Bioaccumulation of polonium-210 in marine copepods

210Po, a naturally occurring radioisotope that is ubiquitous in seawater, is especially enriched in proteinaceous tissues of marine organisms and may therefore be useful as a tracer of organic carbon flux in marine systems. Due in part to its biomagnification in marine food chains, 210Po provides the largest radiation dose to any organism under natural conditions. To better understand the extent to which zooplankton can influence the fluxes of 210Po and serve as a conduit between phytoplankton, which concentrate it greatly from ambient water and higher trophic levels, we conducted a series of laboratory experiments with the calanoid copepod Acartia tonsa. A. tonsa was presented with either dissolved 210Po or with one of eight different phytoplankton species and sterile glass beads, all labeled with 210Po. Assimilation efficiencies (AEs) of ingested 210Po in copepods ranged from 19% to 55% among the phytoplankton diets, and correlated directly with 210Po’s cytoplasmic distributions in the algal cells. The AE of 210Po from ingested glass beads was 0%. The high AE and low efflux rates (mean of 3% d21) of 210Po in copepods can explain its biomagnification in marine food chains. Uptake and loss parameters of 210Po in copepods measured in these experiments were used in a model to quantify the relative sources of 210Po for copepods. Under all realistic scenarios, .90% of 210Po in copepods appears to be taken up through diet. Model-predicted 210Po concentrations in copepods in different ocean regions closely matched independent measurements, suggesting that we understand the processes governing this element’s enrichment in zooplankton and thus can make quantitative predictions of its bioconcentration on a site-specific basis. Zooplankton have long been recognized to influence the cycling and fluxes of elements in the ocean. By producing relatively large, dense fecal pellets that transport material out of surface waters during their descent, zooplankton can reduce the residence time of metals in the mixed layer (Cherry et al. 1978; Fowler and Knauer 1986). In contrast, by concentrating other elements through assimilation in their tissue, zooplankton can increase the time these elements spend in surface waters and in the marine food web (Fisher and Reinfelder 1995). Furthermore, zooplankton are important intermediates between phytoplankton, which can greatly concentrate metals from seawater and higher trophic levels, which are consumed as seafood. The trophic transfer and cycling of many elements by zooplankton have been well studied (Fisher and Reinfelder 1995), but one element that has received relatively little attention in this regard is polonium. 210Po is the final radioactive product in the 238U decay series and is of interest as a potential tracer of organic carbon flux in the ocean (Friedrich and Rutgers van der Loeff 2002). The alpha decay of 210Po accounts for most of the radioactive dose to marine organisms (Cherry 1964), and human consumers of seafood receive their largest dose of natural radiation from the polonium in their food (Cherry 1964; Bulman et al. 1995). Thus, in addition to 210Po’s potential as a geochemical tracer, its interactions with marine biota are of interest to risk assessment modelers in the context of understanding the im1 Corresponding author ([email protected]).