PS 36-11 - Vertical Habitat Use and Metal Accumulation in Dragonfly Nymphs—a Field Caging Experiment

Abstract: Despite all being predaceous, different dragonfly species inhabiting the same wetland can accumulate substantially different metal concentrations. The cause/s of this differentiation is poorly known, but potential influences include differences in habitat use, feeding ecology, or physiology/biology. We previously found aquatic nymphs of the dragonfly Erythemis to accumulate higher concentrations of metals than Anax in a contaminated constructed wetland. Here, we experimentally evaluated the influence of living at different water depths in a winter field-caging study conducted in the same wetland on the Department of Energy’s Savannah River Site, SC, USA. We caged Erythemis simplicicollis and Anax longipes at four water levels ranging from on the bottom to 75 cm above the sediment, near the water’s surface. Peeper water samplers were used to evaluate metal concentrations in water at each depth during the experiment. Sediment metal concentrations were also evaluated near the base of the frames holding the cages. Both Erythemis and Anax accumulated higher Cu and Zn concentrations in the contaminated wetland than in the source reference-wetlands. Previously observed taxonomic differences in metal accumulation were maintained regardless of water depth. However, difference between taxa was reduced in the reference wetlands. Water Cu and Zn concentrations were stratified in the water column, with Cu concentrations highest near the surface and Zn concentrations highest at the bottom. Whole body concentrations of Cu and Zn in Anax did not differ among water levels. In contrast, Cu and Zn concentrations in Erythemis were greatest at the top of the water column and lowest at the bottom. Thus, Cu accumulation in Erythemis appeared to respond to the stratified gradient of Cu in the water, but Zn did not. Exposure to sediment did not increase accumulation of Cu or Zn in either species.