Cladophora glomerata is a ubiquitous species of green algae that hosts a consortium of microorganisms which provide nutrients for grazers and impact element cycles. Not all of these epiphytes have the same functions, thus the structure and activity of this community is consequential for river health. In the Eel River, CA the composition of the algal epiphytes, which are primarily diatoms, changes over the course of the growing season. In early summer Cladophora has few epiphytes and appears green but later the filaments are colonized first by Cocconeis and then Epithemia, changing the mats from green to yellow to red/brown. Although the eukaryotic epiphytes on Cladophora have been well characterized, far fewer studies have examined bacteria which perform important functions including methanotrophy, nitrification, nitrogen fixation and denitrification. Also present is the phylum cyanobacteria, which under some conditions produce toxins capable of killing wildlife and even harming humans. I used quantitative Stable Isotope Probing (qSIP) and sequencing of the 16s rRNA gene to evaluate the activity of prokaryotic functional groups in the Cladophora glomerata microbiome based on their consumption of 18O labeled water. The prokaryotic community increased in species richness and growth rate as the summer progressed while eukaryotic chloroplast growth decreased. Cyanobacterial growth rates increased and taxa that produce toxins became more prominent in late season. Most of the active taxa on the green Cladophora were present the entire summer. The fastest growing bacteria from the late season were unique to that stage and were among the most active taxa overall, suggesting that succession plays a large role in determining the composition of prokaryotes. This research is a first step in understanding the roles of individual taxa in microbiome function.