Abstract
Salinization of freshwater ecosystems, due to the application of road salts, is recognized as a potential threat to aquatic communities. Much of the research on the impact of salinity has focused on performance metrics in vertebrates, including respiration and osmoregulation. Here we focus on immune function in of the dragonfly Anax junius, a top predator in fishless aquatic habitats. Impacts on this top predator have the potential to cascade through the community, and immune function is known to be both plastic and sensitive to stress. We injected larvae with monofilaments (simulating a parasite) and placed them in one of three environmentally relevant concentrations of deicing road salt: control (dechlorinated tap water with no added salt), low (1000 mgL-1), or high salt (3000 mgL-1), for either acute (24hr) chronic or (96hr) exposures. We hypothesized that elevated salinity would suppress the immune response and that longer exposure magnifies this effect. As predicted, chronic exposure to high salt concentrations resulted in a significantly reduced larval immune response, however, there was no detectable treatment effects in larvae exposed to low concentrations of road salt or to acute high concentrations. Our results demonstrate that prolonged exposure to high levels of road salt can compromise the immune response of dragonfly larvae. Our findings suggest that insects in aquatic environments that experience sustained environmental salt pollution will be more susceptible to parasites and pathogens, which in turn may affect the impact of this major predator on aquatic community dynamics.
Highlights
Salinization of freshwater ecosystems, resulting from the transfer of winter road salts into aquatic systems through meltwater (Kaushal et al, 2005; Novotny et al, 2008; Dugan et al, 2017), has become a major concern in freshwater conservation (Cañedo-Argüelles et al, 2013; Herbert et al, 2015)
We found a significantly decreased melanization response in individuals chronically exposed to high (3,000 mgL−1) concentrations of deicing road salt compared to the control treatment (p = 0.034; Table 1, Figure 1)
There was no significant decrease in melanization for individuals exposed to low salt concentrations regardless of exposure type, or for individuals exposed to acute, high concentrations compared to the control treatment (p > 0.05; Table 1)
Summary
Salinization of freshwater ecosystems, resulting from the transfer of winter road salts into aquatic systems through meltwater (Kaushal et al, 2005; Novotny et al, 2008; Dugan et al, 2017), has become a major concern in freshwater conservation (Cañedo-Argüelles et al, 2013; Herbert et al, 2015). Previous research found that salinization of freshwater systems from the increasing use of deicing salt can negatively affect the performance of freshwater organisms (Bernabò et al, 2013; Kearney et al, 2014; Dananay et al, 2015; Hopkins et al, 2016), and threaten the viability of some populations (Silberbush et al, 2005; Petranka and Doyle, 2010; Van Meter et al, 2011; Searle et al, 2016; Castillo et al, 2018; Timpano et al, 2018). Key fitness metrics, including reduced body length and mass, have been reported in fish exposed to high salinity (e.g., Hintz and Relyea, 2017)
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