Lichens as biomonitors of air-borne trace metal pollution in urban area

Lichens and mosses are cryptogamic organisms which occur in almost all terrestrial ecosystems and by virtue of their ability to tolerate long periods of drought may even colonise areas with extreme environmental conditions. Because of their high surface:volume ratio, the simple anatomy and absence of a cuticle, they accumulate heavy metals, concentrating them in tissues. Due to this ability, they may show an elemental composition which reflects, over the long term, the dissolved gases, particulate matter and metal ions of the atmosphere, and can be considered important biomonitors of environmental pollution. Lichens and mosses have been widely used for more than 20 years for assessing the atmospheric deposition of heavy metals and radionuclides in urban areas. Mosses and lichens occurring naturally in the area as well as those transplanted have been used as biomonitoring organisms in a large number of studies. However, rather few investigations have focused on the comparison of the accumulation capacity of the two types of organisms. In urban areas, where mosses and lichens are often scarce or even absent, the ‘‘bags technique'' has been set up and developed in order to monitor city air pollution. Bags consist of a mesh or grid, generally made of nylon, containing waterwashed mosses and lichens. This technique has the following advantages: uniformity of entrapment surface and exposure period, flexibility both in site selection and in the number of stations that can be chosen, known original concentrations of contaminants in the biomonitors and greater collection efficiency for most elements. In addition, bags eliminate the possibility of contamination via root uptake and, in comparison with dust fall jars or bulk samplers, offer lower cost and higher efficiency. The major limitation of the method is in the unknown collection efficiency for different contaminants. Thus, the measured metal concentrations might reflect relative rates of deposition and not the total atmospheric load of contaminants. The duration of exposure is another critical aspect of biomonitoring by bags. Biomonitors may reach a saturation point for the uptake of an element and biomonitoring performance may also be altered by climatic and environmental conditions. The comparability of results obtained with different cryptogamic organisms is another problem associated with biomonitoring studies as a consequence of differences in ecophysiology and mechanisms of metal bioaccumulation. In areas with widespread geochemical natural and anthropogenic sources of metals, epiphytic lichens seem more reliable biomonitors of atmospheric deposition of trace elements than epigeic mosses. In this seminar the results of several experimental biomonitoring works carried out in Europe will be critically analysed and discussed.