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The goal of this study was to evaluate how changes in resource stoichiometry affected the biomass pool size, membership and structure 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 wefurther  explored how shifts in resource C:P affected a) the membership and structure of each community, and b) the rerutiment recruitment  of plankton during biofilm community assembly. Here we highlight three key results that we find important for understanding the assembly of aquatic biofilms. First, biofilm community richness was consistently higher thanthe  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) thanthey were  within a resource treatment. However, for the bacteria both membership and structure of biofilm and 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 (C:P = 500) resulted in a merging of membership in the bacterioplankton and bacterial biofilm communities the same merging of membership 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), communities within a treatment were more similar to each other between timepoints than any other bacterioplankton community (treatment or timepoint). In addition, the control and two lowest carbon treatments (C:P=10 and C:P=100) separated completely from biofilm communities 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 had significant overlapp at the final timepoint (Figure 5). However, the two highest carbon treatment bacterioplankton community snapshots (8 and 17 days) were alsoas  qualitatively as similar as any other community. Thus, variable planktonic community composition among timepoints 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. biofilm as the mechanism for higher diversity in the biofilm compared to the plankton.  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 were rountinely highly abundant within the biofilm community. This was true for both algal and bacterial communities, at all treatment levels and both timepoints. While we did not (could not) specifically measure niche diversity within the biofilm communities our results suggest that the biofilm habitat selected for unique members of the algal and bacterial planktonic community that were in very low abundance in the planktonic habitat but readily 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 three 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. sufficiently similar to merit comparison.  Besemer et al. (2012) concluded that the biofilm community membership was most likely driven by species sorting over mass effects. This is 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 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 three boreal 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 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 relative differences between planktonic and biofilm diversity found in Besemer et al. (2012) compared to this study. In addition, it is important to note that 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 of  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 we are not able to tell whether resources drove productivity that drove changes in diversity or whether resoruces drove diversity which drove productivity.Rather productivity. Rather  we note that, as diversity decreased at the high end of in  the carbon subsidy gradient highest C treatment  bacterial plankton and biofilm membership became increasingly similar. This suggests that enviornments that contain high amounts of labile carbon selected for fewer dominant taxa thatthen also  came to dominate the biofilm community,perhaps  overwhelming the species sorting mechanisms that appeared to dominate biofilm community assembly in all other treatments. While we did not measure extracellular polymeric substances (EPS), direct microscopy showed that planktonic 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 treatments. Thus additional adhesion of the plankton  and the biofilm  may explain, in part also explain  the merging of the planktonic and biofilm bacterial membership in the highest C treatment. \subsection{Sample Class Enriched OTUs} 
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Bacterial OTUs enriched in planktonic samples displayed more dramatic differential abundance patterns than bacterial OTUs enriched in biofilm samples, but, biofilm enriched bacterial OTUs were spread across a greater phylogenetic breadth (Figure 6). This is also consistent with the idea of greater niche diversity in the biofilm environment as opposed to the plankton. Greater niche diversity should select for a more diverse set of taxa but individual taxa would not be as numerically dominant as in the more uniform environment inhabited by the plankton. At the Order level, enriched bacterial OTUs tended to have members in that were enriched in both the plankton and the biofilm suggesting the phylogenetic coherence of this trait is not captured at the level of Order. It should be noted however that taxonomic annotations in reference databasees and therefore environmental sequence collections show little equivalency in phylogenetic breadth between groups at the same taxonomic rank \cite{Schloss_2011}. Unfortunatley, at higher taxonomic resolution (e.g. Genus-level), groups did not possess a sufficient number of OTUs to evaluate coherence between taxonomic annotation and environment type preference.   Biofilm libraries were more even in shape than planktonic libraries (Figure 9) and OTUs enriched in biofilm libraries were enriched less dramatically than planktonic enriched OTUs. It This result  is possible consistent with the idea that  the planktonic environment is has a  more uniformwith respect to  niche space and therefore produces more skewed rank abundance distributions. Similar to the richness results, we found the shape of rank abundance distributions between biofilm and planktonic libraries in our data to be in contrast to that reported by \citet{22237539} although this may be an artifact of each study's different experimental design (see source communities (as discussed  above). \subsection{Enriched Taxa in High Carbon (C:P = 500) Treatment}  Carbon amendments did not affect algal library membership and structure to the same degree as it affected bacterial library composition. As expected, bacterial OTUs enriched in the high carbon amended mesocosm (C:P = 500) include OTUS in classic copiotroph families such as \textit{Vibrionacaea} and \textit{Pseudomonadaceae}. Interestingly, the one OTU depleted in the high carbon treatments is annotated as being in the HTCC2188 order of the \textit{Gammaproteobacteria}. HTCC stands for 'high throughput culture collection' and is a prefix for strains cultured under low nutrient conditions \cite{Cho_2004, Connon_2002}.  \subsection{Conclusion}  In summary this study shows mechanistic links between large scale community level dynamics and the underlying population levels that drive them. We found that autrotrophic pools and heterotrophic pools responded differently to ammendments of labile carbon as hypothesized. Notably while carbon ammendments altered both pool size and membership of the bacterial communities we did not see similar dynamics within the algal communities. Planktonic algal decreased in response to carbon amendments presumably in response to increased competition for P from a larger bacterial community, however there was not a simialr decrease in biofilm algal community. In addition membership of the algal communities betweeen the plankton and biofilm lifestyles did not become more similar in the algae as it did for the bacterial in the highest carbon C  treatment. Consistent with a growing body of work our results suggest that complex environmental biofilms are a unique microbial community that form from taxa that are found in low abundance in the neighboring communities. This membership was affected by resource ammendements for heterotrophic but not autotrophic microbes. microbes but only in the most extreme resource environment.  Ulimately large scale changes in ecosystem processes are driven by composite effects of microbial communities acting as a synthesis of physiological events embedded in a complex biotic and abiotic matrix. Our results suggest that some of these dynamics can be explained by gross level shifts that affect a wide range of taxa in a similar manner.