Modalités de la contamination de deux chaînes trophiques dulçaquicoles par le cobalt 60

Abstract

Introduction

In two freshwater food chains (Chlorella→Daphnia→Carp andElodea→Limnaea→Crayfish), we have observed that contamination through water alone (Amiard-Triquet & Foulquier, 1978) or composite water-food contamination (Amiard-Triquet & Saas, 1979) was responsible for the same⁶⁰Co concentrations in the different species. So, we now have to quantify the role of food alone in the accumulation of⁶⁰Co by freshwater organisms.

Experimental Procedure

The prey were contaminated bothvia water (nearly 4 μCi/l of⁶⁰Co for the food chain of carp; nearly 0.4 μCi/l for the food chain of crayfish) and food. They were delivered to the consumers only when equilibrium between the experimental medium and prey-organisms had been reached.

During this experiment, the animals received food for 5 days (each day: 100Daphnia per carp; 1Limnaea per crayfish,Elodea ad libitum forLimnaea); then they were starved for 2 days. The radioactivity of the living animals was measured at this time. This cycle was repeated for six weeks.

The water was often changed. Different measures of radioactivity were used to calculate the percentages of⁶⁰Co respectively assimilated, excreted as faeces, or excreted as liquids (Table I).

Results and Discussion

Some results indicate that water is a more important vector than food in the contamination ofDaphnia: the radioactivity ofDaphnia is strongly correlated (r=0.7180 to 0.8625) with that of suspended particles i.e., mainlyChlorella); but when the animals have been starved for two days, the transfer factor f.t. from the primary producer to the herbivorous species attained 0.0019 only (Table II); the transfer factor F.T. † from the culture medium of the primary producer to the consumer through food agent is low (mean: 1.08); F.T. is not so important as the overall contamination factor C settled for composite water-food contamination ofDaphnia.

Table II gives the values of f.t. and F.T. for the other species. The percentage of⁶⁰Co retained by the consumers for the whole experiment, the percentage excreted as faeces and the percentage excreted as liquids are collected in Table III.

For carps once the equilibrium is reached, the transfer factor F.T. is neither significantly different (Student-Fisher ś test) from the concentration factor F.C. settled for uptake from water alone (Amiard-Triquet & Foulquier, 1978) nor from the overall contamination factor C settled for composite water-food contamination (Amiard-Triquet & Saas, 1979). We have obtained the same results for the crayfish but equilibrium was reached only during contamination through water alone. On the contrary, forLimnaea, C and F.C. at equilibrium phase are largely higher than F.T. after seven weeks of experiment. This difference probably results from the important part of⁶⁰Co adsorption on the shell during the contamination of this mollusk (Amiard and Amiard-Triquet, 1979).

Conclusions

For all the species studied, we observe that the transfer factor between two successive trophic levels is always low (f.t.=0.0019 to 0.12). On the contrary, the⁶⁰Co concentration in the primary producer is so large that the transfer factor between water and the final consumers (carp or crayfish) through food agent (F.T.) is always higher than 1 (Table II).

Our results also show that the concentration of⁶⁰Co decreases when looking up both the productivity pyramid and the systematic scale.

In previous experiments, we have noticed that the animals immersed in radiocobalt polluted water were no more contaminated when they received radioactive food than when they were starved (Amiard-Triquet & Saas, 1979). However, this study shows that food is an important source of contamination for herbivorous and carnivorous organisms.