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Trace element concentrations in epiphytic lichens and bark substrate
Environmental Pollution (Series B) 11 (1986) 153-160
Trace Element Concentrations in Epiphytic Lichens and Bark Substrate
M. de Bruin & E. H a c k e n i t z Interuniversity Reactor Institute, Mekelweg 15, 2629 JB Delft, The Netherlands
ABSTRA CT The relations were studied between concentrations of 20 trace elements in epiphytic lichens and in the substrate (bark). The bark was separated into two fractions: a 2 mm thick outer layer and the inner layer, not exposed directly to the atmosphere. For most elements a significant correlation was found between the concentrations in the lichen and the concentrations in the outer and inner bark. In general the element concentrations in the inner bark were appreciably lower than those in the outer bark and lichen. For some elements, including Zn and Cd, the concentrations in the inner bark were relatively high, indicating that for those elements the possibility o f uptake from the substrate has to be seriously considered.
INTRODUCTION Lichens are regularly used as monitors for air pollution. The disappearance of certain species is indicative of the SO 2 concentration in the air. The heavy metal concentrations in the lichens are used as an indication of the concentrations of such elements in air and precipitation (Jenkins & Davies, 1966; Nieboer et al., 1972; Saeki et al., 1977; Kauppi, 1980), on the assumption that the elements accumulated by the lichen mainly originate from air and precipitation, and only to a minor extent from the substrate (Manning & Feder, 1980; Rasmussen et al., 1980). However, it is questionable whether this assumption is always valid. For 153 Environ. Pollut. Ser. B. 0143-148X/86/$03.50© ElsevierApplied SciencePublishersLtd, England, 1986. Printed in Great Britain
154
M. de Bruin, E. Hackenitz
terricolous lichens growing on substrates rich in minerals such an uptake cannot a priori be excluded (Seaward, 1980; Goyal & Seaward, 1981; Hanssen et al., 1981). For epiphytic lichens it is less likely to occur because of the relatively low element concentrations present in most plant materials (Rasmussen, 1976). Only when the concentrations in the substrate are extremely high, e.g. due to uptake from heavily polluted soil, can an appreciable contribution from the substrate be expected. Uptake from the substrate bark by epiphytic lichens may take place directly through the rhizines or indirectly from the water flowing along the stem. If elements in this water originate from deposition from the atmosphere on to the bark, their accumulation in the lichen is still related to the atmospheric pollution to be monitored. However, if the elements were already incorporated in the bark material during bark formation and become available on weathering of the bark surface, the concentrations of these elements in the lichen may reflect the soil pollution history rather than the actual air pollution. To evaluate the possibility of uptake from the supporting tree, we have compared the element concentrations in epiphytic lichens with the concentrations in the associated substrate bark. The samples originated from a region known to be polluted by heavy metals. As a comparison of lichen and total bark may lead to inconclusive results (Laaksovirta et al., 1976), the bark was separated into two fractions. The first fraction consisted of the weathered outer layer in which leaching or accumulation of elements due to interaction with the atmospheric environment may occur. The second fraction comprised the lower-lying bark layer, in which the element concentrations will be due primarily to deposition from inside during growth. This study was carried out as part of a heavy metal monitoring programme with biological indicators, currently being carried out in an area along the southern border of The Netherlands. EXPERIMENTAL DETAILS Lichen species used in the study The Netherlands, and especially the area studied, are only sparsely populated by lichens, due to the high degree of environmental pollution. From the few species still found in the area, Parmelia sulcata was selected for these experiments. This lichen is leaf-shaped and can be separated very well from the substrate bark.
Uptake of metals by lichens
155
Sampling The lichen samples were collected in 'De Kempen', a region in The Netherlands close to the Belgian border. The area has been polluted for more than a century by the emissions of Cd and Zn from zinc smelters located in the area. These smelters recently reduced heavy metal emissions, but the Cd and Zn levels are still elevated due to direct emission from the smelters or secondary emission from, for example, piles of Zn ash. The lichens were sampled together with the substrate bark, at a height of about 1.5m above the ground, from different types of trees: oak. poplar, linden, plane tree, willow and birch.
