Performance of Lichen Species as Air Pollution Biomonitors in the Greater Auckland Urban Area, New Zealand

Abstract

Air pollution monitoring devices are often expensive and require high technical skills to operate. Lichens as biomonitors are an effective complementary because they accumulate pollutants in line with atmospheric concentrations. This research assesses the performance of lichens as air pollution biomonitors. The study was conducted in New Zealand, where no such biomonitoring has so far been undertaken. Four contrasting land use sites within the Auckland Region were used. One was located within a relatively clean air-shed, the others within residential, commercial, and industrial areas, characterised by increasingly polluted air-sheds, respectively. Four groups of lichen were monitored over a 24-month period using active and passive biomonitoring methods to assess accumulated concentrations of heavy metals. Short-term transplants were used to quantify heavy metals accumulated by the lichen during each season. Long-term transplants were used to measure how fast lichens accumulate heavy metals, in particular, to better understand how and when heavy metals within the lichen thallus achieve equilibrium with air pollutants over time. In-situ sampling aimed to identify the heavy metal content of the thallus when the lichen was in equilibrium. Transplants from a polluted site were moved to less polluted sites to understand the process of pollutant release from the thalli. The results showed that Parmotrema reticulatum and Ramalina celastri may be successfully used to monitor spatial and temporal pollution patterns caused by even very low concentrations of heavy metals. The in-situ samples at the industrial locations had the highest accumulation of heavy metals, followed by the commercial and residential locations, respectively. The heavy metal accumulation was the same for all these sites, namely, Zn, Pb, Cu, and Cr, in descending order. The lichen continuously accumulates pollutants from the air until equilibrium is reached. Until that point, transplanted lichens are useful for temporal air pollution monitoring. Since pollutant concentration in the transplanted lichen at equilibrium stabilises, at this point the lichen ceases to be useful for monitoring air pollution trends over time, but may useful for spatial air pollution monitoring. Lichens linearly release the accumulated pollutants over time when they are transplanted from a polluted site to a less polluted site.