Mozingo Lake, a reservoir in the mid-continental United States, was sampled 63 times between January 1999 and October 2015. Prior to 2005, phytoplankton biovolume was largely composed of Cryptomonas and cyanobacteria (mostly Aphanizomenon); these taxa correlated with the abundance of Daphnia and juvenile copepods, and were also influenced by competition between cyanobacteria and bacillariophytes. These relationships suggest the zooplankton community was primarily controlled by bottom-up processes during this time. In 2005, gizzard shad (Dorosoma cepedianum) first appeared in the lake, coinciding with the first spined and helmeted forms of Daphnia as well as their decline from 39% to 3% of zooplankton; the relative abundance of juvenile copepods increased concurrently. By 2007 phytoplankton biovolume had decreased by 88%. Although all phytoplankton phyla declined in absolute abundance, some were more heavily impacted. The relative abundance of Aphanizomenon declined from 50% to 6% of the phytoplankton biovolume; Cryptomonas remained common, but bacillariophytes became codominant. Despite the steep decline in phytoplankton biovolume, several important trophic interactions did not change: phytoplankton biovolume continued to control juvenile copepods, and competition continued between cyanobacteria and bacillariophytes. Although juvenile copepods continued to compete with Daphnia, the scarcity of the latter allowed a relative expansion of juvenile copepods and bacillariophytes. Because very few of the standard parameters correlated with phytoplankton, it seems likely that most of the changes were caused by the introduction of gizzard shad, which is an intense consumer of cyanobacteria and Daphnia. Gizzard shad likely initiated a trophic cascade both directly (through consumption) and indirectly (by shifting competition). Although other factors may have played a role, it seems clear that the introduction of gizzard shad can cause dramatic changes in both the zooplankton and phytoplankton communities by altering species relationships in top-down, bottom-up, as well as lateral control processes.