Is it possible to estimate atmospheric deposition of heavy metals by analysis of terrestrial mosses?

Here we present a critical review of diverse research studies involving estimation of atmospheric deposition of heavy metals from the concentrations of the contaminants in terrestrial moss. The findings can be summarized as follows: i) significant correlations between the concentrations of contaminants in moss and bulk deposition were observed in only 40.1% of the cases in which the relationship was studied and in only 14.1% of the cases, the coefficient of correlation was > 0.7; ii) some method-related problems were identified (i.e. small sample sizes, elimination of some data from the regression analyses, large distances between the moss sampling sites and the bulk precipitation collectors, differences in times of exposure of the moss samples and collection times for the bulk precipitation), so that the results of the studies may not be completely valid, and iii) evidence was found in the relevant literature that moss does not actually integrate the atmospheric deposition received. We also discuss the reason why, in accordance with the published data, bulk deposition cannot be correctly estimated by determination of the final concentrations of contaminants in the organism, such as the existence of different sources of contamination, the physicochemical characteristics of the sources of deposition, physicochemical processes to which the organism is subjected and the biological processes that take place in the moss. Taking into account the above findings, it was concluded that, except for certain elements and specific cases (i.e. Pb and Cd), atmospheric deposition of elements cannot be accurately estimated from the concentrations of metals and metalloids in moss tissues. However, the analysis of moss does provide information about the presence of contaminants in the atmosphere, their spatial and temporal patterns of distribution and how they are taken up by live organisms. Use of mosses is therefore recommended as a complementary (rather than an alternative) technique in the conventional analysis of bulk deposition of contaminants.