3C Radionuclides in Biota 
Günter Kanisch 
vTI, Johann Heinrich von Thünen-Institute, Germany 
3C.1 Introduction 
The biota of the Baltic Sea received the most 
significant contribution to their radionuclide 
levels following the Chernobyl accident in 
1986, predominantly in the form of 137 Cs and 
134 Cs. The ratio 134 Cs/ 137 Cs in Baltic biota 
agreed very well with that of the Chernobyl 
fallout. High trophic level species, including 
predators such as cod and pike, showed the 
highest 137 Cs levels, but there was some delay 
in reaching their maximum values after 1986, 
when compared to trends in seawater. In the 
long-term, 137 Cs time trends in biota closely 
follow the trends in seawater. 
Levels of radionuclides in marine biota are 
linked to the corresponding levels in seawater 
and sediments, via accumulation through 
food chains. The complexity of food chains 
increases with the trophic level of the species 
considered. Fish, the biota type in the Baltic 
Sea most important for human consumption, 
accumulate most of their radionuclides from 
food, not from water. Within the existing box 
model (see Chapter 4), which is also used 
for describing activity concentrations in biota, 
however, the simpler concentration factor 
concept is applied. This relates the biota 
activity concentrations directly to those of 
seawater, which is the simplest approximation 
to a food chain sub-model. Concentration 
factors specific to the different model boxes 
are derived from measured data. Results 
of model predictions based on estimated 
concentration factors are included in the 
following summary where appropriate. 
In this report, the evaluation of biota data 
is restricted to fish and the bladder wrack 
Fucus vesiculosus, which is a well established 
biological indicator for radionuclides. 
3C.2 Material 
The number of biota samples collected 
annually by the MORS-PRO group decreased 
slightly through the sampling period 1999- 
2006 from about 120 initially to about 90 
ultimately. Over the whole period the numbers 
of samples collected were 615 for fish, 126 
for Fucus vesiculosus, and 102 for benthic 
animals. 
Fish flesh samples were measured from the 
marine species : herring (138), cod (166) and 
whiting (3), of the round fish; and flounder (68) 
and plaice (24), of the flat fish. 
Fish flesh sample numbers were measured 
from the freshwater species : Pike (48) and 
perch (6). 
Additionally 45 herring samples were analysed 
examining their “edible parts” (fish without 
head and entrails). 
The main artificial radionuclides measured 
in biota during the assessment period were: 
137 Cs, 90 Sr, 239 24 °Pu and 241 Am in fish, and "Tc 
in Fucus vesiculosus. 
3C.3 Use of a box model 
for comparison with measured data 
Trends for the activity values of 137 Cs and 
90 Sr may be also simulated by using the 
box model described briefly in Chapter 4. 
Because 137 Cs showed a sharp peak in 
seawater concentrations in 1986 (HELCOM 
1995), a small sub-model for fish is included 
in the model, which allows the fish to take up 
and excrete the activity with an appropriate 
biological half-life (250 days), thereby 
transforming the sharp seawater peak into a 
smooth increase in fish and a slightly delayed 
decrease after the peak. 
Concentration factors updated from the 
MORS-PRO database are required for this 
purpose. The results of the consequent 
estimations are included in the next section. 
3C.4 Results and discussion 
3C.4.1 Concentration factors 
Because of area-dependent significant