Cladophora glomerata (L.) Kutz. is a green filamentous macroalga common in the South Fork Eel River (SFER), California that supports a diverse biofilm of algal epiphytes. The Cladophora epiphytic microbiome plays an important ecological role in many ecosystems and has recently been identified as a model system for understanding microbiome processes. C. glomerata and its microbiome vary somewhat predictably over time and space in the SFER, facilitating study of factors that influence its form and function. To determine how temperature influences epiphyte structure, we incubated Cladophora in early (green, dominated by sparse Cocconeis), mid (yellow, dominated by denser Cocconeis, Gomphonema, and Rhoicosphenia), and late (red, dominated by Epithemia) stages of succession in microcosms subjected to three temperature treatments (ambient, +2.5ºC, +5ºC) over four-weeks and assessed changes in epiphyte density and composition. In all chambers, the abundance of cyanobacteria and Epithemia, an N-fixing diatom, increased over the course of the experiment. The effect of temperature on epiphyte composition were similar in the green and yellow successional stages. However, higher temperatures in the red stage caused cyanobacteria to increase significantly by week four in both +2.5ºC, +5ºC temperature treatments relative to the ambient control. Early results suggest that Shannon diversity increased with time in green and yellow stages across all temperatures but remained relatively constant in the red stage through time and across treatments. Temperature-induced shifts toward cyanobacteria and/or Epithemia is likely to impact both food quality and N-fixation rates in the SFER. Combined, the changes we observed in the C. glomerata microbiome suggest that warmer conditions are likely to have a significant impact on nitrogen availability in this ecosystem and the ability of this microbiome to support consumers in this nitrogen-limited food web.