Road salt and organic additives affect mosquito growth and survival: an emerging problem in wetlands

Abstract

The global increase in the application rate of road salts such as sodium chloride (NaCl) has led to concern about their negative effects on roadside habitats and freshwater ecosystems. To reduce the application rate of NaCl and minimize the ecological effects of road salts, transportation agencies are continuously seeking alternative salts such as magnesium chloride (MgCl2) and organic additives such as beet juice and distillation byproducts. Yet, there is remarkably little information about how these road salt alternatives and additives affect aquatic communities, including their effects on mosquito populations. Nonetheless, understanding how anthropogenic factors such as road salts and salt additives affect mosquito populations could help minimize threats to human health, especially in urban environments. We used outdoor, freshwater mesocosms to experimentally investigate how the road salt MgCl2 and two organic additives affect mosquito Culex restuans survival and emergence. Additionally, we measured changes to abiotic aspects of the environment that could affect mosquito larvae during development. We found that increased concentrations of MgCl2 reduced mosquito survival while organic additives increased food resources that, in turn, reduced the average time to emergence for mosquitoes. Additionally, the organic additives reduced dissolved oxygen (DO) to hypoxic levels, which might negatively affect mosquito predators such as fish. In the absence of toxic concentrations of MgCl2 or other salts, reduced predation coupled with the faster emergence times, means that organic additives, might increase mosquito population size. More comprehensive studies of multi‐trophic interactions in freshwater ecosystems should be conducted before agencies promote the application of alternative road salts and road salt additives.

Regulations are needed to protect freshwater ecosystems from salinization

“…Human health and wellbeing are negatively affected by the loss of ecosystem functions or services, but salinized drinking water can also cause health conditions such as hypertension [31]. Additionally, urban freshwater salinization can increase the prevalence of salttolerant disease vectors such as mosquitoes [32,33], and salts can mobilize toxic heavy metals into drinking water, increasing the risks to human health [23,34]. Each source of anthropogenic freshwater salinization contains a distinct set of dominant ions [13], meaning that the ion composition of freshwater environments affected by salinization are being altered in unique ways.…”

“…Regardless of which ions have the strongest toxicological effects, the toxicity of specific ions differs among freshwater organisms, and the toxicity of specific ions often depends on the presence, ratio, and concentration of additional ions [28,[40][41][42]. For example, when associated with chloride, magnesium and calcium have been shown to be more toxic to freshwater invertebrates than sodium [33,37,39]. However, the concentration of sodium has been demonstrated to determine the toxicity of potassium salts [40].…”

Tolerance of disease‐vector mosquitoes to brackish water and their osmoregulatory ability

“…, Kaushal et al. , Schuler and Relyea ). This last action has been largely neglected, but has important consequences in temperate countries where desalination is regularly used for anti‐icing or deicing pavements and roads, and could contribute to increase the salt concentration in freshwater bodies.…”

“…Breeding sites of these mosquitoes are alternately prone to dilution by rain, and to salinity increase by evaporation or flooding of coastal marshes. Moreover, human activities can influence the amount of salts in breeding sites by modifying coastal habitats , polluting urban breeding sites (Tene Fossog et al 2013), or by using deicing salts (Muñoz et al 2015, Kaushal et al 2018, Schuler and Relyea 2018. This last action has been largely neglected, but has important consequences in temperate countries where desalination is regularly used for anti-icing or deicing pavements and roads, and could contribute to increase the salt concentration in freshwater bodies.…”

The Impact of Road Salts on Groundwater and Estimation of the Chlorine Ions by Hydrogen Index

“…Taking these circumstances into account, chloride salts used in winter conditions can be considered as potential loads influencing: changes in the biogeochemical water cycle [4]; salinization of drinking water [9][10][11][12]; degradation of aquatic life [13][14][15]; reduction of drinking water supplies [16][17][18][19][20]; on the processes of redox stratification of substances, including dissolved oxygen [10,21]; to increase the accumulation of methane and its release into the atmosphere [21]; toxicological effects on organisms of terrestrial and aquatic environments [22][23][24][25][26][27][28][29]. Accordingly, all deicing agents require certain precautions to be taken.…”