Sample preparation After separation of the lichens from the substrate bark, the remaining bark was separated into two parts: the 2 - 3 m m thick dark-browncoloured and weathered outer layer and the light-brown-coloured inner layer. The samples thus obtained were shaken for a short time with distilled water to remove adhering dust, freeze-dried and ground in a small agate ball mill.
Analysis The samples were analysed by instrumental neutron activation analysis, following the procedure routinely used at IRI (De Bruin et al., 1982). Samples of 10 100mg, depending on the amount of material available, were weighed into small polyethylene capsules and irradiated twice, for 1 min and 1 h respectively, in the Hoger Onderwijs Reactor at Delft at a thermal neutron flux of ~b = 1 x 1013 neutronscm -2 s-1. After the first irradiation, the samples were measured for 5 min after a decay period of 10min. The second irradiation of the samples was followed by two measurements of 1 h after decay times of 5 and 20 days respectively. The gamma-ray spectra of the activated samples were measured with automated spectrometers with sample changers and normal coaxial or well-type Ge(Li)-detectors, depending on the sample activity. The gamma-ray spectra were processed automatically giving a direct yield of the element concentrations. In lichen samples, about 40 elements were detected and quantified in the routine procedure.
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0
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~-'~ -o6
~_
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Uptake of metals by lichens
157
RESULTS AND DISCUSSION Table 1 gives the averages and ranges of the concentrations of 20 elements in the lichens (outer and inner bark), together with the correlation coefficients for the concentrations in lichen and outer bark, lichen and inner bark, and outer bark and inner bark. If significant correlations were found, the relations between the element concentrations in lichen and outer bark and lichen and inner bark were calculated, assuming a proportional relationship between the concentrations in lichen (Ct) and in bark (Cb); C t = A . C b. The values obtained for A are listed in Table 2. The results clearly show that the concentrations in the lichens and in the outer bark are very similar: the average concentrations are very close and a significant correlation is observed for 14 elements out of the 20 studied. This is not surprising as both materials are exposed to the atmosphere. Comparison of the element concentrations in lichens and in inner bark yields a different picture. The correlations are in general appreciably TABLE 2
Ratio A Between Element Concentrations in Lichen and in Outer and Inner Bark Element
Ratio Outer bark
K Ca Sc V Cr Mn Fe Co Cu Zn Cd Sb Ba La Hf W
Inner bark
3"3 0.15 21 1.0 1.5 1.1 1.5 0.99 0.61 0.73 0.55 0-69 1.9
1.3 1.6 1.4
2"9 1.4 17 1.3 0.71 5.2 4.4
.--..
6. E.E C O
(a)
Arsenic
t..
IO 0 tO ¢,.)
"E E 5
•
o oak
•
o poplar
•
o linden
•
= plane-tree
•
v willow
•
0 birch
El O
0
O
m 0 0
V
o|
I
I
5
I0
Lichen
element
•
con centration(mg
kg - I )
V 0 !
Cn
(b)
Cadmium
e-
•
.9 I0
o
o
"E 13
0t-0 0 O
E~P
• 0
E o
|
133
o~
•
I
I
I0 Lichen element concentration(mg 5
kg - I )
Fig. 1. Relation between concentrations of arsenic (a) and cadmium (b) in Parmelia sulcata and in the outer and inner parts of the substrate bark (open symbols, outer bark; solid symbols, inner bark).
