Copepods induce trophic cascades when they consume protists and reduce protists’ grazing pressure on phytoplankton. How the strength of copepod-induced trophic cascades varies under different trophic conditions remains. There are two contrast hypotheses explaining the change of trophic cascades with nutrient supply: 1) high nutrients increase protists abundance; since copepods prefer consuming protists due to their higher quality and larger size, high nutrients supply enhances trophic cascades; 2) high nutrients increase the density of large phytoplankton in the community, so copepods may directly ingest large phytoplankton and weaken the strength of trophic cascades. To investigate this issue, we quantified the intensity of trophic cascades by estimating the difference in phytoplankton concentration reduction using in situ incubations with and without copepods, and tested their relationship with phytoplankton size distribution and the indicators of resource quantity in the East China Sea. Trophic cascades were lower when large phytoplankton is dominant. However, dissolved inorganic nitrogen has a U-shaped rather than negative relationship with trophic cascades, which does not support Hypothesis 2. Perhaps, the optimal phytoplankton size for copepods may occur at intermediate DIN concentration, where copepods mainly feed on phytoplankton and decrease the strength of trophic cascades.