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Gelatinous zooplankton (GZ) populations are sensitive to environmental perturbations, and regional changes in their abundance may be associated with degraded environmental conditions. Two time series of GZ abundances were used to analyze the population dynamics of gelatinous zooplankton in the Chesapeake Bay, USA from 1984 to 2012. Annual and interannual variations in population size and distribution of the scyphozoan medusae Chrysaora chesapeakei, Aurelia aurita, Cyanea capillata, and Rhopilema virrilli, as well as the lobate ctenophore Mnemiopsis leidyi, were compared with environmental conditions and other biological data. Scyphozoan population control by environmental factors was primarily a result of mortality and asexual reproduction by the benthic scyphistomae. C. chesapeakei was present year-round, but biovolume was highest in July-September and in salinities 9-20. M. leidyi populations were primarily controlled by C. chesapeakei predation and were most abundant in June, after waters warmed above 18 degrees C but before C. chesapeakei bloomed. Low bottom-water salinity was negatively correlated with summer C. chesapeakei biovolume, and low bottom dissolved oxygen concentrations delayed the timing of the peak bloom. Total GZ biovolume decreased in both time series (1984-2012), likely due to decreases in C. chesapeakei abundance. This reduction in C. chesapeakei allowed for a concurrent increase in M. leidyi and decrease in copepod abundance. Predicted future increases in spring streamflow and spring hypoxia due to global climate change would further decrease C. chesapeakei abundance, possibly allowing for future increase in M. leidyi populations.