Cladophora glomerata is a green filamentous macroalga commonly found in the South Fork Eel River (SFER), California, where it supports a diverse biofilm of epiphytic algae. The epiphytic microbiome of C. glomerata plays a crucial ecological role in the SFER ecosystem and serves as a model for studying microbiome processes. The microbiome associated with C. glomerata follows a predictable successional sequence in the SFER, with each stage differing in both form and function. To investigate how temperature and nutrient availability influence the composition of these successional stages, we incubated C. glomerata in early- (green, dominated by sparse Cocconeis), mid- (yellow, dominated by denser Cocconeis, Gomphonema, and Rhoicosphenia), and late- (red, dominated by Epithemia) successional stages in microcosms. Each stage was subjected to three temperature treatments (ambient, +2.5°C, +5°C) and three nutrient treatments (ambient, +phosphorus, and +nitrogen) over eight days. We assessed changes in epiphyte density and composition (n=4, resulting in 36 microcosms per experiment).Preliminary results for the green and yellow stages show an increase in the abundance of cyanobacteria and Epithemia (an N-fixing diatom) relative to the ambient treatments. In contrast, Epithemia populations in the red stage were suppressed under elevated nitrogen (+N) treatments and higher temperatures (+2.5°C and +5°C) at ambient nutrient levels. Interestingly, nitrogen addition reduced Epithemia’s response to the temperature treatments in the red stage. Although the cyanobacterial component of the epiphytic community is small, it increased in treatments where Epithemia declined (e.g., +N, higher temperature). Our results suggest that the impacts of temperature and nutrient levels on C. glomerata epiphytes are context-dependent, with distinct effects observed across early-, mid-, and late-successional stages. Temperature-induced shifts toward Epithemia in the early growing season (green and yellow stages) may enhance food quality and nitrogen fixation in the SFER. However, in the late successional stage, elevated temperatures and nitrogen levels reduced Epithemia populations, potentially disrupting this nitrogen-limited food web.