Summary The application of deicing road salts began in the 1940s and has increased drastically in regions where snow and ice removal is critical for transportation safety. The most commonly applied road salt is sodium chloride (NaCl). However, the increased costs of NaCl, its negative effects on human health, and the degradation of roadside habitats has driven transportation agencies to seek alternative road salts and organic additives to reduce the application rate of NaCl or increase its effectiveness. Few studies have examined the effects of NaCl in aquatic ecosystems, but none have explored the potential impacts of road salt alternatives or additives on aquatic food webs. We assessed the effects of three road salts (NaCl, MgCl2 and ClearLane™) and two road salts mixed with organic additives (GeoMelt™ and Magic Salt™) on food webs in experimental aquatic communities, with environmentally relevant concentrations, standardized by chloride concentration. We found that NaCl had few effects on aquatic communities. However, the microbial breakdown of organic additives initially reduced dissolved oxygen. Additionally, microbial activity likely transformed unusable phosphorus from the organic additives to usable phosphorus for algae, which increased algal growth. The increase in algal growth led to an increase in zooplankton abundance. Finally, MgCl2 – a common alternative to NaCl – reduced compositional differences of zooplankton, and at low concentrations increased the abundance of amphipods. Synthesis and applications. Our results indicate that alternative road salts (to NaCl), and road salt additives can alter the abundance and composition of organisms in freshwater food webs at multiple trophic levels, even at low concentrations. Consequently, road salt alternatives and additives might alter ecosystem function and ecosystem services. Therefore, transportation agencies should use caution in applying road salt alternatives and additives. A comprehensive investigation of road salt alternatives and road salt additives should be conducted before wide‐scale use is implemented. Further research is also needed to determine the impacts of salt additives and alternatives on higher trophic levels, such as amphibians and fish.

Around the world, freshwater ecosystems are subjected to numerous stressors that can alter community composition in favour of stress‐tolerant species. Because combinations of stressors often result in non‐additive interactions, elucidating responses to isolated and combined stressors is important to understand the ecological responses to anthropogenic disturbance. In this study, we explored the responses of common macrophyte species to two stressors of increasing concern: elevated salinity from road salt applications and turbidity from human recreational activities and shoreline development. The independent and interactive effects of environmentally relevant salt concentrations and turbidity on macrophyte productivity have received little attention. We hypothesised that both stressors in isolation would reduce macrophyte productivity and that the two stressors combined will lead to a greater (i.e. synergistic) reduction in productivity. To test these hypotheses, we conducted dark‐ and light‐bottle experiments on seven species of native and invasive macrophytes under a factorial combination of three salt concentrations (0, 500 and 3,000 added mg Cl− L−1) and two turbidity conditions (clear and turbid via a disturbance to the sediment). On average, macrophytes exhibited reduced productivity in response to increased salt, but results were highly species‐specific. Several species exhibited a unimodal response to elevated salinity, whereas Elodea canadensis exhibited a positive response to the high‐salt treatment. Similarly, macrophytes exhibited an average reduction in productivity under turbid conditions, but analysis of species‐specific responses revealed both neutral and negative responses. Combining the two stresses yielded non‐additive responses for some species. Specifically, Myriophyllum spicatum appeared to suffer from the combination of salt and turbidity, whereas Elodea canadensis and Ceratophyllum demersum benefited from the combination. Our results suggest that increased anthropogenic disturbance of freshwater ecosystems can dramatically alter macrophyte species metabolism and might stimulate the growth of some species while deterring the growth of others.