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\section{Discussion} The goal of this study was to evaluate the effect of a shifting resource stoichiometry on the biomass pool size, diversity, and membership of planktonic and biofilm communities. Our results suggest that carbon subsidies increased bacterial biomass in both plankton and biofilm communities as predicted. Carbon subsidies also resulted in decreased algal biomass in the plankton community, but there was no signifcant change in algal biomass of the biofilm communities among treatments. These changes in the bulk pool were consistent with changing relationships (commensal to competitive) between the autotrophic and heterotrophic components of the plankton communities but not neccessarily of the biofilm communities.  We Beyond changes in the biomass pool size of each community we further  explored how shifts in C:P affected a) the diversity and membership of each community, and b) the relationship/interaction between the planktonic and biofilm communities using bacterial 16S and plastid 23S rRNA gene surveys. We found that changes in biomass pool size of the various communities were associated with changes in community structure but not neccesarrily in an intuitive manner. This study highlights three key results that we find important for understanding the assembly and function of aquatic biofilms. First, biofilm community richness was consistently higher than the planktonic richness. Second, for the control, C:P = 10 and C:P = 100 resource treatments the membership of the bacterial and plankton communities did not overlap. And third, communities in overlap but membership of  the plankton and biofilm two  communitieswere most similar  in the highest carbon C:P  treatment (C:P =500). = 500) did. Third, while Specifically, the highest level of carbon subsidies resulted in a merging of membership in the bacterioplankton and bacteriofilm communities that was not observed for the analogous algal communities.  This similarity increased over time but was only observed in the bacterial community not in the algal communities. The similarity of the bacterial communities in the highest carbon treatment was not reflected      We propose three potential mechanisms that could result in the increased diversity of the biofilm communities relative to the algal 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 while the plankton community would only represent the community at the moment of each individual sample. Second, the biofilm environment may disproportionately enrich for the tail of the rank-abundance distibution of the planktonic community. In this case it This  is probable that the biofilm would incorporate the most abundant members from consistent with carbon subsidies affecting  the planktonic community heterotrophic  but also select and enrich not  the least abundant members of planktonic autotrophic  communityresulting in a higher level of detectable alpha-diversity. Third, the biofilm enivronment may represent more diverse habitats including multiple oxygen, nutrient  andpH habitats that are not present in  the planktonic environment. While none similar lack  ofthe proposed mechanisms are orthogonal the third mechanisms is  a subset of the second - i.e. the biofilm provides habitat space that does not exist shift  in algal abundance within  the planktonic habitat and thus enriches for certain members biofilm  of the planktonic autotroph  community.   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 while the plankton community would only represent the community at the moment of each individual sample. Second, the biofilm environment may disproportionately enrich for the tail of the rank-abundance distibution 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 diverse habitats including multiple oxygen, nutrient and pH habitats that are not present in the planktonic environment. While none of the proposed mechanisms are orthogonal the third mechanisms is a subset of the second - i.e. the biofilm provides habitat space that does not exist in the planktonic habitat and thus enriches for certain members of the planktonic community. We evaluated the first mechanism by comparing membership among the plankton samples taken 9 days apart (t=8 and t=17). For the bacterioplankton communites while communities were dissimilar between the time points, within a treatment coommunities were more similar to each other between timepoints than any other bacterioplankton community (treatment or timepoint) and the control and two lowest carbon treatments (C:P=10 and C:P=100) separately completely from the biofilm commuities suggesting that they were not a subset of each other during that period (Figure 5). As noted the higheest carbon treatment (C:P=500) biofilm and plankton did begin to resemble each other and were as similar as any other community at the final timepoint.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. 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, a comparison of the rank abundance profiles of each (biofilm and plankton) community (Figure 6) shows that the majority of the OTUs that are not in the top 100 most abundant members of the plankton routinely comprise 1 percent or more of the biofilm community. Thus the lowest 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 the biofilm habitat.  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.    Very few studies have previously evaluated the relationship among 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. That study reported... **dicuss besemer/battin study**    However, the biofilm community in our study had more OTU’s than the plankton community for all paired treatments.   reconcile with Battin’s work*make a conceptual figure of that illustrates tails and membership of each the biofilm and plankton community – color coded for community* work    Interestingly, biofilm community richness peaked at the intermediate treatment (C:P =100) and appeared to decrease over time although with only two time points it was unclear how pronounced this effect was. Since biomass pools for the plankton and the biofilm increased with increasing carbon subsidies the intermediate peak in OTU richness is consistent with a classic productivity-diversity relationship that has been shown for many ecosystems and communities both microbial and otherwise (Refs).   This study highlights three key results that we find important for understanding the assembly and function of aquatic biofilms