Uptake of metals by lichens
159
weaker, and for most of the elements the concentrations are three or more times lower than the concentrations in the lichen. Only five elements--Ca, Mn, Zn, Cd and Ba--show inner bark concentrations comparable to, or even higher than, their concentrations in outer bark and lichen. Figures 1 (a) and 1 (b) show the relationship between the concentrations in lichen and outer bark and between lichen and inner bark for the elements As and Cd, as examples of each of the two groups of elements distinguished. For the first group of elements, with very low element concentrations in the inner bark, it is unlikely that the substrate contributes significantly to the element concentrations in the lichen. Therefore it can be concluded that, in the area studied, the concentrations of these elements indeed reflect the actual air pollution. The situation is less clear-cut for the second group. The inner bark concentrations are relatively high and significant correlations exist between the concentrations in inner bark and lichen. Therefore, it cannot be excluded a priori that these elements have at least in part reached the lichen through the supporting tree. This implies that the element pattern observed in the lichen may be the result of both direct uptake from the atmosphere and indirect uptake from the atmosphere or the soil through leaves or root system. The strong correlations found between the element concentrations in the outer and the inner bark may be the result of insutficient separation between the weathered and the unweathered bark layers. More detailed information on the depth distribution of elements in the bark is required before firm conclusions can be drawn from the observed correlations.
CONCLUSIONS The results presented show that, for most of the elements studied, uptake from the substrate bark into the lichen is not likely to occur. But for Ca, Mn, Zn, Cd and Ba, the possibility of such uptake has to be considered seriously in the area under study. For these elements, high soil concentrations or long-term air pollution may be indirectly reflected in the lichens, thereby obscuring the direct uptake from the atmosphere. This possible interference should be taken into account when using trace element concentrations in epiphytic lichens as a measure for heavy metal air pollution. Thus, measurement of the depth distribution of elements in the substrate bark may provide useful information. The risk of interference by uptake from the substrate can largely be reduced by
160
M. de Bruin, E. Hackenitz
using transplants consisting of pieces of bark with lichens from clear areas.
REFERENCES De Bruin, M., Korthoven, P. & Bode, P. (1982). Evaluation of a system for routine instrumental neutron activation ana~lysis. J. Radioanalyt. Chem., 70, 497.-512. Goyal, P. & Seaward, M. R. D. (1981). Metal uptake in terricolous lichens, I. New Phytol., 89, 631-45. Hanssen, J. E., Ramback, J. P., Semb, A. & Steinnes, E. (1981). Atmospheric deposition of some heavy metals in Norway. Proceedings of the International Conference on Heavy Metals in the Environment, 322-4. Edinburgh, SEP Consultants. Jenkins, D. A. & Davies, R. I. (1966). Trace element content of organic accumulations. Nature, Lond., 210, 1296-7. Kauppi, M. (1980). Methodological contributions on the use of lichens as a tool for studying air pollution. Acta Universitatis Ouluensis A, 104. Laaksovirta, K., Olkkonen, H. & Alakuijala, P. (1976). Observations on the lead content of lichen and bark adjacent to a highway in Southern Finland. Environ. Pollut., 11,247-55. Manning, W. J. & Feder, W. A. (1980). Biomonitoring air pollutants with plants. London, Applied Science Publishers. Nieboer, E., Ahmed, H. M., Puckett, K. J. & Richardson, D. H. S. (1972). Heavy metal content of lichens in relation to distance from a nickel smelter in Sudbury, Ontario. Lichenologist, 5, 292-304. Rasmussen, L. (1976). Uptake of minerals, particularly metals, by epiphytic Hypnum cupressiforme. Oikos, 27, 483 7. Rasmussen, L., Pilegaard, K. & Gydesen, H. (1980). The application of cryptogams as monitoring organisms of metal air pollution in Denmark. Bot. Tidsskrift, 75, 93-9. Saeki, M., Kunii, K., Seki, T., Sugiyama, K., Suzuki, T. & Shishido, S. (1977). Metal burden of urban lichens. Environ. Res., 13, 256--66. Seaward, M. R. D. (1980). The use and abuse of heavy metal bioassays of lichens for environmental monitoring. Proceedings of International Conference on Bioindicators, 3rd, ed. by J. Spal6ny, 375-84. Prague, Academia.
