ABSTRACT: The naturally occurring radionuclides ²¹⁰Po and ²¹⁰Pb 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 ²¹⁰Po and ²¹⁰Pb from phytoplankton to brine shrimp Artemia sp. to euphausiids Meganyctiphanes norvegica under controlled laboratory conditions. The ratio of ²¹⁰Po:²¹⁰Pb within organisms increased 5- to 12-fold with each trophic level (phytoplankton to grazer to carnivore), reflecting a preferential bioaccumulation of ²¹⁰Po over ²¹⁰Pb. M. norvegica assimilated 44% of the polonium ingested but only 3.5% of the ²¹⁰Pb ingested. Because ²¹⁰Pb was unassimilated, the ratio of ²¹⁰Po:²¹⁰Pb 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 ²¹⁰Po has the potential to build up in food chains and be biologically recycled, whereas ²¹⁰Pb 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 ²¹⁰Po to those predators consumed as seafood by humans.