21 
Cs-137 ratio between surface and near-bottom waters 
Figure 4: 
Temporal variations in ratios of 
,37 Cs concentrations in surface 
water to those in near-bottom 
waters at four stations in 
various parts of the Baltic Sea. 
concentrations in surface water to those in 
near-bottom water was 1.3 in the Baltic Proper 
during the period 1993-2006, due to the 
inhibiting effect of the pronounced halocline on 
mixing between different water layers. In the 
Bothnian Sea the average ratio was 1.0, while 
in both the Gulf of Finland and the Bothnian 
Bay, the ratio was 0.8. Vertical exchange is 
much more efficient in these water basins, 
because of the lack of stratification. 
Both the Gulf of Finland and the Bothnian Bay 
are shallow basins, with an average water 
depth of around 40 metres. The Bothnian Sea 
is generally deeper, with an average depth 
of around 70 m. Large freshwater inflows 
may contribute to lower 137 Cs concentrations 
in surface water in the Gulf of Bothnia and 
the Gulf of Finland. No evidence of any 
remobilization of 137 Cs from bottom sediments 
has yet been detected, although long-term 
monitoring may bring more information about 
remobilization in the future. The circulation 
of near-bottom waters in the Baltic Sea can 
also redistribute 137 Cs contamination by 
transferring contaminated near-bottom water 
from the Bothnian Sea to the Baltic Proper. As 
illustrated in Figure 3 , 137 Cs concentrations in 
near-bottom water are highest in the Bothnian 
Sea, and decrease towards the Sound and 
the Kattegat. 
The Western Baltic, as a transitional area 
between the North Sea and the Baltic Sea, 
has special hydrographic conditions different 
from the rest of the Baltic Sea. First of all it is 
shallow, with an average depth of around 20 
m. The bottom water bears oxygen and high 
salinity, and is steadily supplied by currents 
from the North Sea. The surface water has 
a net current out of the Baltic, because the 
great catchment area results in a surplus of 
fresh water into the Baltic Sea. This water 
exchange is not dominated by tidal currents, 
but by wind forces which results in a current 
system of high intra-annual and inter-annual 
variability. As an indicator of the inflow of 
bottom water, mean 137 Cs concentrations from 
seven selected stations are shown in Figure 
5. In addition to the general decreasing trend 
in 137 Cs concentrations, this graph shows 
information about the variability of surface 
and bottom water concentrations, which has 
been much less in recent years in comparison 
with the year 1999. The year with the smallest 
difference between concentrations in surface 
and bottom waters was 2004, when bottom 
water flowing in from the Kattegat with low 
137 Cs concentrations evidently did not reach 
the indicator stations. 
The bottom water from the North Sea 
supplies the Baltic Sea with contaminants 
originating from the La Hague and Sellafield 
Baltic Sea Environment Proceedings No. 117