Trace-Element Enrichment in Epiphytic Lichens and Tree Bark at Pico Island, Azores, Portugal

Abstract

The Azores archipelago (nine islands) stretches across a sizable longitude span and sits directly on the pathway of long-range transport for airborne species over the North Atlantic Ocean. These airborne species include dusts from Africa’s Sahara and Sahel and non-natural contaminants from the United States’ eastern seaboard. Therefore, it is an ideal platform for watching significant deposition episodes in the area and/or simply keeping track of regular downward fluxes at ground level, either through conventional or unconventional (biological) air-monitoring approaches. For this purpose, thalli of the epiphytic lichens Parmotrema bangii (Vain.) Hale, and Parmotrema robustum (Degel) Hale; and outer bark from trunks of Cryptomeria japonica (Thunb. ex L.f.) D. Don—Japanese cedar—were collected in early 2003 at three sampling locations of Pico Island, Azores, Portugal. Lichens’ phorophyte was the cedar, and local soils were sampled as well. Following suitable handling and preparation procedures, all field samples were put through instrumental neutron activation analysis (INAA; k 0-variant) for elemental assessment. Reference data on airborne particulates were provided by a seven-wavelength aethalometer that continuously collects aerosols near the summit of Pico Mountain (observatory: 2225 m above mean sea level [AMSL]; peak: 2351 m AMSL) in quartz filter tapes. The overall results indicate that (1) the patterns of significant elemental enrichment (enrichment factor >10, relative to scandium) in lichen and bark samples are quite dissimilar, with lichen data reflecting more of the local inputs from some characteristic human activities, whereas bark appears predominantly enriched with far-ranged elements arriving in Pico’s atmosphere after long-distance transport; (2) regardless of prospective elemental sources—local or global, anthropogenic or natural—bark displays a more conservative (consistent) pattern through the sampling locations; and (3) not unlike earlier indications, altitude may have a negative influence in lichen enrichment, yet the present data seem insufficient to uphold such a trend.