47
In both the Walton and Marsh plankton samples, marked variation in ratios of the different fission and non-fisaion radioisotopes
is evident and cannot be explained on the basis of pre-
gent evidence.
However, at least two factors may be contribut-
ing to the variation and each of the two could mask the possible
effect of the other.
These factors include,
(1) heterogeneity
in comrosition of the plankton collected at the various stations,
and (2) variation in availatle isotopes in given areas due to
local fallouts from different types of nuclear devices, and from
differences in time of firing.
Plankton is composed of many types of organisms including
dinoflagellates,
diatoms,
protozoa,
copepods,
ostracods,
euphau-
sids, amphipods, coelenterates, siphonophores, worms, pteropods,
heteropods, primitive chordates, and the eggs and larvae of fish.
The fraction of the total mass contributed by any one group can
vary widely in different sreas.
It is known that species differ-
ences in uptake of {sotones do cecur.
Yoshii (1956) ut observed
that radiosctive isotoves in copepods and primitive chordates
collected
in the spring of 1954
in the vicinity of the Marshall
Islands, differed both in bete energy ani rate of decay, and
Kawabata (1956)7° stated “Although the mechanisms of the accumu-
w
letion of radioisotopes in planktons and their action in theog BR
organisms are still vague,
it is, by all means, of importance
thet certain planktons selectively accumulate specific radioactive elements of minute amount in the sea water in their
bodies."