The periphytic green alga Cladophora glomerata is found growing abundantly in dense mats in lakes and rivers worldwide, often co-occurring in eutrophic lakes with near-shore waters saturated in methane. This alga hosts a diverse microbial community, but the spatiotemporal dynamics of the alga's bacterial microbiota over a growth season have not been characterized. In this study, replicate samples of Cladophora were collected in 2014 from multiple locales in Lake Mendota at multiple times during the summer growth season to test the hypothesis that the bacterial community changes over time and is geographically heterogeneous. Genetic sequencing of epibiontic bacteria using the 16S rDNA biomarker showed significant differences in community structure and composition over time and space, suggesting a dynamic microbial community that is strongly influenced by sampling time and weakly by sampling site. Of particular importance are high diversity and relative abundance of likely methane-oxidizing (methanotrophic) bacteria, especially Crenothrix, Methylomonas, and Methylocaldum, which showed distinctive site preferences. Different patterns were observed in many aerobic heterotrophic bacteria, such as Meiothermus, Leadbetterella, and Flectobacillus and non-oxygenic phototrophic bacteria such as Rhodobacter. Comparison to results of a similar 2011 study from the same site revealed a core bacterial assemblage that persists between years and over a growth season, but also opportunistic bacterial genera and ecological guilds whose populations increase, decrease, or peak over different timeframes. Evidence for a highly dynamic microbial community growing on Cladophora glomerata warrants further study to determine the most influential factors and how these factors influence freshwater macroalgae or related submerged photosynthetic organisms in environmental, industrial, or biotechnological systems.