Dartmouth College biology professors Celia Chen and Carol Folt have discovered synergistic interactions of herbicides and natural environmental stressors such as pH and food on wetland zooplankton and larval amphibian. Their findings were published last month in Ecotoxicology and Environmental Safety.
Chen and Folt examined the effects of herbicide Release®, the trade name for the butoxy ethyl ester of triclopyr (TBEE), in combination with two key environmental variables, pH and food availability, on wetland species Simocephalus vetulus (zooplankton) and Rana pipiens tadpoles.
The study involved testing triclopyr at environmentally relevant test concentrations (0.25 and 0.50 mg/L) in conjunction with two levels of algal food availability (low and high) and two levels of pH (pH 5.5 and 7.5). The pH levels chosen represent the pH range commonly encountered in wetlands.
“We did find interactions (synergistic effects) between the stressors suggesting that regulating by chemical concentrations alone is very inadequate. Other factors can affect the concentrations at which chemicals cause mortality or reductions in reproduction in different organisms,” Chen said in an e-mail to the DUJS.
It was found that low pH and low food availability interacted significantly with the herbicide triclopyr to reduce the survival of both the zooplankton and larval amphibian species as well as the reproduction and development time for S. vetulus zooplankton.
“Our research also showed that effects can be seen at concentrations lower than those in the field expected from recommended use,” Chen said.
For both test species, the effects of the herbicide were measured at concentrations significantly lower than the calculated worst-case value for the expected environmental concentration. This raised a red flag.
The results suggest that the recommended “safe” concentration use of herbicide may be far from optimal for the aquatic species inhabiting the wetlands and their situation is particularly exacerbated if natural environmental stresses like habitat acidity and food fluctuations are involved.
The results imply that zooplankton and tadpole populations could be most at risk in low pH, low productivity wetlands sprayed directly or indirectly with Release®. These habitats are most commonly found in northeastern regions of North America where triclopyr herbicides are “made to achieve vegetation management objectives in forestry or industrial rights-of-way,” according to the article.
“These [effects of multiple stressors] are important issues because environmental regulations are based upon single chemical exposures and do not take into account other factors such as varying food availability, pH variation, dissolved oxygen variability, or the presence of other toxins. The question of the interactive effects of multiple chemicals is an area that I am continuing to work on, specifically the combined effects of multiple metal toxins,” Chen said.