Serial introductions modify a trophic cascade and partially mitigate changes in lake ecosystem structure

New species introductions can change lakes rapidly by directly affecting trophic levels through predator–prey interactions or by indirectly affecting ecosystem structure and function. When successive (serial) introductions occur with time between the introductions for each population to become established, there could be permanent shifts in the ecosystem or a return to a prior state if 1 or more introduced species is removed. We used a natural experiment where, in a fishless lake with a simple pelagic zooplankton–phytoplankton food web, the introduction of a zooplanktivorous fish species was followed by the introduction of a piscivorous fish species. Following the loss of the introduced zooplanktivore, the piscivore population shifted from a growing unimodal population dominated by small individuals to a shrinking bimodal population. Zooplankton density remained relatively constant, but zooplankton size increased and composition shifted towards larger-bodied taxa. Phytoplankton biomass decreased, which allowed water clarity to increase and return to pre-fish introduction levels. We used our results to make predictions about the importance of serial introductions in controlling lake food webs and ecosystem structure. Some introductions lead to trophic cascades that that broadly affect lake ecosystems but, following serial introductions of a higher-trophic level, ecosystem structure and function could potentially recover.