The Unforeseen Impact of Road Salt Application & Related Shoreland Development on Freshwater Aquatic Food Webs

By: Jake Knowles

Far from a natural surface, the uniformity of road infrastructure in North America is constantly at odds with the elements. Maintaining safe driving conditions on roads requires a number of amendments – one common example being the widespread and increasing use of de-icing road salts (Kaushal et al., 2005; Chapra et al., 2009). These salts (NaCl and CaCl2), which operate by lowering the freezing point of water, are indiscriminately applied to winter roads. One major consequence of this application is the salinization of freshwater lakes, whereby salts are leached from roads elevating the chloride (Cl-) concentrations of nearby lakes. (Dugan et al., 2017). This reality has far-reaching implications for regions like The Land Between, which has thousands of lakes, as well as heavy winters driven by lake-effect snow. Here, the use of de-icing methods such as road salt is a necessity. 

Concerned with the potential impact of elevated chloride levels on a genus of large zooplankton called Daphnia, Brown and Yan (2015) investigated how food availability influenced their sensitivity to chloride. Critical to freshwater aquatic food webs, Daphnia effectively regulate microscopic aquatic organism populations, consume algae, and provide an essential and excellent food source for fish and benthic invertebrates (Pace and Vaqué, 1994). 

Brown and Yan’s findings indicated that chloride becomes increasingly toxic to Daphnia when concentrations of their food (algal carbon) are lowered. This has serious implications for Daphnia populations in oligotrophic (low-nutrient) lakes with lower concentrations of algal carbon, suggesting they may already be at risk even at chloride levels below the threshold of 120 mg Cl-/L established by the Canadian Water Quality Guideline. Brown and Yan (2015) also indicated the potential for cumulative effects wherein declining concentrations of calcium (Ca) (related to acid rain but also to the removal of native shoreland vegetation) could further increase Daphnia sensitivity to chloride. 

Arnott et al. (2020) further investigated these findings by conducting more ecotoxicological tests under conditions similar to the oligotrophic, low-calcium lakes commonly found across The Land Between. Their findings suggest that multiple species of Daphnia exhibited lower rates of reproduction and higher rates of mortality between 5-40 mg Cl-/L. Their research also reveals that 22.7% of recreational lakes in Ontario currently fall within this range.

With a reduction of Daphnia to the extent found recently in lakes of The Land Between, increased algal blooms can be expected, as well as reductions in fish population recruitment due to declines in nursery habitat health. 

It can’t be denied that road salts improve winter road conditions and promote safe travel. However, it is important to consider the full spectrum of environmental and economic implications associated with their use. Additionally, new roads and developments that both remove native vegetation around lakes and introduce new sources of salt from direct applications or from hardened surfaces and increased runoff, are of increased and also significant concern. 

The economy of The Land Between bioregion is intrinsically linked to the quality of its lakes and also the related fisheries– a reality which requires those with authority, as well as community members of the region, to consider aquatic ecosystem health. While the findings discussed here highlight an unforeseen impact of road salts on lake health, they also perhaps reveal future avenues of conservation planning and research into more sustainable and targeted salt application methods, as well as development site plans, which could take into account at-risk, low-nutrient lakes. Directly, regulations concerning salt-use limits or utilizing safe salt alternatives in areas where high road density, major roadways, and sensitive lakes overlap should be considered within The Land Between.