A biogeochemical approach to understanding the accumulation patterns of trace elements in three species of dragonfly larvae: evaluation as biomonitors

The accumulation patterns of different trace elements (As, Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn) were studied in three species of dragonfly larvae (Cordulegaster boltoni, Boyeria irene and Onychogomphus uncatus). Additionally these species were assessed as potential biomonitors in a lotic ecosystem (Louro River, Spain). Element concentrations were determined by inductively coupled plasma optical emission spectrometry (ICP-OES) in both dragonfly larvae and river sediments. The surface of the larvae was observed and analyzed with scanning electron microscopy (SEM). A desorption test was used to establish the percentage of element adsorbed into the exoskeleton. Field biomonitoring studies were made considering the first part of the river as a control area in relation to the second part, which was severely polluted by industrial and domestic activities. Upon application of principal component analysis (PCA), two different element groups were found in relation to element bioaccumulation. Cu, Cd and Zn were mainly associated with the inner part of the larvae. As, Cr, Fe, Mn, Ni and Zn were found on the outer body parts, related with deposition of oxyhydroxides in the hydrocycle. SEM revealed a layer of inorganic particles, similar in composition to fine bed sediments. Significant relations between the element content of this group and that of sediments at the sampling site were found. Differences in bioaccumulation for each of the three species, except for As, were observed. Dragonfly larvae revealed their ability to reflect environmental concentrations of As in freshwater media.