Print PageAmphibian populations are in decline worldwide in wetlands and agricultural contaminants may be part of the cause. Based on their widespread use, glyphosate-based herbicides and fertilizers are logical contaminants of interest. However, these are not known to be directly toxic to larval amphibians at environmentally relevant concentrations. Amphibians exist in a complex ecosystem, where changes in the abundance of other organisms in the food web can cause indirect effects that reverberate throughout the entire food web. Herbicides and nutrients may have such indirect effects on the wetland ecosystem by affecting emergent plants and aquatic algae. We can investigate indirect food web effects by examining stable isotope ratios; contaminants can indirectly affect the stable isotope signatures of an organism through effects on their food source. In 2009 at the Long-term Experimental Wetlands Area, New Brunswick, Canada, we determined if mixtures of glyphosate-based herbicides (2 treatments, low-0.21mg/L a.e. and high-2.89mg/L a.e.) with inorganic nutrients at environmentally relevant concentrations would affect the food web in natural wetland ecosystems. Six wetlands were split with impermeable plastic barriers to allow for direct comparison between treated and untreated halves. Amphibians, aquatic invertebrates, and basal food sources were collected from treated and control sides of each wetland.
Results/Conclusions
Phytoplankton communities of the high-glyphosate/nutrient treatment recovered from an initial depletion in δ13C (-1.8‰±1.1) possibly due to a shift in the community composition from the effects of the glyphosate, compared to their control counterparts. Phytoplankton and benthic periphyton of the low-glyphosate/nutrients treatments demonstrated immediate differences in δ15N (0.5‰±0.4 and -0.5‰±0.3, respectively) and a later enrichment in δ13C (2.9‰±1.1 and 1.6‰±1.4), possibly due to the change in nutrient source, and an eventual response of the algal community. Zooplankton stable isotope ratios in both treatments closely mimicked trends in the phytoplankton signatures; green frog tadpoles closely mimicked changes in benthic periphyton signatures. Specialized secondary consumers, such as damselfly larvae reflected the changes in zooplankton stable isotopes ratios, whereas generalized predators only showed increased variability in their stable isotope ratios. These findings will be correlated with data collected on the gut contents, development and abundance of these wetland organisms to identify possible links between the changes in stable isotope ratios and the effects of glyphosate-herbicides and nutrients on the food web.
