Experimental evidence for 234Th bioaccumulation in three Antarctic crustaceans: Potential implications for particle flux studies

234Th is considered a valuable and useful tracer of oceanic biogeochemical processes occurring over timescales of days to weeks. While the geochemical behaviour of this radionuclide in the marine environment is well known, relatively few studies have explored its interactions with biota. To better understand biologically related 234Th dynamics, bioaccumulation of 234Th from the dissolved phase and its subsequent retention in small Antarctic crustaceans (the isopod Natatolana oculata and the amphipods Orchomenella ultima and Uristes stebbingi) was determined under controlled laboratory conditions. Despite morphological and behavioural differences, all three species displayed comparable concentration factors (CF ≥ 80) and very long retention of 234Th (biological half-life not significantly different from infinity). From 16% (isopod) to 49% (both amphipods) of accumulated 234Th was associated with the animal soft parts, which is substantial when compared with reported values for other particle-reactive transuranic elements. The relevance of zooplankton as a potential modulator of 234Th distribution in the water column is discussed in light of these findings. CF-based computations suggest that, for typical zooplankton biomass, biologically mediated interactions with particle flux models can be neglected. In contrast, in waters with very high crustacean biomass, such as krill schools, 234Th distribution in the water column would be largely determined by these organisms. In such waters the biological compartment should be addressed as it could confound the reliability of vertical particle flux assessment using 234Th as a proxy.