δ¹³C and δ¹⁵N of crustacean zooplankton and size-fractionated suspended particulate organic matter (SPOM), ranging from bacteria to >200 μm plankton, were studied seasonally in the large shallow eutrophic Taihu Lake in China. Stable isotopic ratios of carbon and nitrogen both showed temporal variation with heaviest values in summer accompanying the progression of cyanobacterial bloom, and tended to be low in winter and early spring. During four seasons, the δ¹⁵N values of plankton (>50 μm) were generally enriched more than those of small size classes (<20 μm), which indicated size-dependent consumption. But the regressions between δ¹⁵N and the logarithmic organism size were only significant at 3 of 7 occasions, reflecting more numerous trophic interactions per size-class. The results indicated that a size-structured approach was partially successful in the depiction of the microbial food web structure in Taihu Lake. Zooplankton was largely depleted in ¹³C relative to the size-fractionated SPOM. There existed a continuum in the δ¹³C differences between SPOM and zooplankton from highly productive summer to poorly productive winter. These differences probably reflected the high contribution of nonalgal allochthonous sources of organic carbon in winter and the less detritus assimilation efficiency by zooplankton.