Are wetland treatment systems suitable habitat for a common amphibian? Examining fitness-related traits, copper bioaccumulation, and tolerance in southern toads (Anaxyrus terrestris)

Anthropogenic contaminant release has altered freshwater ecosystems, where pollutants often accumulate to concentrations that threaten aquatic and amphibious inhabitants. Copper (Cu) is a widespread contaminant that is toxic to amphibians at low concentrations. Constructed wetland treatment systems, designed to sequester Cu and meet water quality criteria, provide a unique opportunity to investigate ecological risk in amphibians inhabiting Cu contaminated environments. We collected adult southern toads from natural, reference wetlands and from constructed wetland treatment systems processing Cu-contaminated wastewater to investigate potential divergences in fitness-related traits (e.g. body condition, clutch volume). We further conducted an embryo exposure trial comparing mortality responses between populations across four treatments: a soft water control, a 15 μg L -1 total Cu soft water solution, water from a reference wetland, and water from a wetland treatment system (∼ 21 μg L -1 total Cu). Our results show that males and females from constructed wetland populations have elevated body condition, and therefore greater clutch volume, compared to those from reference populations. In the embryo exposure trial, there was no evidence for Cu tolerance in constructed wetland treatment system populations, and significant embryo mortality was only observed in the 15 μg L -1 total Cu soft water treatment, but not the ∼ 21 μg L -1 Cu constructed wetland treatment. However, surviving larvae from both Cu treatments bioaccumulated Cu, suggesting that conditions in the wetland treatment system do not prevent biological uptake of Cu species, but may reduce Cu lethality in southern toad embryos. These findings highlight complex individual- and population- level effects of constructed wetland inhabitation and further research will be essential for responsible implementation and conservation in their contaminated habitats.