PHYSIOLOGICAL AND STATISTICAL-METHODS TO IDENTIFY BACKGROUND LEVELS OF METALS IN AQUATIC BRYOPHYTES - DEPENDENCE AN LITHOLOGY

We determined levels of Cd, Cr, Cu, Co, Ni, Pb, Zn, Fe, and Mn in aqua tic bryophytes [Fontinalis antipyretica Hedw., Fissidens polyphyllus W ils., Brachythecium rivulare Schimp., Rhynchostegium riparioides (Hedw .) Card., and Scapania undulata (L.) Dum.] sampled from 247 sites on 3 6 rivers in Galicia (northwest Spain). We then used four alternative m ethods to estimate background levels of these metals in Fontinalis ant ipyretica and Fissidens polyphyllus. One method involves prior identif ication of putatively undisturbed sites on the basis of a physiologica l stress criterion (the D665/D665a pigment index) applied to all five bryophyte species; the other three methods use statistical analyses of the metal level data to identify undisturbed sites for each metal-bry ophyte pair. Considerable differences were found between the outcomes of the four methods. We favor the physiological stress criterion, sinc e it requires no assumptions about natural variability, and therefore used this method to estimate background levels for each metal in each of the five bryophytes. Scapania undulata and Rhynchostegium riparioid es had the highest background levels, significantly exceeding those of Fontinalis antipyretica and Fissidens polyphyllus for almost all meta ls. The influence of substrate lithology on background levels was clea rly significant for some metals in Scapania undulata and Fissidens pol yphyllus.