systems and, on one case at least, an essential vitamin. The levels of radioisotopes present in the plankton vary with time after release of the radioactive materials, mainly because of the variation in availability due to physical decay of the individual radioisotopes (Bonham, 1958 ). In general, however, plankton contain the three radio- active isotopes of cobalt, Co°7, Co°8 and Co80, ata level of 11 to 50 percent of the total radioactivity. percent; Fe°5-59 at a level of zn§5 is present at a level of 12 to 47 1 to 40 percent; Mn° in trace amounts, and the fission products Zr?5-Nb2° at levels of 3 to 44 percent; Ru106Rh106 ig present from 0 to 7 percent; Cel44_py 144 0 to 13 percent; and Cs!37 in trace amounts, if present at all (Lowman 1958 ). gr90- ¥90 has not been found in plankton. Once the radioactive materials have been absorbed or adsorbed by the plankton, their distribution is likely to be greater both vertically and horizontally thanif distribution were solely dependent upon the surface currents. One reason for a greater distribution would be that absorption by plankton makes the radionuclides available to larger organisms which can move beyond the current's boundaries. Similarly these materials also become available to the local resident populations and, as they are recycled through the food chain, the effect is a delay in their distribution away from the original area of contamination. Another factor influencing the distribution of radioactive materials by plankton is their diurnal vertical migration. If this migration were great enough to take the plankton below the current stream, it would extend the vertical