Spatiotemporal environmental change can produce phenotypic differences within and between populations. For scyphozoans, the effect of environmental variation on phenotype has been unclear because of multiple challenges, including difficulties delimiting populations. Marine lakes, bodies of seawater entirely surrounded by land, provide an opportunity to study discrete populations and capture responses to perturbations. We use this opportunity to compare Mastigias papua (Lesson, 1830) medusae before and after a demographic and environmental perturbation. We reconstructed mitochondrial DNA haplotype networks, measured morphological variation, and assessed swimming behavior of pre- and post-perturbation samples to evaluate two hypotheses about the source of variation: recolonization from an alternate location or endemic phenotypic variation. We found significant differences between samples in morphology (F > 9.5, P < 0.001) and in two of three behaviors (F > 8.45, P < 0.005) but no substantial genetic differentiation (Φ_ST = 0.03, P = 0.09). We reject the hypothesis of recolonization because pre- and post-perturbation lake medusae were genetically similar to each other and also significantly different from any potential source locations (Φ_ST > 0.48, P > 0.001). We could not distinguish the source of endemic variation; this will require genomic or experimental analyses. Increasing climatic variability emphasizes the need for understanding population-level responses to environmental change and how responses may be modified by sources of intraspecific variation.