Effect of age on the heavy metal concentration in segments of Pseudoscleropodium purum and the biomonitoring of atmospheric deposition of metals

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We measured growth of the terrestrial moss Pseudoscleropodium purum during several well defined periods (monthly and quarterly between January 2008 and December 2009) at seven sampling sites affected by different climatic conditions. We also measured the concentrations of Cd, Cu, Hg, Pb and Zn in different moss segments comprising the tissue grown during each period. The concentrations of heavy metals in the different portions of moss shoots were highly variable, probably because of the high spatial and temporal variability in the growth rates of P. purum and the differences in the cation uptake/retention capacities of old and new tissues. The error associated with measurement of the concentrations of heavy metals in individual segments of the moss shoots ranged between 18 and 41%. The concentrations of Cd, Cu, Hg, Pb and Zn in the moss tissues appear to be more closely related to the physicochemical characteristics and the moss metabolism than to any preferential accumulation in young or old tissues. We conclude that comparison of the results of different biomonitoring studies is not valid, even when the same parts of the moss shoots are analyzed. Therefore, we recommend the use of green parts of moss shoots, as this would reduce the effect of tissue ageing on the cation uptake/retention capacity, although this prevents the establishment of a chronological relationship between the concentrations of heavy metals in moss tissues and atmospheric deposition.