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The goal of this study was to evaluate how changes in resource stoichiometry affected 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 resource treatments. The changes in the biomass pool size that did occurr 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.  \subsection{Biofilm and Plankton Alpha and Beta Diversity}   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 and interaction between the planktonic and biofilm communities.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.  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 community richness. Second, for the control, C:P = 10 and C:P = 100 resource treatments the membership and structure of the bacterial biofilm and plankton communities were less more  similar to each other relative to other communities of the same within a  lifestyle (i.e. biofilm or planktonic) (plankton vs. biofilm) than they were within a treatment,  despite growing within the same mesocosm. However, for the Bacteria  both membership and structure ofthe bacterial  biofilm and plantonic communities in the highest C:P treatment (C:P = 500) at day 17 was more similarrelative  to memebership and structure of each  other relative to  communities from the same lifestyle but under different C:P other  treatments (Figure 5). Third, carbon subsides acted differently on the algal and bacteria communities. Specifically while the highest level of carbon subsidies resulted in a merging of membership in the bacterioplankton and bacterial biofilm communities the same membership pattern was not observed for the algal biofilm and plankton communities had distinct membership in all treatments.   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 a more 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 environmental habitats (i.e. 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 treatment 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.