Contrasting transfer of polonium-210 and lead-210 across three trophic levels in marine plankton

Abstract

The naturally occurring radionuclides 210 Po and 210 Pb can be used as geochemical tracers in marine systems, but their interactions with biota in surface waters need to be understood before oceanographic data can be interpreted unambiguously. We compared the food chain dynamics of these radionuclides in plankton assemblages by measuring the uptake and trophic transfer of 210 Po and 210 Pb from phytoplankton to brine shrimp Artemia sp. to euphausiids Meganyctiphanes norvegica under controlled laboratory conditions. The ratio of 210 Po: 210 Pb within organisms increased 5-to 12-fold with each trophic level (phytoplankton to grazer to carnivore), reflecting a preferential bioaccumulation of 210 Po over 210 Pb. M. norvegica assimilated 44% of the polonium ingested but only 3.5% of the 210 Pb ingested. Because 210 Pb was unassimilated, the ratio of 210 Po: 210 Pb was 1 to 2 orders of magnitude smaller in zooplankton fecal pellets than in the animals producing them. These results suggest that in surface waters 210 Po has the potential to build up in food chains and be biologically recycled, whereas 210 Pb would not build up in marine food chains and would display shorter residence times. Since euphausiids comprise an important link between small plankton and larger predatory animals in many marine ecosystems, they may serve as an important conduit of 210 Po to those predators consumed as seafood by humans.