by Alana Coulombe, Education programs administrator

Nature sustains itself through food web interactions that support long-term ecological balance. Every organism is part of multiple food chains that overlap and connect to form larger food webs within a single ecosystem. Each web is woven with lines that trace the potential flow of nutrients and energy from one organism to another within an ecosystem. Environmental sustainability depends on these balanced interactions between organisms in a food web.

Excessive road salt applications have led to the salinization of freshwater ecosystems, including lakes, rivers, streams, ponds, and wetlands, worldwide. Although road salt is applied during the cold winter months, evidence suggests that salts are a year-round problem since they are stored in soil and groundwater. This raises the notion that road salt needs to be recognized and studied as a potential direct contributor to various emerging environmental issues regardless of the time of year. For example, road salt contamination in freshwater environments can cause food web alterations, water quality degradation, accelerated surface water eutrophication, and an increased risk of harmful algal blooms. 

When salt-contaminated runoff enters a freshwater body, increased chloride concentrations alter aquatic communities through direct toxic effects on sensitive freshwater species (Jones et al., 2017). Road salt pollution may also indirectly affect freshwater food web structure and interactions, potentially threatening ecosystem services (Van Meter et al., 2011). This impact typically progresses from the bottom of the food web, working its way up by altering the direction and strength of interactions between producers and consumers in freshwater communities (Van Meter et al., 2011). 

Microscopic zooplankton are a critical component of freshwater food webs and serve as a major consumer of algae and a food source for many fish species. Elevated chloride concentrations can significantly reduce zooplankton abundance and decrease the reproduction of surviving populations (Jones et al., 2017). These lethal and sublethal effects on zooplankton populations influence community resilience and have cascading effects that can alter freshwater food webs (Jones et al., 2017). Reduced zooplankton abundances can lead to trophic cascades that change freshwater community compositions and the behaviour, physiology, and morphology of residing organisms (Jones et al., 2017). Identifying these long-term food web impacts from road salt pollution is one way to identify and protect salt vulnerable areas.

A study by Van Meter et al. (2011) suggests that as road salts directly reduce zooplankton populations, tadpoles may indirectly benefit from the loss of zooplankton competitors and increased availability of algal resources. The results also showed that tadpoles reared in chloride-contaminated waters grew larger and metamorphosed sooner than those reared in environments without salt contaminants (Van Meter et al., 2011). 

The reduction in zooplankton abundance subsequently causes an increase in the abundance of phytoplankton, including the development of toxic blue-green algae, or cyanobacteria, since zooplankton are a major consumer of algae (Lind et al., 2018). These altered trophic interactions can lead to novel food web patterns in freshwater habitats, even when pollutants occur at relatively low levels (Van Meter et al., 2011). Cyanobacterial blooms negatively impact the water quality, ecosystem biodiversity, drinking water supply, aesthetic value, and recreational use of the affected waterbody. Algal blooms can also have cascading effects that alter the food web structure in freshwater ecosystems by reducing light transmission and depleting dissolved oxygen supplies (Lind et al., 2018). 

Both the eutrophication and salinization of freshwater contribute to the development of harmful algal blooms through direct and indirect mechanisms: excess nutrient loading directly increases the rate of algae growth, while road salts indirectly cause algal blooms by reducing the abundance of its primary consumer, zooplankton (Lind et al., 2018). In a study by Radosavljevic et al. (2022), the researchers link the deteriorating water quality trends in Lake Wilcox of southern Ontario to watershed urbanization, mainly due to increased salinization. Radosavljevic et al. (2022) conclude that the rising salinity intensifies lake stratification which reduces levels of dissolved oxygen and enhances internal phosphorus loading to the water column, contributing to eutrophication and algal blooms. 

It is our collective responsibility to keep our freshwater food webs intact by reducing the use of road salts and other pollutants and finding sustainable alternatives, and by planting salt tolerant native species along shoreline areas to help reduce the pollution that enters the waterway. Share the information and resources from Watersheds Canada’s salt pollution toolkit with those you know to help advocate for more sustainable and eco-friendly de-icing practices to protect our freshwater ecosystems for generations to come.

References

Jones, D. K., Mattes, B. M., Hintz, W. D., Schuler, M. S., Stoler, A. B., Lind, L. A., Cooper, R. O., & Relyea, R. A. (2017). Investigation of road salts and biotic stressors on freshwater wetland communities. Environmental Pollution, 221, 159-167. https://doi.org/10.1016/j.envpol.2016.11.060
Lind, L., Schuler, M. S., Hintz, W. D., Stoler, A. B., Jones, D. K., Mattes, B. M., & Relyea, R. A. (2018). Salty fertile lakes: How salinization and eutrophication alter the structure of freshwater communities. Ecosphere 9(9). https://doi.org/10.1002/ecs2.2383
Radosavljevic, J., Slowinski, S., Shafii, M., Akbarzadeh, Z., Rezanezhad, F., Parsons, C. T., Withers, W., & Van Cappellen, P. (2022). Salinization as a driver of eutrophication symptoms in an urban lake (Lake Wilcox, Ontario, Canada). The Science of The Total Environment, 846, 157336. https://doi.org/10.1016/j.scitotenv.2022.157336
Van Meter, R. J., & Swan, C. M. (2014) Road salts as environmental constraints in urban pond food webs. PLoS ONE, 9(2). https://doi.org/10.1371/journal.pone.0090168
Van Meter, R., Swan, C., Leips, J., & Snodgrass, J. (2011). Road salt stress induces novel food web structure and interactions. Wetlands, 31(5), 843-851. https://doi.org/10.1007/s13157-011-0199-y