Epiphytic lichens as biomonitors of airborne heavy metal pollution

► In situ lichen Parmotrema reticulatum is useful for spatial air pollution monitoring. ► Transplanted lichens reliably measure trends in heavy metal concentrations. ► Lichen–air equilibrium time is determined by air pollution level at transplant site. ► After equilibrium transplanted lichen is no longer a useful biomonitor of trends. ► We show lichens to be low cost biomonitors of weak concentrations of heavy metals. The research aims to assess the performance of the lichen Parmotrema reticulatum as an air pollution biomonitor of four heavy metals, namely, chromium (Cr), copper (Cu), lead (Pb) and zinc (Zn). Four contrasting land use sites within the greater Auckland region of New Zealand are used. One site is located within the relatively clean air shed of conservation land, the others within residential, commercial, and industrial areas, characterised by increasingly polluted air sheds, respectively. Three groups of lichens are monitored over a two-year period using active and passive biomonitoring methods to assess ‘on-thallus’ and ‘in-thallus’ concentrations of heavy metals. Seasonal transplants are used to quantify heavy metals accumulated by the lichen during each season. Long-term transplants are used to measure how fast lichens accumulate heavy metals and to better understand how and when heavy metals within the lichen thallus achieve equilibrium with air pollutant concentrations over time. The results show that the lichens continuously accumulates pollutants from the air until equilibrium is reached, thus transplanted lichens are useful for monitoring air pollution concentrations over time. Since pollutant concentration in the transplanted lichen at equilibrium stabilises, at this point the lichen ceases to be useful for monitoring temporal trends in air pollution, but may useful for spatial air pollution monitoring. The industrial location has the highest total accumulation for all four heavy metals, followed by the commercial and residential locations, respectively. Overall, the results show that the lichen P. reticulatum may be successfully used to monitor spatial and temporal pollution patterns caused by even very low concentrations of Cr, Cu, Pb and Zn.