\begin{abstract}
The influence of resource availability on planktonic and biofilm microbial
community membership is poorly understood. Heterotrophic bacteria derive some
to all of their organic carbon (C) from photoautotrophs while simultaneously
competing with photoautotrophs for inorganic nutrients such as phosphorus (P)
or nitrogen (N). Therefore, C inputs have the potential to shift the
competitive balance of aquatic microbial communities by increasing the resource
space available to heterotrophs (more C) while decreasing the
resource space available to photoautotrophs (less mineral nutrients due to increased
competition from heterotrophs). To test how resource dynamics
affect membership of planktonic communities and assembly of biofilm communities
we amended a series of flow-through mesocosms with C to alter the
availability of C among treatments. Each mesocosm was fed with unfiltered
seawater and incubated with sterilized microscope slides as surfaces for
biofilm formation. The highest C treatment had the highest planktonic
heterotroph abundance, lowest planktonic photoautotroph abundance, and highest
biofilm biomass. We surveyed bacterial 16S rRNA genes and plastid 23S 
rRNA genes to characterize biofilm and planktonic community
membership and structure. Regardless of resource additions, biofilm communities had
higher alpha diversity than planktonic communities in all
mesocosms. Heterotrophic plankton communities were distinct from heterotrophic
biofilm communities in all but the highest C treatment where heterotrophic
plankton and biofilm communities resembled each other after 17 days. Unlike the
heterotrophs, photoautotrophic plankton communities were different than
photoautotrophic biofilm communities in composition in all treatments including the highest
C treatment. Our results suggest that although resource amendments affect
community membership and structure, microbial lifestyle (biofilm versus
planktonic) has a stronger influence on community composition.

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\keyFont{ \section{Keywords:} microbial ecology, 16S, 23S, planktonic,
biofilm, carbon subsidies, resource stoichiometry }

\end{abstract}
