The Chernobyl accident : Can lichens be used to characterize a radiocesium contaminated range ?

Many of the lichen species that are important in the lichen dominated plant communities in the Norwegian mountains are important reindeer winter forage. They are also organisms that collect fall-out from the atmosphere. The Chernobyl accident brought, among other, radioactive Cesium, and from lichens this follow the food chain to reindeer and finally man. From region to region this fall-out was unevenly distributed and methods are needed to compare winter ranges and to monitor the developement of radioactive levels in the lichen carpet. Cornicularia divergens, Alectoria ochroleuca, Cetraria nivalis, Cladina mitis, C. stellaris and Stereocaulon pa¬schale was collected in the Dovre mountains to compare species levels and to study collection methods. We found that from spot to spot there is a very large variation between samples, even within the same species. Because of this we are not able to show significant species differences. We found, however, that species from more or less snow free ridgetops, Cornicularia divergens, Alectoria ochroleuca, Cetraria nivalis and Cladina mitis showed less variation and thus must be recommended as the best species for monitoring and comparison of ranges. Tsjernobyl-ulykken: Kan lav nyttes til karakterisering av et radioaktivt forurenset reinbeite? Abstract in Norwegian / Sammendrag: Mange av de lavarter som er viktige i lavdominerte plantesamfunn i de norske fjell er viktige som vinterfor for rein. Disse organismer samler ogsa nedfall fra atmosfaeren. Ulykken i Tsjernobyl brakte, blant annet, radioaktivt cesium, og fra lav folger dette naeringskjeden til rein og endelig mennesket. Fra omrade til omrade var dette nedfallet ujevnt fordelt, og det kreves metoder for a sammenligne vinteromrader og for a overvake utviklingen av det radioaktive niva i lavmattene. Cornicularia divergens, Alectoria ochroleuca, Cetraria nivalis, Cladina mitis, C. stellaris og Stereocaulon paschale ble samlet pa Dovrefjell for a sammenligne artsnivaene og for a studere innsamlings-prosedyrer. Vi fant at det fra sted til sted var en svaert stor variasjon, til og med innen samme art. Pa grunn av dette var vi ikke i stand til a vise statistisk sikre forskjeller mellom artene. Men vi fant at arter fra mer eller mindre snofrie rabbetopper, Cornicularia divergens, Alectoria ochroleuca, Cetraria nivalis og Cladina mitis viste mindre variasjon og derfor ma anbefales som de beste arter for overvakning og sammenligning av omrader.

This means that about 2 500 reindeer owners (Saami) and 23 000 hunters may face a disaster: no one wants to eat the meat they harvest.
Variation in radiocesium consentrations in reindeer from different ranges is thought to relate to «hot spot» areas in Scandinavia caused by uneven distribution of rainfall in the period 28.April to 5. May 1986 (Saltbones 1986).In addition, a large area received dry fallout, but this was less contaminated.Hvinden and Lillegraven (1961) first drew attention to risk of contamination with radioactive Cesium and Strontium passed along a food chain from lichens to reindeer to man.Lichen collect and store radionuclids in the ecosystem.These organisms absorb water through their whole surface, and store the dissolved mineral ions with great efficiency (Tuominen and Jaakkola 1973, Kershaw 1985, Ellis 1987).They do this mainly by strongfield ionic exchange in the fungal cell wall, and not by incorporating them as part of the living cell as plants with roots do.In addition it is important to note that particles in non-ionic form also will be sucked into the thallus, provided they are small enough, 1-2 fim.Different species of lichens may differ in this respect.Knowledge about accumulation and retention of Cesium in the alpine ecosystem is badly needed, and a radioecological research program was started in May 1986.We need methods to compare the burden of contamination in different winter ranges.Forage lichen have been collected for this before appropriate sampling procedures were developed.Results reported here show how sampling may be done to minimize uncertainty due to stochastic and systematic variation.

Methods
We have two set of samples taken to study the variation in radiocesium content between and within species from different habitats.The first comes from lichens put out originally for growing experiments (Gaare 1986a).We have two study sites: one in the western part of the Dovre mountain range at 62°22,5' n.L, 8°33' e.l., the Aursj0 growth field, and another further east on the Dovre plateau proper at 62°16' n.l., 9°36' e.l., the Gr0nbakken growth field.The western site has 800 mm of precipitation per annum, the eastern site has 400 mm.The difference falls mainly as snow in the winter period.
The lichens were set out in 1979 in perforated plastic trays placed tight together covering 1.7 sq.m under what is considered equal environmental conditions.Two species is studied viz.Cladina stellaris and Cetraria nivalis, in 8 to 9 replicates from each site.The plants were taken in for measurement of radioactivity in October, 1986.The second set of samples were collected in September 1987, from a location 200 m from the eastern study field.We sampled the following species, listed according to their increasing requirement for snow protection: Cornicularia diver gens y Alectoria ochroleuca y Cetraria nivalis, Cladina mitis, C. stellaris and Stereocaulon pa¬ schale.One sample consisted of 10-15 subsamples picked from an area of 1 ha (10000 sq.m).This is a sampling procedure practiced in connection with comparison of reindeer winter pastures.We took 5 replicate samples for each species so that a variance and standard deviation could be calculated.
In the laboratory the samples were sorted and the lichen thalli was divided in an upper living part and a lower rotting part.Samples were dryed and weighed at 70°C before they were sent to The Norwegian Technical University where radioactivity was measured with a Canberra multi-channel reading system with a GeLi-detector.Only the readings of the living part are shown here.Results for Cs-134 and Cs-137 have been combined.

