Pre- and post-Chernobyl accident levels of 129I and 137Cs in the Southern Baltic Sea by brown seaweed Fucus vesiculosus

129I is a very long-lived radionuclide (T1/2 = 15.7 × 106 years) that is present in the environment both because of natural and anthropogenic sources. In this work 129I concentration and 129I/127I ratio have been determined in seaweed Fucus vesiculosus collected in the Southern Baltic Sea during 1982 and 1986 (post-Chernobyl accident). The resulting data were evaluated in terms of 129I concentrations, 129I/127I and 129I/137Cs ratios. 129I concentrations were found to be in the order of (0.82–5.89) × 109 atoms g−1 in 1982 and (1.33–38.83) × 109 atoms g−1 in 1986. The 129I/127I ratios ranged from (22.7–87.8) × 10−10 for seaweed collected in 1982 and from (26.1–305.5) × 10-10 for seaweed collected in 1986. Also a linear relationship was established for 127I concentrations in seawater and salinity in this area, enabling the estimation of concentration factors for 127I in F. vesiculosus. The high levels of 129I and 129I/127I in the Kattegat and their gradually decreasing trend to the Baltic Sea indicates that the most important contribution to the 129I inventory in the Baltic Sea area comes from Sellafield and La Hague reprocessing plants. With respect to Chernobyl accident, 129I concentrations in samples collected in 1986 were not much higher than those expected in less contaminated samples from 1982. This supports the view that the contribution of the Chernobyl accident to 129I in the Baltic region was not significant.