Trace Element Concentrations in Epiphytic Lichens and Bark Substrate
M. de Bruin & E. H a c k e n i t z Interuniversity Reactor Institute, Mekelweg 15, 2629 JB Delft, The Netherlands
ABSTRA CT The relations were studied between concentrations of 20 trace elements in epiphytic lichens and in the substrate (bark). The bark was separated into two fractions: a 2 mm thick outer layer and the inner layer, not exposed directly to the atmosphere. For most elements a significant correlation was found between the concentrations in the lichen and the concentrations in the outer and inner bark. In general the element concentrations in the inner bark were appreciably lower than those in the outer bark and lichen. For some elements, including Zn and Cd, the concentrations in the inner bark were relatively high, indicating that for those elements the possibility o f uptake from the substrate has to be seriously considered.
INTRODUCTION Lichens are regularly used as monitors for air pollution. The disappearance of certain species is indicative of the SO 2 concentration in the air. The heavy metal concentrations in the lichens are used as an indication of the concentrations of such elements in air and precipitation (Jenkins & Davies, 1966; Nieboer et al., 1972; Saeki et al., 1977; Kauppi, 1980), on the assumption that the elements accumulated by the lichen mainly originate from air and precipitation, and only to a minor extent from the substrate (Manning & Feder, 1980; Rasmussen et al., 1980). However, it is questionable whether this assumption is always valid. For 153 Environ. Pollut. Ser. B. 0143-148X/86/$03.50© ElsevierApplied SciencePublishersLtd, England, 1986. Printed in Great Britain
154
M. de Bruin, E. Hackenitz
terricolous lichens growing on substrates rich in minerals such an uptake cannot a priori be excluded (Seaward, 1980; Goyal & Seaward, 1981; Hanssen et al., 1981). For epiphytic lichens it is less likely to occur because of the relatively low element concentrations present in most plant materials (Rasmussen, 1976). Only when the concentrations in the substrate are extremely high, e.g. due to uptake from heavily polluted soil, can an appreciable contribution from the substrate be expected. Uptake from the substrate bark by epiphytic lichens may take place directly through the rhizines or indirectly from the water flowing along the stem. If elements in this water originate from deposition from the atmosphere on to the bark, their accumulation in the lichen is still related to the atmospheric pollution to be monitored. However, if the elements were already incorporated in the bark material during bark formation and become available on weathering of the bark surface, the concentrations of these elements in the lichen may reflect the soil pollution history rather than the actual air pollution. To evaluate the possibility of uptake from the supporting tree, we have compared the element concentrations in epiphytic lichens with the concentrations in the associated substrate bark. The samples originated from a region known to be polluted by heavy metals. As a comparison of lichen and total bark may lead to inconclusive results (Laaksovirta et al., 1976), the bark was separated into two fractions. The first fraction consisted of the weathered outer layer in which leaching or accumulation of elements due to interaction with the atmospheric environment may occur. The second fraction comprised the lower-lying bark layer, in which the element concentrations will be due primarily to deposition from inside during growth. This study was carried out as part of a heavy metal monitoring programme with biological indicators, currently being carried out in an area along the southern border of The Netherlands. EXPERIMENTAL DETAILS Lichen species used in the study The Netherlands, and especially the area studied, are only sparsely populated by lichens, due to the high degree of environmental pollution. From the few species still found in the area, Parmelia sulcata was selected for these experiments. This lichen is leaf-shaped and can be separated very well from the substrate bark.
Uptake of metals by lichens
155
Sampling The lichen samples were collected in 'De Kempen', a region in The Netherlands close to the Belgian border. The area has been polluted for more than a century by the emissions of Cd and Zn from zinc smelters located in the area. These smelters recently reduced heavy metal emissions, but the Cd and Zn levels are still elevated due to direct emission from the smelters or secondary emission from, for example, piles of Zn ash. The lichens were sampled together with the substrate bark, at a height of about 1.5m above the ground, from different types of trees: oak. poplar, linden, plane tree, willow and birch.
