Seasonal Variation of Caesium-137 from Fallout in a Clam, Rangia cuneata Gray

Abstract

IN the northern temperate zones, fallout of long lived radionuclides, including caesium-137, varies seasonally, with a maximum during May–June and a minimum during November–December¹. The concentration of caesium-137 in caribou flesh and in Alaskan Eskimos shows a similar seasonality². Because of its half life of 30 yr and the huge stratospheric reservoir of radioactive debris, the amount of caesium-137 on the surface of the Earth is still increasing, despite the moratorium on atmospheric testing³. Although caesium is not a biologically important trace element, chemically it resembles potassium, which is the principal cation of cytoplasm. The bioaccumulation of caesium has therefore aroused considerable interest^4,5. Whereas the concentration of caesium in marine organisms is usually about three to thirty times higher than in the surrounding water, the concentration for similar organisms in fresh water may be higher by a factor of 100–1,000 (ref. 5). In laboratory experiments with isopods and snails from brackish water, Bryan⁶ showed that the biological concentrations of caesium and potassium were directly related and that the concentration factors of both elements were inversely related to the salinity of the water. In the field study reported here, a bivalve mollusc, Rangia cuneata Gray, was collected from six stations in the Trent–Neuse estuary in eastern North Carolina (Fig. 1), and the amount of caesium-137 in the soft tissues was measured periodically for 18 months.