sas eS 30 DISCUSSION OF SURVEY DATA The ratio of the radioactivity in plankton to that in water on a basis of equal weights is highly variable from station to station and from collection date to collection date. Some of the variability is probably due to inaccuracies water sample values. in A single value for water is probably less reliable than a single value for plankton because of the method of sampling and the lower level of activity in the water; when the water value is based on the average of seversl observations the comparison 1s fairly reliable, but a comparison based on a single water value is less reliable. The average of ratiosfor all stations of the Marsh survey was 2500 (minimum 29, maximum 18,000). The average ratios for the Walton and Troll surveys were 7,000 and 300 respectively. Several factors may enter into the reduction of the plank- ton-water ratio with time. One factor may be that selective up- take by the plankton is less evident from relatively fresh fall- out meterial, where a wide spectrum of radioisotopes is available. A decreasing ratio with time after fallout, comparing the Walton, Marsh and Troll data, bility. evident from supports this possi- Furthermore, the western stations (14 - 42) of the Marsh survey, which would tend to have older fallout material than the eastern Stations, have a lower ratio of plankton activity to water activity than do the eastern stations. The values and their standard errors are 475 + 98 and 3716 * 754 respectively.