Bioaccumulation of polonium-210 in marine copepods

Po-210, 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, Po-210 provides the largest radiation dose to any organism under natural conditions. To better understand the extent to which zooplankton can influence the fluxes of Po-210 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 Po-210 or with one of eight different phytoplankton species and sterile glass beads, all labeled with Po-210. Assimilation efficiencies (AEs) of ingested Po-210 in copepods ranged from 19% to 55% among the phytoplankton diets, and correlated directly with Po-210's cytoplasmic distributions in the algal cells. The AE of Po-210 from ingested glass beads was 0%. The high AE and low efflux rates (mean of 3% d(-1)) of Po-210 in copepods can explain its biomagnification in marine food chains. Uptake and loss parameters of Po-210 in copepods measured in these experiments were used in a model to quantify the relative sources of Po-210 for copepods. Under all realistic scenarios, >90% of Po-210 in copepods appears to be taken up through diet. Model-predicted Po-210 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.