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Biofilms are diverse and complex microbial habitats that are the rule rather than the exception for microbial lifestyle in many environments (Battin per. comm.). Large and small-scale architectural features of biofilms play an important role in their ecology and influence their role in ecosystem level biogeochemical cycles (Battin et al. 2007). While fluid mechanics have been shown to be important drivers of biofilm structure (Besemer et al. 2009, Battin et al. 2003), it is less clear how resources and other abiotic factors affect biofilms. Biofilms are initiated with seed propagules that originate within the planktonic community (Hoedl et al. 2011, REF). Thus how resource amendments influence planktonic communities should influence the formation of microbial biofilms. In a crude sense, biofilm and planktonic microbial communities can be broken into two key groups: phototrophic eukaryotes (hereafter algae) and heterotrophic bacteria and archaea. This dichotomy, while somewhat artificial, has been shown to be a powerful paradigm for understanding community shifts across ecosystems of varying trophic state (Cotner and Biddanda 2002).   Heterotrophic bacteria meet some to all of their organic C requirements from algal produced C while simultaneously competing with them algae  for limiting nutrients such as P. The presence of external C inputs, such as terrigenous carbon leaching from the watershed (Jansson et al. 2008, Karlsson et al. 2012) or C exudates derived from littoral macrophytes (Stets and Cotner 2008a and 2008b), can mitigate bacterioplankton reliance on algal derived carbon and shift the relationship from commensal and competitive to strictly competitive (Figure 1, Stets et al. 2008a). Under this mechanism increased carbon supply should increase the resource space available to the bacteria while and lead to  increased competition for P should decrease decreasing  P available for algal biosynthesis – assuming that bacteria are superior competitors for P as has been suggested (Cotner and Wetzel 1992, Figure 1). These dynamics should result in the increase in bacterial biomass relative to the algal biomass along a gradient of increasing carbon inputs. While these gross level dynamics have been discussed conceptually (Cotner and Biddanda 2002) and demonstrated empirically (Stets et al. 2008a), the effect that these shifts in the biomass pool size have on community algal and bacterial community composition and diversity has not been evaluated in plankton or biofilm communities. In addition, how resource amendments to planktonic communities are propagated to newly formed biofilms is not well understood. Intuitively, shifts in planktonic community composition should alter the available pool that can be recruited into a biofilm. Therefore, if bacterioplankton diversity increases , increases,  the number of potential bacterial species that can be recruited to the biofilm should increase, potentially increasing diversity within the biofilm. Similarly, a decrease in P available to algae should decrease algal pool size, decreasing candidate algae that are available for biofilm formation. In addition, carbon in excess of resource requirements may increase the production of extra cellular polysaccharides (EPS) by planktonic cells (REFS) thus increasing the probability that planktonic cells are incorporated into a biofilm by adhesion. If alpha diversity increases with increasing biomass pool size, these mechanisms suggest that an increase in labile carbon to the system should result in increased alpha diversity in bacterial plankton that should propagate to the bacterial biofilm communities while decreasing alpha diversity within both algal communities. Thus we designed this study to test a) if carbon subsidies shifted the biomass balance between autotrophs and heterotrophs within the biofilm or its seed pool (plankton) and b) if these putative changes in pool size altered all populations within the community similarly, or if the relative abundance among the algal and bacterial populations in both the plankton or the biofilm were affected differently.