Investigation of selected natural and anthropogenic radionuclides in reindeer (Rangifer tarandus tarandus) and lynx (Lynx lynx)

Abstract

The present thesis investigates a range of dietary, physiological and environmental factors influencing transfer and long-term behaviour of radionuclides in reindeer and lynx. Feeding experiments were designed to provide details on factors related to absorption, retention and secretion of radionuclides in reindeer, while concentrations and time trends of radionuclides in the environment were studied in two reindeer herding districts during the period 2000 – 2003. Data on 137Cs in reindeer from 1986 onwards and samples of lynx were obtained from national archives.

In the first feeding experiment, reindeer calves were offered either combined diets of fallout contaminated lichens and pelleted concentrates applicable for clean feeding, or a pure lichen diet. Constant daily radiocaesium (134Cs and 137Cs) administration rates were used throughout the experiments. Due to lower dietary potassium and crude fibre intake, reindeer on the pure lichen diet had significantly lower excretion of radiocaesium and a 40 % longer biological half-time for radiocaesium in red blood cells than reindeer fed a diet with a higher mineral element content. The study showed that the bioavailability of Chernobyl radiocaesium in lichen was 35 % of that of 134Cs as chloride in aqueous solution. The results were in agreement with previous studies demonstrating the effect of seasonal variations in potassium intake on radiocaesium retention in reindeer. A literature review of the many factors influencing transfer of radiocaesium to ruminants suggested that additional seasonally affected factors need to be taken into account in studies of reindeer, particularly the effects of digestibility and metabolic rates on absorption and endogenous faecal excretion of radiocaesium.

In the second feeding experiment, four pregnant reindeer on a concentrate diet were given daily constant quantities of 134Cs and 85Sr as chlorides in aqueous solution during the last part of gestation. The experiment showed that similar fractions of the administered activities of 134Cs and 85Sr were transferred to the foetus, and 1.4 – 2.5 % of the total administered activities were deposited in the calves at birth. The distribution of the nuclides in different tissues of newborn calves was comparable to that reported in older calves. The transfer coefficients (Fm) for 134Cs and 85Sr from diet to reindeer milk were as expected from extrapolated Fm values observed in other ruminants, with Fm for 134Cs 8.5 times higher that of 85Sr. The influence of the mineral element intake, particularly K and Ca, on absorption of Cs and Sr in reindeer, suggests that transfer of the nuclides to foetus and milk of free-ranging reindeer may be considerable higher than observed in this experiment. Furthermore, secretion of Sr accumulated prior to the lactation period will probably cause higher Sr concentration in milk of free-ranging reindeer than indicated by the Fm estimated in this experiment. No differences in halftimes for 134Cs and 85Sr secretion in milk were observed, with both nuclides secreted with short- and long-term half-times of 1 – 2 and 12 – 19 days, respectively.

The study of 137Cs, 90Sr, 210Po and 210Pb in the environment included sampling of soil, vegetation and reindeer tissues in two climatically different reindeer herding districts, Østre Namdal and Vågå. These districts were among the areas in Norway most affected by fallout from the Chernobyl accident. The study showed that 137Cs concentrations in reindeer from both areas declined by effective ecological half-times (Tecol) of 3 – 5 years in autumn and winter up to the mid and late 1990s, when the rates of decline decreased possibly due to a reduced role of lichens as sources of absorbed 137Cs in reindeer. Future time-trends may well be governed simply by physical decay, although incidences of high abundances of fungi may potentially cause elevated 137Cs concentrations in reindeer in autumn for many years to come.

Higher 137Cs concentrations in vascular plants, lichens and reindeer in Østre Namdal compared to Vågå (relative to the 137Cs deposition density), suggested that climatic influences on soil properties that influence the availability of 137Cs for plant uptake and on lichen growth and abundance may have a larger impact on long-term transfer of radiocaesium in the soil-plant/lichen-reindeer food chain than has been previously observed. Furthermore, the results of this thesis suggest that various proportions of lichens and vascular plants in the diet may cause appreciable differences in transfer of radiocaesium to reindeer across Norway due to differences in radiocaesium, potassium and crude fibre intake.

The samples of bone and antlers of reindeer of different ages showed that 90Sr concentrations in bone of older females were 40 % higher than in calves due to higher dietary 90Sr intake during their periods of growth (< 2 year) and continuous 90Sr intake thereafter. Combined with previously reported data, a Tecol of 9.03 ± 0.06 years was estimated for 90Sr concentrations in antlers of reindeer calves in Vågå for the period 1988 – 2002. The study supports the use of antlers as monitors of 90Sr concentrations instead of bone, since concentrations in bone appear to be significantly influenced by age and constant bone renewal. Concentrations of 90Sr were 50 – 80 % higher in reindeer from Vågå compared to those from Østre Namdal.

Age did not appear to have a major effect on muscle and liver tissue 210Po and 210Pb concentrations in reindeer from Østre Namdal and Vågå. Concentrations of 210Po and 210Pb were similar in the two districts and to previously reported values from other Nordic areas. Thus, climatic differences did not have noticeable effects on 210Po and 210Pb concentrations in reindeer in this study.

The study of 137Cs in muscle samples of 747 lynxes killed in Norway from the 1986 Chernobyl accident up to the year 2001 showed that a model with 137Cs deposition density, the year lynxes were killed, age, and extent of reindeer grazing area in the lynxes’ home ranges could account for 50 % of the variability in observed 137Cs concentrations. The analyses were equivocal regarding the lynxes’ specialization in prey species. Further work on the possible use of radiocaesium as a tracer of reindeer predation by lynxes require experimental data on Cs retention in lynx and better estimates of deposition density in the lynxes’ home ranges.

The calculated absorbed doses to reindeer from both anthropogenic and natural nuclides suggested that some of the most exposed individuals received dose rates approaching 1 mGy d-1 after the Chernobyl fallout. No effects on morbidity, mortality or reproductive capacity of reindeer would be expected from these doses. Only the lynxes with the highest radiocaesium concentrations received doses comparable to those received by reindeer.

The assessment of doses to humans from the studied radionuclides showed that 137Cs continues to be the most important contributor to ingestion doses by South Saamis. The time trend in 137Cs concentrations in the studied reindeer herding districts suggests that ingestion doses by persons with average Saami consumption rates of reindeer meat will continue to exceed the 1990 recommendation by the International Commission on Radiological Protection for many years to come, if countermeasures are not applied.