The potential of shells from <i>Mytilus edulis</i> for retrospective analysis of marine ¹⁴ C discharges from nuclear power plants

Abstract Carbon-14 ( 14 C) is an important contributor to the collective effective dose to the public due to releases from nuclear power plants (NPPs). In Sweden, only airborne emissions of 14 C from NPPs are currently routinely monitored, and the existing data on waterborne 14 C discharges are limited. A recent study of 14 C in brown algae ( Fucus spp.) in Swedish coastal waters showed higher F 14 C values collected at Ringhals NPP, on the Swedish west coast, than expected. Therefore, this study aimed at assessing if blue mussels ( Mytilus edulis ) could be used to retrospectively estimate the 14 C concentration of dissolved inorganic carbon (DIC) in seawater at three sites. A method was developed to extract the fibrous layer that forms visible annual structures in the shells. All samples were analyzed with accelerator mass spectrometry and the results compared with 14 C data from Fucus spp. For one of the analyzed shells (structures from 1974–1978), from the site Särdal, F 14 C in Fucus spp. and M. edulis agreed very well. For another shell (1972–1978), shell structures from some of the earlier years displayed up to 6% lower F 14 C than Fucus spp. F 14 C in one shell from a remote site, Båteviken, only had small annual variations (2017–2022: F 14 C = 1.070 ± 0.015 (1 σ)). Two shells from Ringhals NPP had higher average F 14 C, and a significant temporal variability (2014–2022: F 14 C = 1.427 ± 0.268 (1 σ)). Difficulties in unambiguous identification of the annual structures in the shells, as well as the future potential of this method, are discussed.