The Potential Role of Bryophytes as Biomonitors

Antarctic bryophytic vegetation is often dominated by small acrocarpus mosses belonging to the genus Bryum, which form flat, short turfs. These mosses have an ephemeral life strategy and produce vegetative propagules capable of rapid development on unstable and wet substrata; under dry conditions the moss turfs form plate crusts, which are readily detached and redistributed by winds. Owing to their short lifetime and scarce biomass interspersed with large amounts of soil particles, these mosses cannot be used to investigate the relative composition of dry, wet and occult atmospheric deposition. However, in sheltered and more stable substrata, which receive enough water from melting snow during summer, several moss species tend to have a persistent life strategy and may survive for many years. These mosses can form large stands (up to some square metres), and cushions may be up to several centimetres thick, sometimes showing a thin layer of peat. Some cosmopolitan or circumpolar species were first investigated in Sweden (Rühling and Tyler 1968), and there is evidence that these organisms can be very useful for large-scale biomonitoring surveys.

As a rule, surveys for trace metal deposition are based on the analysis of total element concentrations in unwashed mosses (cryptogam samples are not washed so as not to remove soluble intercellular elements; Brown 1984). The high surface-to-mass ratio of mosses provides a highly effective trap for airborne particles, and this property is very useful in studying long-range atmospheric transport and deposition of pollen and spores in Antarctica (e.g. Linskens et al. 1993). However, the same property complicates the interpretation of data from trace element biomonitoring surveys. In dry and barren Antarctic terrestrial ecosystems, "raw" concentrations of elements in moss samples often reflect the biogeochemical nature of soils and rocks rather than atmospheric input of elements. As a rule, levels of Al, Fe, Cr, Fe and other lithophilic elements are higher in Antarctic mosses than in samples of related species from forest ecosystems in the Northern Hemisphere. By comparing regional background values in the literature, Bargagli et al. (1995) found that concentrations of many elements increase from Amazonian or temperate rainforests to agricultural or barren and desert areas, independently of the adopted moss species or analytical procedure. The more evident exceptions were Cd and Hg (elements with a low crustal abundance) and sometimes Pb, which arises mostly from anthropogenic sources. For a more reliable comparison between the elemental composition of mosses from sites with different climatic and environmental conditions, and to establish reliable background concentrations, it was necessary to minimise the effects of soil contamination. This was done (e.g. Bargagli et al. 1995) by assuming that the lowest concentration of Al measured in Antarctic mosses (about 250 |g g-1 dry wt.) was the non-particulate fraction of the element. By ascribing the excess of Al above this value to entrapped soil particles, the amount of soil contaminating each sample was then evaluated (taking soil samples at the same site, and digesting and analysing them following the same procedure used for mosses). More realistic background concentrations of trace metals in Antarctic mosses were subsequently estimated by subtracting the amount attributed to entrapped soil particles from the total measured concentration. Table 7 reports total concentrations of trace elements in the Antarctic moss Bryum pseudotriquetrum from northern Victoria Land, together with estimated background concentrations (assuming 250 |g g-1 dry wt. of Al as a standard for the non-particulate fraction). Baseline total concentrations of trace elements in samples of Hylo-comium splendens from Alaska (Wiersma et al. 1986) and Greenland (Pile-gaard 1987) are reported for comparison. The amount of soil particles in mosses from Alaska were probably slightly higher than in Antarctic samples, while the average Fe concentration in Greenland samples seems to indicate a low element contribution from soils and rocks. If we bear in mind the different input of lithophilic elements, the data summarised in Table 7 show that concentrations of most trace elements in Antarctic mosses are in the same range as those measured in the Arctic. As for Antarctic lichens, the main difference is a significantly lower Pb content (the most widespread metal pollutant in the Northern Hemisphere). Concentrations of Hg and Cd in B. pseudotriquetrum are in the same range or slightly higher than those in mosses from control areas in Europe (Bargagli et al. 1998 c). Through a detailed study of the accumulation of these metals in Antarctic mosses growing along nutrient and moisture gradients in coastal ice-free areas, it was found that the marine environment is the main source of Cd. Concentrations of this metal and those of P increased in samples collected near a beach with a penguin rookery. Besides direct deposition through marine aerosol, it was found that seabird excrements increased the environmental bioavailability of Cd and P. On the contrary, Pb and Hg contributions from guano were negligible. The moss Campyloplus pyriformis growing on the warm fumarolic

Table 7. Total concentrations and estimated background concentrations ((_ig g 1 dry wt.) of trace elements in Antarctic mosses compared with data for mosses collected in other remote areas

Region

Species

Al

Ba

Cd

Cr

Cu

Fe

Hg

Mn

Ni

Pb

Zn

Northern Victoria Landa

Bryum pseudotriquetrum

5,100

138

0.10

8.2

12

3,980

0.12

268

5.5

1.9

68

Estimated background

250

47

0.09

2.3

7.1

205

0.12

134

2.5

1.1

35

Alaskab

Hylocomium splendens

4,300

200

20

15

4,900

530

9.0

6.8

65

Greenland0

Hylocomium splendens

-

-

0.15

3.8

-

1,450

0.09

-

-

6.8

47

a Data extracted from Bargagli et al. (1995) b Data extracted from Wiersma et al. (1986) c Data extracted from Pilegaard (1987)

ground of the Mount Melbourne crater had a rather high Hg concentration (1.52 |g g-1 dry wt.), thus sustaining the hypothesis that active volcanoes and fumaroles contribute to the enhanced accumulation of Hg in Victoria Land cryptogamic organisms.

As in the case of lichens, the average concentration of 137C in mosses from King George Island (23±14 Bq kg-1) was much higher than that in surface soils (Godoy et al. 1998). During summer, sublimating snow and high evaporation determine deposition of soluble salts on moss and lichen surfaces. Electrical conductivity measurements (Bargagli et al. 1999) in moss-supporting soils (protorankers) show that during summer salts tend to migrate and concentrate in mosses, which have a much higher CEC and evaporation surface than soil. In addition to major and trace elements, radionuclides may also be transferred from soil to mosses, as there is evidence of their mobilisation from radioactive particles deposited on soils (Salbu et al. 1998).

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