclude that examination of increased SO42- concentrations and fluxes at different spatial scales is urgently needed as the ongoing global perturbation of the S cycle is likely to be accelerated by climate change and human development activities. The adverse effects of this on freshwater organisms worldwide may prove detrimental to the future well-being of humans and ecosystems. Field-scale research to estimate the ecotoxicological effects of elevated SO42- concentrations is recommended as is widespread implementation of large-scale wetland restoration and bioremediation systems to reduce SO42- loads on freshwater ecosystems.