Tracing metals through urban wetland food webs

Constructed wetlands are commonly used to remove contaminants such as heavy metals from stormwater. However, metals can become bioavailable and be transferred into and along wetland food chains. Our study sought to establish whether urbanisation affected concentrations of metals in water, wetland sediments, freshwater crayfish and fish tissues. Samples were taken from constructed wetlands in western Melbourne. There was no relationship between catchment total imperviousness (TI, an index of urbanisation) and water column and sediment concentrations of metals with the exception of Zn, which was higher in more urbanised catchments. Concentrations of metals in fish tissues were highest in benthic species but declined with increasing body size and trophic level. This suggests that metals are not bioaccumulating or biomagnifying along food chains in these systems. Metabolic activity can differ between smaller and larger fish, or smaller fish may also be feeding on different food sources. Our results suggest that there is not a major human health risk associated with accumulation of metals in large-bodied fish in these wetlands, and that most of the metal load is retained in sediments, or the lowest trophic levels.