A study of natural radionuclides in zooplankton collected during 1990−1992 from the low productivity waters of French Polynesia has demonstrated the presence of enhanced uptake of Po-210 by zooplankton when zooplankton biomass is low. Po-210 in zooplankton increases exponentially to previously unreported levels up to 3200 Bq/kg dry weight, as their biomasses decline to levels as low as 0.14 mg dry weight/cubic meter. A validated mathematical model, incorporating the established role of zooplankton in the removal of Po-210 from the water column, captures the shape of this empirical relationship and also explains this biomass-related mechanism that increases Po-210 concentrations in zooplankton. Our results and analysis point to the enhanced vulnerability of such low productivity marine systems to environmental contamination following potential leakage of radionuclides from former weapons test sites and radioactive waste repositories that have been recently proposed for the Pacific.