Contamination modalities of the planktonic crustacean Daphnia magna Straus with 134Cs. Uptake and retention studies
Considering the potential radiological risk, due to the normal operation of the spanish nuclear power plants (Jose CABRERA, TRILLO and ALMARAZ) sited in the Tejo (Tagus) River watershed, the radioecological study of the river is of the utmost interest. Aiming for a better understanding of radionuclides behaviour in the Fratel dam, a particular freshwater ecosystem in the Tejo River located a few kilometers from the border, several studies have been carried out. Besides the field follow up, an experimental study concerning the radioactive cesium transfer in a simplified freshwater trophic chain was developed, under the Contracts (CEC) n° BI6-B-1 98-P and n° BI6-0245-P, whose objectives were respectively the knowledge of Tejo River radioecology and the radiocesium behaviour in a natural ecosystem. In a radiological assessment, the use of site specific data is always recommendable, instead of using default values, what is another perspective of the present work. The importance of cesium behaviour in the environment, is related to its chemical similarity with potassium, which is a wide-spread element in the living tissues. Following a hypotetical ingestion of radiocesium contaminated foodstuff or water by man, the contamination may then be extended to the whale body, mainly the muscles. Recently, the accident in the ucranian Chernobyl power plant, has alerted the international scientific community to the problem of the environmental dispersion of this radionuclide. 134Cs, a gamma radiation emitter, was the isotope selected for the experiments. Its usual concentration in the radioactive liquid effluents released by PWR plants, is similar to the one of 137Cs, an isotope with a half-life of 30.15 years. 134Cs has a half-life of only 2.07 years, which is an advantage regarding the production of radioactive wastes resulting from the experiments. Our objective is to understand the mechanisms of the 134Cs transfer along the trophic chain, and to determine the parameters that might integrate e radioecological model, namely the concentration factors between organisms and water, and the trophic transfer factors between consumers and nourishment. The simplified trophic chain concerned : a primary producer, the green microalgae Selenastrum capricernutum Printz (CORISCO and CARREIRO, 1990 a); a primary consumer, the planktonic crustacean Daphnia magna Straus; and a secondary consumer, the omnivorous fish Tinca tinta L. (CORISCO and CARREIRO, 1990 b). In this paper the 134Cs uptake and retention by Daphnia magna Straus, (Crustacea, Cladocera) via different pathways, are presented. D. magna is a filter-feeding organism very common in freshwater ecosystems and frequently used as a test organism in toxicology. Several authors have been using it, as well, in the field of experimental radioecology. These small crustaceans, hardly exceeding a 3 mm mesh size, reproduce predominantly by parthenogenesis and, in laboratorial conditions, this process may be permanent if optimum temperature, abundant feeding and water quality are maintained. The main chemical characteristics of Fratel water, which was used in this study, are presented. The average stable cesium concentration, determined by instrumental neutron activation analysis, is 6 x 10-5 ppm. During the experiments, the temperature was kept at 20 ± 2 °C and artificial light was maintained for 15 h a day, using daylight fluorescent tubes (36 watt). The radiocesium used was a 134Cs chloride in a 0.1 M solution, with a total cesium content of 1 μg ml-1. The radioactive measurements were performed in e well type 1 1/4" diameter and 2 1/2" deep sodium iodine (thalium activated) detector of 4" x 4", connected to monochannel analyser. The direct uptake from 134Cs Iabelled river water was carried out using an initial radionuclide concentration in the water near to 10 Bq ml-1. The concentration factor (CF), referred to wet weight of daphnids, evolved according to the function : CF (t)=57.4 (1 - e-0.275t) , with time (t) referred to days. When the contamination of D. magna was carried out through the nourishment, 134Cs Iabelled Selenastrum capricornutum, the trophic transfer factor (TF) followed the exponential function : TF(t)=5.1 (1 - e-0.245t). Combining both pathways simultaneously, water appears to be the main contamination source. The number of cells and 134Cs concentration in microalgae S. capricornutum being approximately stabilized, nourishment was responsable for 28.5 ± 8.3 % of the radionuclide concentration in D. magna. However, using the functions issued from the separated pathways experiments, the assessed contamination of D. magna suggests that the trophic pathway would be predominant. The evaluation of the relative importance of the contamination pathways, would very likely be dependent on the experimental conditions. Therefore, experimental results must be carefully taken in consideration, and an approach as best as possible to the environmental conditions should be attempted. The elimination of the radionuclide , after the uptake from water, is expressed by the retention function : R (t)=8.5e-12.211t + 91.5e-0.441t, meaning two biological retention compartments with half-lives Tb1 = 0.06 day and Tb2 = 1.6 days. Following the uptake from the food pathway, retention is expressed by an exponential with only one term : R(t)=100 e-0.494t and, therefore, only one hall-life, Tb = 1.7 days. From the retention study after 134Cs uptake from water, it appears that the shorter hall-life might correspond to the surface desorption, whereas the longer one might correspond to the loss of assimilated 134Cs. Following uptake from ingestion of labelled food, retention study lead to a single hall-life, very similar to the longer one in the previous case. These results would suggest that water and food pathways act differently when tested together or separately, and assimilation must be the main concentration mechanism of 134Cs for the crustacean Daphnia magna. Considering the importance of these planktonic crustaceans as a link in the freshwater trophic chains, the concentration factor at equilibrium (57 ± 3) and mainly the trophic transfer factor et equilibrium (5.0 ± 0.6), a significative contamination concerning the 134Cs, could be reached.