A review of recent advances and future challenges in freshwater salinization

In spite of being a worldwide phenomenon that can have important ecological, economic and social consequences, freshwater salinization (i.e. the increase in ion concentrations in freshwaters) has been poorly studied when compared with other environmental issues. However, it is receiving increasing attention, with significant scientific advances being made during the last two decades. Here I review the current knowledge on the topic and propose future research directions within the context of human welfare and global change. Freshwater salinization is caused by a wide range of human activities, with agriculture and resource extraction being the most widely documented. Different studies suggest that it could be affecting around 1/3 of freshwater bodies, and this number will very likely increase in the future due to climate and land use changes. The increase in the salinity of freshwaters is known to cause adverse effects on the fitness and survival of many aquatic organisms, however the osmoregulatory mechanisms underlying these effects are still poorly understood. Moreover, it has been proved that different ions can have different toxicities to aquatic organisms, but most of the research has focused exclusively on Na+ and Cl-. Thus, more investigations on the potential effects of increasing concentrations of other specific ions (e.g. K+, SO42-) are needed. The impact of salinization on freshwater biodiversity can alter ecosystem functioning, although only a few functions have been studied (e.g. leaf litter decomposition). Also, freshwater salinization can affect the delivery of ecosystem services and have direct economic (e.g. infrastructure corrosion) and social (e.g. human health) impacts that have rarely been assessed. The management of this urgent environmental issue needs to be improved through mitigation (e.g. backfilling of mine tailings), remediation (e.g. enhanced wastewater treatment), prevention (e.g. using alternative deicers for roads) and monitoring (e.g. estimating salt loads to freshwater ecosystems) actions.