Influence de la température sur l'accumulation par la voie directe du 60Co chez un poisson dulçaquicole

L'influence du facteur thermique sur la contamination de la carpe par le 60Co a été étudiée pour 4 températures : 8 °C, 15 °C, 20 °C et 25 °C. Pour chaque expérience, un lot homogène de 10 individus, d'une masse initiale d'environ 2,3 g et nourris ad libitum, est introduit dans 5 litres d'eau, renouvelée tous les 3 à 4 jours et contaminée à raison de 1,2.105 Bq.l-1.Entre deux renouvellements, la concentration du 60Co dans l'eau décroît sensiblement et l'état ionique du radionucléide, introduit à 100 % sous forme cationique, évolue vers des formes anioniques et neutres. Ces deux faits sont liés au métabolisme des poissons dont la croissance pondérale est corrélée à la température, la distribution de la nourriture étant adaptée aux besoins des individus.Globalement, la quantité totale de radiocobalt fixée par les carpes apparaît très fortement liée au facteur thermique mais, selon le niveau des températures, des situations différentes se dégagent pour des mêmes écarts. Il n'y a pas proportionnalité comptéte entre élévation de température et accumulation du 60Co mais une augmentation discontinue témoignant de l'existence de paliers, dont le plus important se situe à 20 °C.Le facteur de concentration du 60Co est, par contre, peu influencé par le facteur thermique pour les écarts inférieurs à 10 °C ou quand la température maximale est inférieure à 20 °C. L'incidence de la température devient sensible quand les écarts dépassent 10 °C et notamment lorsque sont atteintes des valeurs correspondant à l'optimum thermique des carpes. A l'équilibre, le facteur de concentration moyen du 60Co est de 3,6 à 8 °C, 4,4 à 15 °C, 5,1 à 20 °C et 6 à 25 °C.The problems of simultaneous thermal and radionuclide pollutions have led to a number of laboratory and field studies of the temperature effects on the radioactive contamination of aquatic organisms. AMIARD and KHALANSKI (1981) concluded from their bibliographic study that a temperature rise generally has a positive effect on biological accumulation of radionuclides in freshwater organisms. Due to insufficient data, however, this conclusion does not appear to be substantiated for 60Co uptake by fish. An experimental study was therefore undertaken to assess the effect of temperature on 60Co uptake by Caprinus carpio.The importance of 60Co results from the fact that, apart from tritium, radioactive cobalt isotopes represent the major components of liquid wastes discharged from pressurized water reactors, which are widely operated in France (BARDIN et al., 1982; PICAT et al., 1986). The common carp (Caprinus carpio L.) was selected as the subject of this experimentation because of its wide geographic distribution and the frequency with which it is found in field samples, making it a suitable biological indicator (D'AUBENTON and SPILLMANN, 1978; FOULQUIER and PALLY, 1982; FOULQUIER et al., 1985).The effect of temperature on 60Co contamination of the carp was investigated by conducting the same experiment at four temperatures (8 °C, 15 °C, 20 °C and 25 °C) determined by ecological and physiological criteria. In each experiment, young carps with a mean weight of 2.3 g were placed in a tank containing 5 liters of water contaminated with 1.2 x 105 Bq.l-1 of 60Co. The water was renewed at intervals of 3 - 4 days. Modifications in the ionic form of the radio-active cobalt dissolved in the water were monitored by percolating filtered water samples through ion exchange resin columns. The carps were fed to satiation with an industrial food containing 12 % minerals, 8 % fats and 58 % proteins (dry weight) together with vitamins and trace elements including 0.2 mg.kg-1 of cobalt. Radioactivity levels in the fish were measured, after mild anesthesia, with a single-channel amplitude selector equipped with a thallium-activated flat sodium iodine probe.Between successive water renewals, the 60Co concentration diminished (Table 1) and the ionic form of the radionuclide remaining in solution was significantly modified. The 60Co was introduced in chloride form and was initially found exclusively as a cation; anionic or neutral forms appeared and became dominant after a few days (Figure 1). These two phenomena are probably related to the fish metabolism which is temperature dependent. Thus, food was distributed according to the individual requirements, and an exponential correlation was observed between the temperature and the weight gain in the carps (Table 1). The appearance of anionic and neutral forms may be attributed to excessive amounts of organic matter due to the presence of the fish and food (FUKAI and MURRAY, 1973; KILLEY et al., 1984). This evolution also results from a significant increase in ions such as HCO3-, Cl-, SO4--, PO4-- and NH4+ (FRITSCH and BAUDIN, 1984) which may combine with Co++ ions to form neutral molecules and anionic complexes (TRISHAN et al., 1981). Radioactive decay in the mater is also temperature-dependent : the amount of fecal matter, which strongly adsorbs radioactive cobalt, is proportional to the metabolic intensity in the fish.The overall 60Co uptake by the carps (Table 2) appears to be strongly related to the temperature, but the effect of a given temperature rise varied with the initial temperature (Table 4). The 60Co uptake was not fully proportional to the temperature rise, but was observed to increase in stepwise fashion indicating the existence of various thresholds. The most significant of these was noted at 20 °C, which is the lower limit of the optimum temperature range for carps.The 60Co concentration in th fish (Table 3) was influenced to a much lower extent by the temperature than the total uptake. Small temperature variations (5 - 7 °C) had little or no effect on the concentration factor; this was also true for larger variations, from 8 °C to 20 °C (Table 4). Temperature effects were only significant between 8 °C and 25 °C, and between 15 °C and 25 °C.The 60Co concentration variation in time was similar for all four temperatures, with two successive phases (Figure 2). It can be expressed mathematically by an exponential model based on the existence of several compartments in living organisms characterized by the influx and efflux rates for the element under consideration (GOLSTEIN and ELWOOD, 1971; AOYAMA and INOUE, 1973; PENTREATH, 1975; GONTIER, 1983). The maximum value and the time required to reach equilibrium were determined by extrapolation from the equations for the concentration factor variations. The results were 3.6 and 510 days at 8 °C, 4.4 and 220 days at 15 °C, 5.1 and 125 days at 20 °C, and 6 and 110 days at 25 °C. These values indicate that a temperature rise diminishes the time required to reach equilibrium and slightly increases the concentration factor by an amount that is statistically significant only for major temperature differences (Table 4).Detailed investigations of various freshwater species, notably Caprinus carpio, have demonstrated the decisive effect of temperature on metabolic activity in these animals (LEVESQUE, 1980). The results presented here indicate that the 60Co uptake rate in the carp is not proportional to the temperature rise as is generally the case for physiological processes. From a radiation protection standpoint, this observation suggests that the temperature is not an essential factor in 60Co contamination of freshwater fish