Spatial and temporal variations of ¹⁴C in Fucus spp. in Swedish coastal waters

Carbon-14 (¹⁴C) dominates the collective effective dose from globally dispersed long-lived radionuclides produced by and released from the nuclear power industry. Literature data on the discharge of ¹⁴C to the marine environment from nuclear power plants (NPPs) and its dispersion in the marine ecosystem are sparse. The local marine ¹⁴C background must be determined before the ¹⁴C enrichment in the marine environment from a NPP can be estimated. This is not trivial since marine activity concentrations of ¹⁴C vary spatially, partly due to long-range transport of ¹⁴C from other anthropogenic sources. We have analysed ¹⁴C in samples of several species of brown algae (Fucus spp.) collected at 45 sites along the Swedish coast in 2020. At sites remote from NPPs, the ¹⁴C activity concentrations per unit mass of carbon (here expressed as Fraction Modern, F¹⁴C) were significantly higher on the west coast than on the east coast (F¹⁴C up to about 1.10 in Skagerrak, and about 1.01, close to atmospheric levels, in the Baltic Sea and the Gulf of Bothnia). On the west coast, F¹⁴C showed a strong correlation with salinity, both of which increased towards the north. This indicates that ¹⁴C is carried from other anthropogenic sources (e.g. from the nuclear fuel reprocessing plants at La Hague and Sellafield). The highest value of F¹⁴C observed was close to the Ringhals NPP on the west coast, F¹⁴C ≈ 1.3, which is higher than expected in the terrestrial environment of this NPP. We also report on temporal variations of F¹⁴C in Fucus spp. collected at Särdal on the Swedish west coast during the period 1967-2020. The values of F¹⁴C in the Särdal marine samples collected after the 1990s are clearly higher than F¹⁴C in clean air CO₂, indicating contributions of ¹⁴C of anthropogenic origin.