(In this guest post, Aaron Stoler, a postdoctoral researcher in the lab of Rensselaer biologist and Jefferson Project at Lake George director Rick Relyea, discusses research results recently published in the journal Environmental Toxicology and Chemistry. The research tests the effects of road salt alone and in combination with a common insecticide on wetland 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.)
With the approach of winter, salt trucks are getting ready to wage battle on icy roads and slick driving conditions. Every year, more than 22 billion kilograms (49 billion pounds) of road salt are applied on our roadways. In the spring, rains flush this salt into fields, forests, wetlands, streams, rivers, and lakes. As a consequence, the level of salt in our environments has gradually risen over the past several decades. However, scientists know very little about how road salt affects aquatic food webs.
A rise in salt pollution is symptomatic of a much larger problem facing natural ecosystems: human development. A slew of factors associated with human activity threaten our natural landscapes, such as conversion of forests to agriculture, habitat fragmentation, and chemical contamination. For example, 450 million kilograms of pesticides are used in the United States annually. Like salt, these pesticides are easily washed into natural landscapes and waterways, and pose a threat to the stability and function of food webs.
Scientists are only beginning to understand the effects of individual contaminants such as road salt and pesticides on food webs, and very little is known of their combined effects. Although low levels of any contaminant are unlikely to kill organisms, researchers have found that such sub-lethal levels can still severely affect organisms by changing their behavior, growth, and development. Combining contaminants can lead to even further changes and can ultimately stress organisms to the point where death is a concern. Too much stress is never a good thing.
With this background in mind, researchers in the Relyea lab at Rensselaer questioned how road salt and the insecticide carbaryl act alone and in combination to affect the structure and function of wetland food webs (carbaryl [commercial name: Sevin] is one of the most commonly applied insecticides in the United States). To do this, they created miniature wetlands using plastic wading pools. After filling the pools with water, researchers put in the ingredients commonly found in wetlands, including leaf litter, bacteria, fungi, algae, snails, small clams, tadpoles, and amphipods (small crustaceans, commonly referred to as scuds).
After creating 60 of these miniature wetlands – which takes up about a fourth of an acre – researchers added road salt (sodium chloride) and carbaryl, either alone or in combination. Because these contaminants can be found within a wide range of concentrations, their treatments included four concentrations of road salt crossed with three concentrations of carbaryl. The concentrations of salt mimicked concentrations found in nature, ranging from the low concentrations found in drinking water (30 and 80 milligrams chloride per liter), to higher concentrations that are more typical of stormwater runoff ponds (230 and 780 milligrams chloride per liter). Similarly, the concentrations of carbaryl also mimicked concentrations found in nature, ranging from 0 to 50 micrograms insecticide per liter. After contaminating the artificial wetlands, the researchers let the food web respond over eight weeks. During that time, they measured everything.
Although each one of the 60 wetland communities started out exactly the same, things were pretty different by the end of the experiment. By itself, salt had numerous effects on the community. The abundance of some zooplankton (i.e. microscopic crustaceans in the water that eat algae) was lower at higher salt concentrations, but not low enough to cause a bloom of algae. Similarly, the abundance of clams – which also filter the water to eat algae – decreased at the highest salt concentration. However, the abundance of snails increased with more salt. Although this might seem like a good thing, snails excrete a lot of nutrients that might cause algal blooms. Without the clams and zooplankton to filter out the algae, this could destabilize the system over long time periods.
Carbaryl also had both positive and negative effects. The insecticide decimated the populations of some larger zooplankton species, resulting in a bloom of algae. At the same time, carbaryl also killed off many of the snails and amphipods in the wetlands, which allowed their tadpole competitors to grow more quickly.
Despite the numerous independent effects of salt and carbaryl, we actually found that the combination of salt and carbaryl produced very few additional effects. Among all of the organisms tested in our experiment, only one showed any sign of compounded stress when the two contaminants were combined, and this only occurred at our highest concentrations of both contaminants.
All of this is bad news for wetlands. In addition to providing further evidence that pesticides can dramatically change the structure and function of aquatic ecosystems, our experiment is among the first to demonstrate how road salt can have similar effects. Moreover, although we saw only one interaction, it is very likely that more severe interactions could occur with higher concentrations of pollutants.
We still have a long way to go toward understanding the effects of salt and pesticides in aquatic systems. Despite the widespread use of salt, this study is among the first and few to actually test the effects of road salt on wetland communities, and far more work has to be done to fully understand these effects. In addition, while sodium chloride road salt is the most widely used type of road salt, it is increasingly being replaced by alternatives that contain different types of salts or different additives such as beet juice. As little as we know about the effect of traditional road salts on aquatic food webs, we know almost nothing about the effects of these alternatives that are promoted as being safer for the environment. Lastly, carbaryl is only one out of hundreds of pesticides regularly applied to the landscape, and only one out of many stressors that wetland communities experience. Understanding the interaction between all of those stressors is a daunting task, but it is a challenge that must be met to improve our conservation and management efforts. All members of the Relyea lab at Rensselaer are working hard to address these and other issues facing our freshwater systems.