We examined the influence of macronutrient (nitrate or phosphate) additions on the bioaccumulation of Ni in two species of phytoplankton (Prorocentrum donghaiense Lu and Skeletonema costatum), and its subsequent assimilation and physiological turnover in marine copepod (Calanus sinicus) after ingestion, using a radioactive-tracer method. The results showed that N or P addition significantly influenced intracellular Ni accumulation in the algal cells after 24 h of exposure, with higher ambient N or P enhancing the biological uptake of Ni in the algal cells. Ni assimilation efficiency by the copepod generally increased with an increase of nutrient concentration in two algal species, due to higher N or P concentration facilitating the uptake of Ni by algae. However, ambient nutritional conditions had insignificant effect on the physiological turnover rate of Ni by the copepod, which seemed to be affected by the physiological conditions of animal individuals. An extremely significant positive-correlation was found between Ni assimilation efficiency by the copepod and % intracellular Ni in algal diets (p < 0.001), testifying the hypothesis “copepod only assimilates metal in the cytoplasmic pool”. Therefore, eutrophication may affect the transfer of Ni along the food-chain by facilitating the biological uptake of it by phytoplankton.