Definition and number of subsamples for using mosses as biomonitors of airborne trace elements.

We collected 50 subsamples of the moss Scleropodium purum from each of three sampling sites and determined the concentrations of As, Cd, Cu, K, Hg, Ni, Pb, Se, and Zn in each subsample. We then calculated the number of subsamples required to determine significant differences in the mean concentrations of two sampling sites. We found that to differentiate between an uncontaminated sampling site and another, slightly contaminated site, 30 subsamples are required from each. On the basis of these results and because, to date, there are no studies that justify the application of the previously proposed recommendations, studies of local variability in other areas, and under different conditions, with other contaminants and moss species must be undertaken. For posterior comparison of data on variability as well as application of the recommendations associated with such results, the general use of a single definition of a subsample is required. We propose the following definition of a subsample: "1 g dry weight (approx.), collected within a 25-cm radius of a node, selected at random from a 1 x 1-m sampling grid placed in a sampling site."