Sample preparation After separation of the lichens from the substrate bark, the remaining bark was separated into two parts: the 2 - 3 m m thick dark-browncoloured and weathered outer layer and the light-brown-coloured inner layer. The samples thus obtained were shaken for a short time with distilled water to remove adhering dust, freeze-dried and ground in a small agate ball mill.
Analysis The samples were analysed by instrumental neutron activation analysis, following the procedure routinely used at IRI (De Bruin et al., 1982). Samples of 10 100mg, depending on the amount of material available, were weighed into small polyethylene capsules and irradiated twice, for 1 min and 1 h respectively, in the Hoger Onderwijs Reactor at Delft at a thermal neutron flux of ~b = 1 x 1013 neutronscm -2 s-1. After the first irradiation, the samples were measured for 5 min after a decay period of 10min. The second irradiation of the samples was followed by two measurements of 1 h after decay times of 5 and 20 days respectively. The gamma-ray spectra of the activated samples were measured with automated spectrometers with sample changers and normal coaxial or well-type Ge(Li)-detectors, depending on the sample activity. The gamma-ray spectra were processed automatically giving a direct yield of the element concentrations. In lichen samples, about 40 elements were detected and quantified in the routine procedure.
66666666666666666666 I
I
I
I
I ,.Q o 0 0 0 0
0
vv
~-'~ -o6
~_
~ 0 ~ o = o
v
v
0
vv 0
0
0 o o o o o
N ~ 0
0
0 0 0 0
~
0
0
~ o~
o,5
Uptake of metals by lichens
157
RESULTS AND DISCUSSION Table 1 gives the averages and ranges of the concentrations of 20 elements in the lichens (outer and inner bark), together with the correlation coefficients for the concentrations in lichen and outer bark, lichen and inner bark, and outer bark and inner bark. If significant correlations were found, the relations between the element concentrations in lichen and outer bark and lichen and inner bark were calculated, assuming a proportional relationship between the concentrations in lichen (Ct) and in bark (Cb); C t = A . C b. The values obtained for A are listed in Table 2. The results clearly show that the concentrations in the lichens and in the outer bark are very similar: the average concentrations are very close and a significant correlation is observed for 14 elements out of the 20 studied. This is not surprising as both materials are exposed to the atmosphere. Comparison of the element concentrations in lichens and in inner bark yields a different picture. The correlations are in general appreciably TABLE 2
Ratio A Between Element Concentrations in Lichen and in Outer and Inner Bark Element
Ratio Outer bark
K Ca Sc V Cr Mn Fe Co Cu Zn Cd Sb Ba La Hf W
Inner bark
3"3 0.15 21 1.0 1.5 1.1 1.5 0.99 0.61 0.73 0.55 0-69 1.9
1.3 1.6 1.4
2"9 1.4 17 1.3 0.71 5.2 4.4
.--..
6. E.E C O
(a)
Arsenic
t..
IO 0 tO ¢,.)
"E E 5
•
o oak
•
o poplar
•
o linden
•
= plane-tree
•
v willow
•
0 birch
El O
0
O
m 0 0
V
o|
I
I
5
I0
Lichen
element
•
con centration(mg
kg - I )
V 0 !
Cn
(b)
Cadmium
e-
•
.9 I0
o
o
"E 13
0t-0 0 O
E~P
• 0
E o
|
133
o~
•
I
I
I0 Lichen element concentration(mg 5
kg - I )
Fig. 1. Relation between concentrations of arsenic (a) and cadmium (b) in Parmelia sulcata and in the outer and inner parts of the substrate bark (open symbols, outer bark; solid symbols, inner bark).
