Trophic cascade studies have so far mostly focused on changes in the abundance,biomass, or average size of prey and predators. In contrast, individual size-based interactions, playing a key role in the trophic structure and functioning of aquatic ecosystems, have been less explored.We conducted a 3-month in situ experiment in Lake Myvatn, Iceland, with two fish treatments (with and without fish, Gasterosteus aculeatus). After the first month of the experiment, Anabaena blooms appeared in the lake. We studied the effects of fish predation and occurrence of cyanobacteria blooms on the individual size structure (i.e. the distribution of the number of organisms over a size range) of zooplankton and phytoplankton. We also assessed the potential consequences for trophic transfer efficiency (TTE) (measured as the predator to prey biomass ratio) in the planktonic food web. Our results showed that fish predation and cyanobacteria bloom had a negative relationship with size diversity of zooplankton, which became dominated by small-sized individuals in both cases. The phytoplankton size diversity changed over time particularly due to the blooming of large-sized Anabaena, and its increase was apparently mainly driven by changes in resources. Low zooplankton size diversity related to fish predation reduced TTE, particularly in the enclosures with fish. This may be because low zooplankton size diversity represents a lower partition of resources among consumers, thereby decreasing the trophic energy transfer. With the occurrence of Anabaena bloom, high phyto- plankton size diversity coincided with a lower energy transfer in all enclosures likely due to reduced zooplankton grazing when large-sized colony-forming Ana- baena dominated. In conclusion, our results indicate that both top-down and bottom-up forces significantly influence the size structure of planktonic communities. The changes in size structure were related to shifts in the energy transfer efficiency of the Lake Myvatn food web. Thus, our study underpins the importance of taking into account size-based interactions in the study of trophic cascades, particularly in a warming climate where strong planktivorous fish predation and frequent cyanobacteria blooms may occur.