(In this guest post, Devin Jones, a graduate student in the lab of Rensselaer biologist and Jefferson Project at Lake George Director Rick Relyea, discusses research results recently published in the journal Environmental Pollution. The research tests the effects of road salt and road salt alternatives alone and in combination with natural stressors on vernal pond communities. This research is part of the Jefferson Project – a collaboration between Rensselaer, IBM Research, and The FUND for Lake George – founded to develop a new model for technologically enabled environmental monitoring and prediction to better understand and protect the Lake George ecosystem and freshwater ecosystems around the world.)
Vernal pools—seasonal water bodies formed by the collection of water in small depressions of the terrestrial landscape—are vital habitats for wildlife and are an important energy source for the ecosystem. For example, the amphibians and invertebrates that use vernal pools as breeding habitats transfer energy from the aquatic environment to the terrestrial environment following metamorphosis. Also, vernal pools contain many species that interact through predator-prey and competitive interactions to form complex food webs.
Like other ecosystems, vernal pools are affected by human activities, and researchers working with the Jefferson Project at Lake George are interested in how they may be affected by de-icing road salts. More than 50 percent of all de-icing materials used on roads are made of sodium chloride (NaCl), and salt overspray and runoff into adjacent natural systems can change the physical and chemical environment. While the Jefferson Project is focused on the influence of increased salinity in Lake George, wetlands, streams, ponds, and vernal pools in the surrounding watershed have links to the Lake George food web.
Runoff carrying road salts from adjacent roads increases chloride concentrations in vernal pools. Much of our knowledge about the direct sublethal and lethal effects of chloride contamination comes from single-species toxicity tests conducted under laboratory conditions. Yet, we don’t fully understand how the increased salinity in vernal pools will interact with natural stressors (e.g., predator-prey and competitive interactions) of aquatic communities under natural conditions.
To investigate those effects, researchers turn to mesocosms – large tanks of water that can be used to simulate natural freshwater systems – at the Rensselaer Aquatic Facility. The collaborative effort explored how various concentrations of NaCl or a road salt alternative influenced vernal pool communities under different stressors.
To create semi-natural vernal pool communities in our outdoor mesocosms, we filled the tanks with water, and then added leaf litter to simulate the forest floor and algae and zooplankton collected from local ponds. Once the communities were established, we added wood frog and American toad tadpoles at constant densities.
Once we established the vernal pool communities, we altered two-thirds of them to create “stressor treatments.”
In the “competitive environment,” we doubled the density of tadpoles for each species. Amphibian egg masses contain anywhere from 600 to 1,200 embryos, and wood frogs may deposit over 50 egg masses in a single vernal pool. By doubling the density of wood frog and American toad tadpoles, we mimicked these high-density environments to simulate increased competition for food resources among tadpoles.
In the “predator environment,” we added caged dragonfly larvae. Dragonfly larvae are voracious predators of aquatic organisms and release kairomones — chemical signals released by predators following the digestion of prey— that permeate throughout the aquatic community. Caging the dragonfly larvae allows kairomones to saturate the mesocosm while preventing the dragonfly larvae from eating the tadpoles.
The remaining third set of mesocosms was a “no-stressor environment” and was not altered.
We then contaminated our vernal pool communities with three realistic concentrations of either NaCl or a road salt alternative made of NaCl, magnesium chloride (MgCl2), and potassium chloride (KCl). We compared the effects of these six road salt treatments to the outcomes of communities in mesocosms that received no salt additions.
If you’re doing the math, this experimental design included seven salt treatments crossed with three stressor treatments. These 21 treatment combinations were replicated four times for a total of 84 outdoor mesocosms.
Our experiments ran for 49 days, and during that time we discovered that NaCl and the salt alternative reduced the pH of our communities as salt concentration increased. At the highest concentration of the salt alternative and the two highest concentrations of NaCl, we observed lethal effects on the zooplankton of our communities. In communities exposed to the highest concentration of NaCl, the lower abundances of zooplankton led to an increase in the abundance of floating algae (their food source). Though we did not find a full trophic cascade in our systems, previous research has shown increased floating algae decreases the availability of light to the attached algae below, thus decreasing the abundance of important resources for grazers like tadpoles and snails. The below image shows a high salt pool on the left (with an increase in floating algae) and a no salt pool on the right.
We also observed reduced American toad tadpole activity in communities exposed to the highest NaCl concentration, a sublethal effect. Decreased activity may negatively impact resource consumption, slow time to metamorphosis, or even have long-term fitness consequences that appear following metamorphosis.
Although we found main effects of salt concentration and natural stressor within our communities, we found no evidence of road salts interacting with the stress of predation or competition.
This research was published as the “Investigation of Road Salts and Biotic Stressors on Freshwater Wetland Communities” in the February 2017 issue of Environmental Pollution.
Freshwater ecosystems are commonly contaminated with multiple chemicals and pollutants. Understanding how the toxicity of contaminants changes with the presence of natural stressors allows researchers to predict how communities will be impacted under realistic conditions. By investigating the effects of road salts on vernal pool communities, members of the Jefferson Project are gaining a further understanding of how contaminants can impact freshwater ecosystems beyond single species, laboratory toxicity tests. Understanding what happens in the vernal pools may improve future predictions for the dynamics of the watershed, and how human activities will affect other freshwater communities worldwide.