Uptake of metals by lichens
159
weaker, and for most of the elements the concentrations are three or more times lower than the concentrations in the lichen. Only five elements--Ca, Mn, Zn, Cd and Ba--show inner bark concentrations comparable to, or even higher than, their concentrations in outer bark and lichen. Figures 1 (a) and 1 (b) show the relationship between the concentrations in lichen and outer bark and between lichen and inner bark for the elements As and Cd, as examples of each of the two groups of elements distinguished. For the first group of elements, with very low element concentrations in the inner bark, it is unlikely that the substrate contributes significantly to the element concentrations in the lichen. Therefore it can be concluded that, in the area studied, the concentrations of these elements indeed reflect the actual air pollution. The situation is less clear-cut for the second group. The inner bark concentrations are relatively high and significant correlations exist between the concentrations in inner bark and lichen. Therefore, it cannot be excluded a priori that these elements have at least in part reached the lichen through the supporting tree. This implies that the element pattern observed in the lichen may be the result of both direct uptake from the atmosphere and indirect uptake from the atmosphere or the soil through leaves or root system. The strong correlations found between the element concentrations in the outer and the inner bark may be the result of insutficient separation between the weathered and the unweathered bark layers. More detailed information on the depth distribution of elements in the bark is required before firm conclusions can be drawn from the observed correlations.
CONCLUSIONS The results presented show that, for most of the elements studied, uptake from the substrate bark into the lichen is not likely to occur. But for Ca, Mn, Zn, Cd and Ba, the possibility of such uptake has to be considered seriously in the area under study. For these elements, high soil concentrations or long-term air pollution may be indirectly reflected in the lichens, thereby obscuring the direct uptake from the atmosphere. This possible interference should be taken into account when using trace element concentrations in epiphytic lichens as a measure for heavy metal air pollution. Thus, measurement of the depth distribution of elements in the substrate bark may provide useful information. The risk of interference by uptake from the substrate can largely be reduced by
160
M. de Bruin, E. Hackenitz
using transplants consisting of pieces of bark with lichens from clear areas.
REFERENCES De Bruin, M., Korthoven, P. & Bode, P. (1982). Evaluation of a system for routine instrumental neutron activation ana~lysis. J. Radioanalyt. Chem., 70, 497.-512. Goyal, P. & Seaward, M. R. D. (1981). Metal uptake in terricolous lichens, I. New Phytol., 89, 631-45. Hanssen, J. E., Ramback, J. P., Semb, A. & Steinnes, E. (1981). Atmospheric deposition of some heavy metals in Norway. Proceedings of the International Conference on Heavy Metals in the Environment, 322-4. Edinburgh, SEP Consultants. Jenkins, D. A. & Davies, R. I. (1966). Trace element content of organic accumulations. Nature, Lond., 210, 1296-7. Kauppi, M. (1980). Methodological contributions on the use of lichens as a tool for studying air pollution. Acta Universitatis Ouluensis A, 104. Laaksovirta, K., Olkkonen, H. & Alakuijala, P. (1976). Observations on the lead content of lichen and bark adjacent to a highway in Southern Finland. Environ. Pollut., 11,247-55. Manning, W. J. & Feder, W. A. (1980). Biomonitoring air pollutants with plants. London, Applied Science Publishers. Nieboer, E., Ahmed, H. M., Puckett, K. J. & Richardson, D. H. S. (1972). Heavy metal content of lichens in relation to distance from a nickel smelter in Sudbury, Ontario. Lichenologist, 5, 292-304. Rasmussen, L. (1976). Uptake of minerals, particularly metals, by epiphytic Hypnum cupressiforme. Oikos, 27, 483 7. Rasmussen, L., Pilegaard, K. & Gydesen, H. (1980). The application of cryptogams as monitoring organisms of metal air pollution in Denmark. Bot. Tidsskrift, 75, 93-9. Saeki, M., Kunii, K., Seki, T., Sugiyama, K., Suzuki, T. & Shishido, S. (1977). Metal burden of urban lichens. Environ. Res., 13, 256--66. Seaward, M. R. D. (1980). The use and abuse of heavy metal bioassays of lichens for environmental monitoring. Proceedings of International Conference on Bioindicators, 3rd, ed. by J. Spal6ny, 375-84. Prague, Academia.