The evolutionary and ecological effects of road deicing salt

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Coldsnow, Kayla Dawn
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Third, I investigated the salt tolerance of an invasive mollusk, the Asian clam (Corbicula fluminea), which is found in a variety of habitats that vary in salinity. Since most freshwater organisms are more tolerant to NaCl than to other chloride-based alternatives, I hypothesized that Asian clams are more tolerant to NaCl road salt than to calcium chloride (CaCl2) and magnesium chloride (MgCl2). I also predicted Asian clams would be relatively tolerant to all salts given their tolerance to other stressors. I performed a 4-day and 8-day LC50 experiment using the three different road salts. I found that Asian clams were extremely tolerant to NaCl road salt with an estimated LC504-day > 22,581 mg Cl-/L. The estimated LC504-day for MgCl2 and CaCl2 were significantly lower, but still high at 2162 and 3554 mg Cl-/L, respectively. After 8 days, the estimated LC50’s for NaCl, CaCl2, and MgCl2 were 10,069, 2235, and 1769 mg Cl-/L, respectively. The high salt tolerance of Asian clams allows them to be easily transported in ballast water and to invade a variety of saline habitats. Additionally, they will be more resistant to road salt contamination, which might allow them to outcompete native species.
Second, I investigated the consequences of evolved salt tolerance. Contaminant exposure, including salt, results in a variety of genetic and physiological consequences. Some contaminants can even disrupt the circadian rhythm. Therefore, I hypothesized that the evolution of road salt tolerance alters the circadian rhythm, as well as other important genes. Using salt-tolerant D. pulex produced in the previous experiment, I completed qRT-PCR on samples from a 24- and 48- hr time course. I found that the control population exhibited a ~20-hr rhythm in period, a core clock gene. D. pulex populations previously exposed to moderate concentrations of NaCl were similar to the control, while high salt populations exhibited an ablated rhythm. Additionally, moderate and high salt populations also exhibited increases in clock (a core clock gene), actin, and a subunit of the Na+/K+-ATPase pump (involved in salt tolerance). Therefore, the evolution of salt tolerance comes with diverse genetic consequences. These trade-offs could affect a variety of bodily functions, as well as Daphnia’s ability to perform important daily activities.
First, I investigated whether organisms can adapt to road salt. Zooplankton species in the genus Daphnia can adapt to other chemicals including pesticides, heavy metals, and brackish water (from seawater intrusion). Therefore, I hypothesized that Daphnia pulex populations exposed to road salt evolve increased salt tolerance. To test this, D. pulex were exposed to a no-salt control (15 mg Cl-/L). and sodium chloride (NaCl) road salt (100 - 1000 mg Cl-/L) for 2.5 months in outdoor mesocosms. After this exposure and a period of indoor husbandry, I exposed D. pulex to a no-salt control (30 mg Cl-/L) and high salinities (1300 - 1900 mg Cl-/L) in a time-to-death assay. I found that D. pulex populations evolve increased salt tolerance. Since Daphnia consume phytoplankton and are a food resource for many fish species, evolved tolerance might be vital for road salt contaminated communities.
Overall, this thesis goes beyond the traditional single-organism, naïve-tolerance toxicity tests to enhance the road salt literature about what might be occurring in our freshwater ecosystems. The evolution of salt tolerance might mitigate some of the effects of salinization, however it may come with unfavorable consequences to important genes that influence a variety of daily functions and behaviors. It remains unknown whether organisms can evolve tolerance to NaCl alternatives and, if so, if there are any associated consequences. It is always the hope that alternatives to common chemicals will improve not only the lives of humans, but also the health of the environment. Thus far, however, chloride-based NaCl road salt alternatives have not shown to be safer for the environment. While some of these alternatives come with vital nutrients, their toxicities exceed the toxicity of NaCl. For invasive species, it is reassuring that road salt alternatives with vital ions might not facilitate their invasions. However, for native species, it means that chloride-based alternatives will not diminish the threat of freshwater salinization but rather, alternatives might increase the consequences. While much more research is needed on freshwater salinization from road salt, these studies open up new avenues of research that will ultimately help preserve fresh water.
Lastly, I investigated the interactive effects of invasive bivalves and CaCl2 road salt on native freshwater communities. CaCl2 road salt can enter fresh water where it might serve as a source of calcium. The calcium concentrations of a water body can determine the success of invasive bivalves and their impacts on freshwater communities. I hypothesized that invasive bivalves, in the absence of increased CaCl2, remove phytoplankton via filtration and reduce zooplankton and native clams via competition; but they increase benthic algae via excretion. I also hypothesized low to moderate concentrations of CaCl2 benefit native mollusks and result in few negative consequences to the community. Lastly, I hypothesized that CaCl2 increases invasive bivalve biomass and density, which results in a greater negative impact on native communities from invasive bivalves. To investigate these hypotheses, I conducted an outdoor mesocosm study with invasive Asian clams and zebra mussels (Dreissena polymorpha), as well as native organisms, all exposed to CaCl2 (35 - 187 mg Ca2+/L). I found that invasive bivalves decreased phytoplankton, periphyton, zooplankton, and native clams, but increased filamentous algae and isopods. Zebra mussel survived poorly across all CaCl2 concentrations, but 91 and 187 mg Ca2+/L substantially reduced Asian clam survival. The reduction in survival, in turn, reduced impacts of Asian clams on other species in the community. The highest CaCl2 treatment also reduced zooplankton densities. Therefore, CaCl2 did not promote invasive species and more research is needed to understand the substantial differences in tolerance of Asian clams in lab versus mesocosm experiments.
Organisms around the world are exposed to a variety of anthropogenic stressors. One stressor is the salinization of fresh water from road deicing salt. Huge increases in road salt application over the decades have increased the salinity of typically low-salt ecosystems. Studies have investigated the effects of road salt at the organismal and community level, however many focus on single organism lethality or single stressors. Many questions remain including those about evolved tolerance and subsequent trade-offs, road salt alternatives, invasive species, and the interaction of multiple stressors. The goal of my dissertation was to investigate the evolutionary and ecological effects of road salt.
May 2020
School of Science
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Rensselaer Polytechnic Institute, Troy, NY
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