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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. 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 more similar within a lifestyle (plankton vs. biofilm) than they were within a treatment, despite growing within the same mesocosm. However, for the Bacteria bacteria  both membership and structure of biofilm and plantonic planktonic  communities in the highest C:P treatment (C:P = 500) were more similar to each other relative to communities from 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 which 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 highest carbon treatment (C:P = 500) the bacterial biofilm and plankton community membership became increasingly similar over time and were as similar as any other community to each other had significant overlapp  at the final timepoint (Figure 5). However, the two highest carbon treatment bacterioplankton community snapshots (8 and 17 days) were also as qualitatively as similar as any other community. Thus, the variable  planktonic community compositionwas 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 C 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 were  rountinely highly abundant within the biofilm community. This was true for both algal and bacterial communities, at all treatment levels and both timepoints where community composition was analyzed. timepoints.  While we did not (could not) specifically measure niche diversity within the biofilm communities our results suggest that the biofilm habitat selects selected  for unique members of the algal and bacterial planktonic community that are were  in very low abundance in the planktonic habitat but readily become became  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, the analyses and questions addressed in each study were very similar. Both studies Besemer et al. (2012)  concluded that the biofilm community membership was most likely driven by species sorting over mass effects. effects consistent with what we report here.  However, in the \citet{22237539} study the authors reported that planktonic diversity was significantly higher relative to biofilm diversity (the opposite of what we found in our study). Given the differences in the study systems, source of the planktonic community among studies,  this result is not suprising. While biofilm communities were establishsed on glass beads 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 three  boreal stream streams  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, as residence time of the system slows the relative influence of species sorting increases. Thus, in headwater ecosystems stream plankton communities can often be composed primarily of soil organisms \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 as compared to the current study where the plankton community was sampled and analyzed only at two independent timepoints. Indeed, when we pool OTU counts from all planktonic libraries and compare the rarefaction curve of the pooled planktonic libraries (algae and bacteria) against sample-wise biofilm libraries, we find more total bacterial and algal planktonic OTUs than in any given single biofilm sample. It appears, however, that some sample-wise bacterial biofilm rarefaction curves may exceed the integrated planktonic curve upon extrapolation and most exceed the integrated planktonic curve at sampling depths where data is present for the biofilm and itegrated planktonic library. This result is consistent with our conclusion that temporal heterogenetity in the plankton was not sufficient to explain the higher diversity in the biofilm sample but would explain the differences between planktonic diversity in each study. In addition, we evaluated how resource subsidies potentially alter altered  the seed pool (the plankton) and the biofilm community. community assembly.  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. However, with our experimental design it is important to note that whether resources drove productivity that drove changes in diversity or whether resoruces drove diversity which drove productivity cannot be decoupled. Interestingly, as diversity decreased at the high end of the carbon subsidy gradient bacterial plankton and biofilm membership became increasingly similar. This suggests that enviornments that contain high amounts of labile carbon selected for fewer dominant taxa that then also came to dominate the biofilm community. While we did not measure extracellular polymeric substances (EPS), direct microscopy showed that cells in the highest carbon treatment (C:P = 500) were surrounded by what appeared to be EPS. Because biofilm EPS appeared also to increase moving from the low to high carbon treatments (Figure 3) it is possible that more abundant  planktonic cells were more readily incorporated into biofilms due both to increased "stickiness" of the planktonic cells as well as the biofilm itself. While we did not observe floculating DOC which has been shown to dominate high DOC environments in nature, we did measure a substanial increase in DOC in the C:P = 500 treatment which was more than 2-fold higher than any of the other treatments. \subsection{Sample Class Enriched OTUs}