Results and discussion
The consentration of Cs-134 and Cs 137 in lichens in the Dovre mountains, Norway increases with altitude and with increasing wind exposure (Gaare 1986b).Species growing on the ridge top viza.Alectoria ochroleuca and Cetraria nivalis, showed larger values than those dominating the lee sides, e.g.Cladina mitis, C. stellaris and Stereocaulon paschale.
In contrast to fallout from nuclear bomb tests, the fallout from the Chernobyl accident varied considerably not only from contry to contry, but also over very short distances -a meter or so.Paired samples of Cetraria nivalis collected less the 1 m apart in the Dovre mountains in August 1986, for example, ranged from 4700 to 18200 Bq/kg.The range in similarly paired samples of Cladina stellaris was from 6900 and 66000 Bq/kg.At first we suspected a specific variation in the absorption of Cesium, but the species we studied had small, but important differences in their habitat ecology.Alectoria ochroleuca and Cetraria nivalis dominate the top of ridges that have nil or very shallow snow cover in winter.Cladina species dominates on slightly more protected places, and Stereocaulon often covers depressions which may become flooded in the thaw in spring.There may also be differences in moisture regimes in the snow free months.Ridgetops lichens dry out much earlier than those growing in depressions.Small topographic irregularities is all that is needed to create large differences in moist periods for poikilohydric organisms like lichens or mosses.So far it seemed that variation of radioactivity might have been due to habitat rather than species differences.I concluded that the physical reason for differences in the radiocesium content in these lichens was mostly due to a different moisture content at the time when the fallout from Chernobyl came.A dry sponge may absorb more water and contamination than a wet one.And lichens (and many mosses) work very much like sponges in this respect.
Lichens with the highest consentrations of ra-dionuclids came from habitats judged to be the dry est.The main differences which I observed are thus partly explained by the sponge hypothezis.However, there are exceptions and there must be other sources of variation besides conditions on the ground.
Table 1 shows radiocesium content of Cetraria nivalis and Cladina stellaris grown under near identical environmental conditions from each of the two experimental sites Gr0nbakken in the east and Aursj0 in the west.We see that even in samples of lichens of the same species from the same 1.7 sq.m the variation have a standard deviation of 64 and 83 percent of the mean (100 * coefficient of variation) at the Gr0nbakken site and 18 and 39 at the Aursj0 site.Based on a simple t-test (Tab 1 B) between pairs of species and sites no significant differences exist among these samples, the number of samples is too small.Meat from reindeers harvested in the western part of the range, near the Aursj0 growth field, shows significantly lower values than those from Gr0nbakken (Skogland 1987a).The sampling undertaken to study variation between species is shown in table 2. Each sample was made up of 10 to 15 subsamples picked Weather conditions may explain this.When contaminated rain fell on 28.April, 1986 the mountains were partly snow covered.According to information from a nearby meteorological observation site, it is very likely that the former species was snow free whereas the latter two was partly covered.This would add to the heterogenous dryness thought to explain the uneven distritubion of contamination shown for non covered species (Gaare 1986b and1987).Data from Table 1 appear to contradict this.The coefficient of variation in samples from the eastern site is larger than in samples from the western one.But there experimental field in the west was covered by snow, while the eastern one was snow-free.There it might, therefore, have been differences in thallus moisture due to minor variations in wind and sun exposure.Details known about the weather in April 1986, prior to the accident, support this (Gaare 1987).
From the analyses we have done, we conclude that lichens growing on the ground may be used to compare the load of radiocesium on ranges from the Chernobyl accident.The best results is to be expected if one uses species growing on ridgetops avoiding prolonged snowcower, viz.Cornicularia divergens, Alectoria ochroleuca, Cetraria nivalis, Cladina mitis.Cladina stellaris and Stereocaulon paschale are less suitable.In order to obtain reliable results, the sample must be composed of not less than 10-15, preferably more, subsamples picked from a large area.It is doubtful whether this is precise enough to allow determin biological halflives of radioactive contamination in the species we study.

Table 1 .
A. Radiocesium content (Cs-134 plus Cs-137) of lichens: Identical conditions.Samples grown under identical environmental conditions at the Aursjø (W) and Grønbakkien (E) growth field.Readings in Bq/kg dry weight of the upper 3 cm (living part) of the thallus.