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  We propose three potential mechanisms that could result in the increased diversity of the biofilm communities relative to the planktonic communities. First, it is possible that the planktonic community composition of our flow through incubators was dynamic in time. In this case the biofilm community would represent a temporally integrated sample of the planktonic organisms moving through the reactor resulting in higher apparent alpha diversity. Second, the biofilm environment may disproportionately enrich for the least abundant members of the of the planktonic community. In this case it is probable that the biofilm would incorporate the most abundant members from the planktonic community but also select and enrich the least abundant members of planktonic community resulting in a higher level of detectable alpha-diversity. Third, the biofilm enivronment may represent more a diverse habitat including sharply delineated oxygen, nutrient and pH gradients that are not present in the planktonic environment. In this case the more diverse habitat would be able to support a more diverse community due to an abundance of additional niches. We evaluated the first mechanism by comparing membership among the plankton samples taken 9 days apart (t=8 and t=17). While bacterioplankton communities were not indentical between the time points (Figure 5), coommunities within a treatment were more similar to each other between timepoints than any other bacterioplankton community (treatment or timepoint). In addiition, the control and two lowest carbon treatments (C:P=10 and C:P=100) separated completely from biofilm commuities in principle coordinate space (Bray-Curtis distance metric). This suggests that the biofilm community was not integrating variable bacterioplankton community membership, but rather selecting for a unique community that was composed of distinct populations when compared to the plankton community. As noted above, in the higheest carbon treatment (C:P = 500) the biofilm and plankton community membership became increasingly similar over time and were as similar as any other community to each other at the final timepoint (Figure 5). However, the two highest carbon treatment bacterioplankton community snapshots (8 and 17 days) were qualitatively as similar to each other as the day 17 high carbon bacterioplankton community was to the day 17 high carbon biofilm community. Thus, the consistency of the planktonic community composition was not highly variable among timepoints (relative the treatmet effects) suggesting that temporal heterogeneity in the planktonic community would not explain the higher diversity observed in the biofilm compared to the planktonic community. Rather, two results point to enrichment of planktonic community members within the biofilm. The first is the increasing similarity between the plankton and the biofilm communities over time in the highest resource treatment. This suggests that selection pressure of the \textit{in situ} conditions were sufficient to alter the relative abundance of the populations within each community. Second, an analysis of the OTU relative abundance in biofilm and planktonic libraries where OTUs are sorted by planktonic sample rank (Figure 6) shows that the least abundant members of the plankton community are rountinely highly abundant members of the biofilm community. This was true for both algal and bacterial communities, at all treatment levels and both timepoints where community composition was analyzed. While we did not (could not) specifically measure niche diversity within the biofilm communities our results suggest that the biofilm habitat selects for unique members of the algal and bacterial planktonic community that are in very low abundance in the planktonic habitat but readily become major constituents of the biofilm community.    Very few studies have previously evaluated the relationship among membership and or diversity of the plankton and the biofilm community from complex environmental microbial communities. One notable study looked at planktonic community composition and biofilm formation on glass beads placed for three weeks in two boreal freshwater streams \cite{22237539}. While that study system is markedly different than our study study,  the analyses and questions addressed in each study were very similar. Both studies concluded that the  biofilm community assemblage membership  was most likely driven by species sorting over mass effects. However, in the \citet{22237539} study the authors reported that planktonic diversity was significantly higher relative to biofilm diversity - the (the  opposite of what we found in our study. study).  Given the differences in the study systems systems,  this result is not suprising. While biofilm communities were establishsed on glass beads \cite{22237539} in \citet{22237539}  and glass slides (this study) over a similar time period (~21 days, \citet{22237539} and ~17 days this study) the origin of the planktonic community in each study was very different. The \citet{22237539} study was conducted in a boreal stream during snow melt when connectivity between the terrestrial and aquatic habitats was high and potentially highly variable depending on how hydrologic pathways differed among precipitation events. A separate study conducted in alpine and sub-alpine streams of the Rocky Mountains clearly showed that stream plankton communities reflected localized precipitation events and could be traced largely to sources of soil communities of drainages within the watershed \cite{22626459}. While planktonic communities in lake ecosystems can be linked to soil communities in the watershed watershed,  as residence time of the system slows the relative influence of species sorting increases. Thus Thus,  in headwater ecosystems stream plankton communities can often be composed primarily of soil organismsand drive community composition  \cite{22378536}. In addition to the diverse source communities the \cite{22237539} study sampled the plankton community at multiple timepoints and integrated the samples before sequencing further increasing community richness while in as comparted to  the current study where  the plankton community was sampled and analyzed only at two independent timepoints - providing a snapshot of community composition at each timepoint. timepoints.    **If we combine the total OTUs present in the planktonic communities at each timepoint we note that our study shows X OTUs. This is (Lower - Higher) than the Besemer study and....*** CP-R