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To discern controls on particulate organic carbon (POC) : natural radionuclide (RN) ratio variability in order to enhance the accuracy of water column radionuclide-based carbon flux estimates, 234Th, 210Po, and POC were analyzed in seven size classes of mixed micro- and mesoplankton (1-1,500-µm size range), in larger zooplankton from different taxa (salps, euphausiids, copepods, pteropods), and in freshly produced feces from zooplankton collected during spring in the NW Mediterranean. POC:RN ratios in zooplankton ranged between 120 and 11,600 and between 89 and 9,200 µmol dpm-1 for 234Th and 210Po, respectively. In fecal pellets, POC:RN ratios were one to three orders of magnitude lower for 234Th and 3-fold to 30-fold lower for 210Po; the only exception was euthecosome pteropods, which had a higher POC: 210Po ratio in their pellets than in their whole bodies. Significant increases in POC:RN ratios with organism size were best described by a power relationship for POC: 234Th (p < 0.0006) and a saturation exponential equation for POC: 210Po (with a constant POC: 210Po ratio above 70 µm; p < 0.004), suggesting that the observed trend most likely results from surface adsorption processes for 234Th and food chain bioaccumulation for 210Po. This inference is further supported by the observation that, for the >33-µm size classes, 210Po specific activity correlates negatively with the surface : volume ratio, while 234Th correlates positively with it (p < 0.004 and p < 0.001, respectively). POC:RN ratios vary greatly among species and to a lesser extent among fecal pellet types, most probably due to differences in zooplankton feeding strategies. Partial removal of most zooplankton "swimmers" from trap samples would not likely confound assessment of 234Th flux; however, it could considerably bias similar measurements of 210Po flux as well as those of POC:RN ratios. [ABSTRACT FROM AUTHOR]