[en] The concentrations of fallout ²³⁹²⁴⁰Pu in the surface waters from all the Great Lakes were slightly lower in 1976 samples than in those from 1973. The same trend of higher concentrations in the surface waters of the upper lakes as in the surface waters of the lower lakes was observed for both years. In addition, the ²³⁹²⁴⁰Pu concentration in samples of deep water collected during the summer of 1976 was higher than in the surface waters but was similar to the surface water values of the 1973 spring samples. This observation is significant in that it suggests that the surface waters of all the Great Lakes undergo a seasonal decrease in plutonium concentration similar to that already observed in Lake Michigan