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Physiological and Statistical Methods to Identify Background Levels of Metals in Aquatic Bryophytes: Dependence on Lithology

Ecología, Facultad de Biología, Universidad de Santiago de Compostela, 15706 Santiago de Compostela, Spain;

J. López^*

Ecología, Facultad de Ciencias, Universidad de Vigo, 36200 Vigo, Pontevedra, Spain.

^* Corresponding author (jlopez{at}uvigo.es).

ABSTRACT

We determined levels of Cd, Cr, Cu, Co, Ni, Pb, Zn, Fe, and Mn in aquatic bryophytes [Fontinalis antipyretica Hedw., Fissidens polyphyllus Wils., Brachythecium rivulare Schimp., Rhynchostegium riparioides (Hedw.) Card., and Scapania undulata (L.) Dum.] sampled from 247 sites on 36 rivers in Galicia (northwest Spain). We then used four alternative methods to estimate background levels of these metals in Fontinalis antipyretica and Fissidens polyphyllus. One method involves prior identification of putatively undisturbed sites on the basis of a physiological 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-bryophyte pair. Considerable differences were found between the outcomes of the four methods. We favor the physiological stress criterion, since 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 riparioides had the highest background levels, significantly exceeding those of Fontinalis antipyretica and Fissidens polyphyllus for almost all metals. The influence of substrate lithology on background levels was clearly significant for some metals in Scapania undulata and Fissidens polyphyllus.