33 
Species 
Model box 
Time range 
Mean ± 1s SD 
n 
ns 
min value 
max value 
ratio 238 Pu / »«-»Pu 
Sprat 
75 
1991 - 2006 
0.041 ±0.018 
9 
6 
0.030 
< 0.14 
(81,83) 
1991 - 2006 
0.038 ± 0.008 
10 
4 
0.031 
< 0.13 
Blue mussel 
75 
1991 - 1997 
0.065 ± 0.024 
10 
7 
0.056 
< 0.17 
83 
1989 - 2005 
0.044 ±0.016 
12 
2 
0.027 
0.078 
Bladder wrack 
75 
1988 - 1998 
0.043 ± 0.008 
9 
0 
0.034 
0.057 
83 
1995 - 2006 
0.034 ± 0.007 
16 
0 
0.019 
0.044 
ratio 24, Am / 239240 Pu 
Sprat 
75 
1995 - 2005 
0.47 ±0.11 
6 
0 
0.36 
0.63 
(81,83) 
1995 - 2005 
0.41 ±0.07 
9 
0 
0.34 
0.53 
Blue mussel 
75 
1995 - 1997 
0.25 ± 0.09 
8 
0 
0.15 
0.35 
83 
1995 - 2005 
0.28 ± 0.07 
10 
0 
0.15 
0.37 
Bladder wrack 
75 
1995- 1998 
0.24 ±0.14 
6 
0 
0.12 
0.50 
83 
1995 - 2006 
0.12 ±0.09 
14 
0 
0.04 
0.33 
Table 5: 
Results from a statistical 
evaluation of radionuclide 
activity ratios measured in 
samples of sprat, blue mussel 
and Fucus vesiculosus; n 
and ns are the numbers of 
significant and non-significant 
values. All samples were 
measured as whole body 
samples. 
for the activity ratio ^Pu/ 239 ' 24 ^. p ata sets 
containing “less-thans” are called “censored”. 
Their statistical evaluation requires special 
statistical procedures (Helsel and Cohn, 1988; 
Lee and Helsel 2005). The former method by 
Helsel and Cohn was used for this purpose, 
programmed in FORTRAN 90. Table 5 shows 
the results of this evaluation. 
Average values found for the ratio 
238 Pu/ 239 ' 240 Pu are in the range of 0.034 
to 0.065. Earlier (1988-1990), in Fucus 
vesiculosus from the Belt Sea a few values 
around 0.054 had been found (HELCOM 
1995). Global nuclear weapons fallout can 
be assumed to be the major contribution to 
this ratio, for which values around 0.034 (as 
of 1971, i.e. 0.026 decay-corrected to 2006) 
are typical for this hemisphere (c.f. Pentreath 
1988). Discharges from the fuel reprocessing 
plants of Sellafield and La Hague lead to 
activity ratios between 0.1 and 0.5 in North 
Sea water (BMU 2006). Small contributions 
from Sellafield and La Hague to western 
Baltic Sea biota are to be expected; this may 
be indicated by the measured data showing 
slightly larger averages in Box 75 (the Belt 
Sea), closer to the North Sea, compared to 
those from the Box 83 (Baltic Sea east). 
Regarding these two major sources 
for plutonium and americium, a similar 
argumentation for the ratio 241 Am/ 239 24 °Pu 
is not as easy, because the two involved 
radionuclides, 241 Am and 239 249 Pu, are 
expected to have different CF values for 
accumulation from seawater. While for fish 
the same CF of 100 for both, Pu and Am, 
is recommended by the IAEA (2004), the 
corresponding recommended CF values 
for macroalgae are 4x10 3 and 8x10 3 for 
Pu and Am, respectively. Additionally, the 
geochemical behaviour of Pu and Am in the 
sea is different. 
3C.4.4 Trends in activity concentrations 
137 Cs, the most important radionuclide in 
fish since Chernobyl, is continuing to show 
generally slowly decreasing trends, as 
demonstrated by Figure 1, which shows such 
trends for herring muscle (Clupea harengus). 
In the western parts of the Baltic Sea, i.e. the 
Kattegat, the Sound, the Belt Sea and the 
Arkona Sea, the values already show levels 
slightly below the target value of 2.5 Bq kg- 
1 wet weight. In the remaining Baltic Sea 
basins, the target value is still exceeded, in 
the Bothnian Bay and in the Gotland area, by 
a factor of up to 5. 
The diagrams show comparisons between 
calculated (red lines and squares) and 
observed (black circles) concentrations of 
137 Cs or 90 Sr in Baltic Sea sub-regions, i.e. the 
model boxes. Figure 1 in Chapter 4 shows the 
geographical locations of these boxes. Each 
Figure consists of a collection of model box 
related graphs. The observed concentrations 
are annual average values of data collected 
by MORS-PRO. The vertical bars represent 
the variability of the observed concentrations 
within a single year. The modeled curves 
are included here in order to demonstrate 
agreement or disagreement of the model with 
measured data. The box- and species-specific 
concentration factors (CF, in I kg 1 ) are shown 
in the title lines of the individual graphs, which 
also specify the values used for the biological 
half-lives in the model (T-bio, in days). 
Figure 2 shows measured and modeled 
137 Cs activity concentrations in muscle tissue 
from (marine) round fish (cod, herring and 
whiting). A slight increase in the values is 
observed when moving from the Kattegat 
(box 71) over the Belt Sea (box 75) to the 
southeast boxes 81 and 83 (Baltic Sea west 
and east). Furthermore, in the Bothnian 
Sea and Bothnian Bay, boxes 85 and 87 
Baltic Sea Environment Proceedings No. 117