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Here we highlight three key results that we find important for understanding the assembly of aquatic biofilms. First, biofilm community richness was consistently higher than the planktonic richness. Second, for the C:P = 10 and C:P = 100 resource treatments and the control the membership of the bacterial and plankton communities did not overlap but membership of the two communities in the highest C:P treatment (C:P = 500) did. Third, while carbon subsides increased the bacterial biomass pool size in both the plankton and the biofilm and decreased the algal abundance in the plankton community as hypothesized, the resource treatments did not have similar affects on membership among the two communities. Specifically, the highest level of carbon subsidies resulted in a merging of membership in the bacterioplankton and bacteriofilm communities that increased over time but the same membership pattern was not observed for the analogous algal communities.     To address the first key result 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 sequences retrieved from the biofilm community would have a longer residence within each mesocosm relative to the planktonic community and thus the biofilm community would represent a temporally integrated sample of the organisms moving through the reactor. 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. 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), within a treatment coommunities 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) did not overlapp from the biofilm commuities. This suggests that the biofilm community was not inegrating 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 eachother at the final timepoint (Figure 5). However, even the highest carbon treatment bacterioplankton community was more similar to itself than any other community over time. Thus,the consistency of the planktonic community composition was not highly variable among timepoints suggesting that a consistency in the flow planktonic community could 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. Secondly, Second,  a comparison of the rank abundance profiles of each (biofilm and plankton) community (Figure 6) shows that the least abundant members of the plankton community routinely comprise 1 percent or more of the biofilm community. Thus the lowest least  abundant members of the plankton community are routinely overrepresented in the biofilm community for algae and bacteria, 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 components within constituents of  the biofilm habitat. community.  Very few studies have previously evaluated the relationship among membership and or diversity of the plankton and the biofilm community. One notable study looked at biofilm formation on glass beads over X weeks and the planktonic community composition of the overlying water in high DOC boreal freshwater streams. In this study the authors found that planktonic diversity was enriched relative to biofilm diversity - the opposite of what we found in our study.    **dicuss besemer/battin study**