Abstract Changes in seasonal cycles (i.e., phenology), can cause mismatches throughout aquatic food webs and often are used as the primary indicator of zooplankton response to environmental changes. However, zooplankton phenology has been largely overlooked in Lake Michigan despite the altered phenology of phytoplankton, including the disappearance of the offshore spring phytoplankton bloom following Dreissena mussel invasions. Multiple measures of phenology were evaluated for the major zooplankton groups in offshore southeastern Lake Michigan both before the loss of the spring bloom (1983–2003), and after the spring bloom had disappeared (2007–2019). Water temperatures in spring did not differ between time periods. Standardised biomass declined between time periods for all zooplankton groups in spring and for total zooplankton and the cladoceran Daphnia in the summer. Changes in zooplankton phenology varied across groups and depended on the phenological measures used. Month of peak biomass shifted later for all groups, but the central point of the growing season did not change for any group between time periods. The onset of population development was delayed only for the copepod Limnocalanus. Spring chlorophyll concentrations combined with spring water temperatures provided the best predictions of population onset for Limnocalanus and Daphnia. The central point of the growing season varied with spring chlorophyll concentrations for Limnocalanus, but with spring water temperature for Daphnia. Changes in zooplankton phenology were relatively subtle compared to the large changes observed for spring chlorophyll concentrations. By contrast, there were fairly consistent declines in zooplankton biomass across groups that were associated with the declines in spring chlorophyll either alone or in combination with spring water temperature. This may mean that food limitation has a stronger influence on zooplankton biomass or production than on zooplankton phenology in offshore Lake Michigan. Based on our results from Lake Michigan, alterations such as nutrient reduction or invasive dreissenid mussel filtering that lead to declines in spring phytoplankton blooms in other large freshwater lakes could result in subtle delays in zooplankton phenology for some groups along with stronger declines in zooplankton biomass.