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\section{Discussion} 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. These The  changes in the bulk 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. 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. 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 C:P = 10 and C:P = 100 resource treatments and the control treatment  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 algal biofilm and plankton communities.   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 casesequences retrieved from the biofilm community would remain within each mesocosm longer than the planktonic community. In this case  the biofilm community would represent a temporally integrated sample of the planktonic organisms moving through the reactor. 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 more a 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 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) separted separated completely  from biofilm commuities in principle component space. 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 temporal heterogeneity in the 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. 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 least abundant members of the plankton community are routinely overrepresented in 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. 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 3 weeks in two boreal freshwater streams (Besemer et al. 2012). While that study system is markedly different than our study the analyses and quetsions addressed in each study were very similar. Both studies concluded that biofilm community assemblage was most likely driven by species sorting over mass effects. However, in the Besemer et al. 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 this result is not suprising. While biofilm communities were establishsed on glass beads (Besemer) and glass slides (this study) over a similar time period (~21 days, Besemer and ~17 days this study) the origin of the planktonic community in each study was very different. The Besemer study was conducted in a boreal stream 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 (Portillo et al. 2012). 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 and drive community composition (Crump et al. 2013). In addition to the diverse source communities the Besemer study sampled the plankton community at multiple timepoints and integrated the samples before sequencing further increasing community richness while in the current study the plankton community was sampled and analyzed only at two independent timepoints - providing a snapshot of community composition at each timepoint.    **If we combine the total OTUs present in the planktonic communities at each timepoint we note that our study shows X OTUs. This is (Lower - Higher) than the Besemer study and....*** CP-R    communities were completely separate    While there are only a few studies that link the abundance of biofilm community composition and the overlyng planktonic community abundance, those studies that have, have found community composition among the two habitats often overlapp little to not at all (Besemer 2007, Besemer 2012, Jackson 2001, Lyautey et al. 2005). This is consistent with our findings in this experimental system with a natural marine planktonic source commmunity. Our study also evaluates algal community composition which showed a similar result suggesting that both the algal and bacterial biofilm communities are form from phylogenetically unique organisms that exist in low abundance in surrounding habitat but are readily enriched in the biofilm lifestyle.    Very few studies have previously evaluated the relationship among membership and or diversity of the plankton and the biofilm community from complex environmental plankton communities. One notable study looked at biofilm formation on glass beads over 3 weeks in a streambed and the planktonic community composition of the overlying water in two boreal freshwater streams (Besemer et al. 2012). While the study systems in that study is markedly different than our study the analyses and quetsions were very similar. Both studies concluded that biofilm community assemblage was most likely driven by species sorting over mass effects. However, in the Besemer et al. 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 this result is not suprising. While biofilm communities were establishsed on glass beads (Besemer) and glass slides (this study) over a similar time period (~21 days, Besemer and ~17 days this study) the origin of the planktonic community in each study was very different. The Besemer study was conducted in a boreal stream 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 (Portillo et al. 2012). 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 and if the discharge of the system is sufficiently high are most likely making little contribution to stream metabolism (Crump)      From Besemer et al. 2012 "Besemer et al. (2007) compared community succession in