\section{Results} \subsection{Bulk community characteristics} We first assessed
the effect of the resource treatments on the dissolved chemistry and bulk
community characteristics of the plankton and the biofilms. In the control
treatment the mean DOC level was 0.12 $+/-$ 0.02 $\mu$moles L$^{-1}$. The lower
C treatment (C:P $=$ 10) was statistically indistinguishable from the control at
0.10 $+/-$ 0.02 $\mu$moles C L$^{-1}$. The intermediate treatment (C:P $=$ 100)
increased in DOC to 0.70 $\mu$moles C L$^{-1}$ before decreasing to 0.12
$\mu$moles C L$^{-1}$ at the end of the experiment, with a mean of 0.28 $+/-$
0.16 $\mu$moles C L$^{-1}$ over the course of the experiment. Only the highest
C treatment (C:P $=$ 500) had DOC levels that were significantly higher (2.53
$+/-$ 1.6 $\mu$moles C L$^{-1}$) than the control treatment, over the course of
the experiment. The high DOC levels in the highest C treatment were consistent
with C being supplied in excess of the metabolic requirements of the community
(i.e. C saturation), but not higher than what has been observed in coastal
marine ecosystems.

% Fakesubsubsection:This increase in DOC
This increase in DOC in the higher C treatments was associated with decreases
in planktonic Chl \textit{a} in each treatment (Figure~\ref{fig:pool_size}A),
however there was no significant difference in biofilm Chl \textit{a} among
treatments (Figure~\ref{fig:pool_size}B). In combination with the decrease in
planktonic Chl \textit{a} on the 6th day of the experiment the highest
C treatment had approximately 4-fold higher planktonic heterotroph abundance
than the control and the 10 $\mu$M C treatment (Figure~\ref{fig:pool_size}D).
Similarly, biofilms had significantly higher total biomass in the high
C treatment compared to the other treatments (Figure~\ref{fig:pool_size}D).
Thus the shift in resource C:P altered the pool size of both the photoautotroph
and heterotroph communities. Clear differences in heterotroph and
photoautotroph pool size among treatments allowed us to address how shifts in
pool sizes were related to community membership and structure within and among
plankton and biofilm communities.

\subsection{Planktonic and biofilm community structure} \subsubsection{Alpha
diversity} We used rarefaction curves to evaluate alpha diversity in all
treatments for both the plankton and the biofilm communities. Rarefaction
curves showed heterotroph and photoautotroph OTU richness was consistently
higher in the biofilm compared to the planktonic communities
(Figure~\ref{fig:rarefaction}). For both the photoautotroph and heterotroph
sequence datasets the biofilm and plankton communities had the fewest OTUs in
the highest C treatment (C:P $=$ 500) (Figure~\ref{fig:rarefaction}). When
planktonic rRNA gene sequences from all planktonic samples were pooled,
individual biofilm heterotroph community richness still generally exceeded the
pooled planktonic heterotroph richness. For photoautotrophs, the individual
biofilm richness was similar to the pooled photoautotroph planktonic richness
(Figure~\ref{fig:integ_rarefaction}).

\subsubsection{Community membership biofilm versus plankton} Heterotroph
community membership between the plankton and biofilm communities was notably
different for all treatments except for the highest C treatment where the
plankton and biofilm communities during the second sampling event (day 17) were
more similar to each other than any other community (Figure~\ref{fig:pcoa}).
Photoautotroph plankton and biofilm communities were also different in OTU
composition, however, the similarity among photoautotroph plankton and biofilm
communities in the highest C treatment was not observed as it was for the
heterotroph communities (Figure~\ref{fig:pcoa}).

% Fakesubsubsection:In bacterial libraries, sequences
In heterotroph libraries, 19,978 sequences were distributed into 636 OTUs and
58\% of quality controlled sequences fell into the top 25 OTUs in order of
decreasing sum of relative abundance across all samples. In photoautotroph
libraries 44,719 23S plastid rRNA gene sequences were distributed into 359 OTUs
and 71\% of sequences fell into the top 25 OTUs sorted by mean relative
abundance across all samples.

% Fakesubsubsection:To investigate differences in
To investigate differences in community structure and membership between
the heterotroph biofilm and overlying planktonic communities we identified the
most enriched OTUs in biofilm compared to the planktonic communities and vice
versa. The most enriched OTUs were enriched in planktonic samples (with respect
to biofilm) as opposed to biofilm samples (Figure~\ref{fig:l2fc}). This is
consistent with the higher alpha diversity in biofilm communities compared to
planktonic communities and evidence that sequence counts were spread across
a greater diversity of taxa in the biofilm libraries compared to the planktonic
libraries (i.e. biofilm communities had higher evenness than planktonic
communities). Of the top 5 enriched heterotroph OTUs between the two lifestyles
(biofilm or plankton), 1 is annotated as \textit{Bacteroidetes},
1 \textit{Gammaproteobacteria}, 1 \textit{Betaproteobacteria},
1 \textit{Alphaproteobacteria} and 1 \textit{Actinobacteria} and all 5 were
enriched in the planktonic libraries relative to biofilm
(Table~\ref{Tab:01}). Of the 25 most enriched OTUs among lifestyles only
2 heterotroph OTU centroid DNA sequences shared high sequence identity ($>=$
97\%) with cultured isolates ("OTU.32" and "OTU.48", Table~\ref{Tab:01}). 

% Fakesubsubsection:We similarly assessed membership
We similarly assessed membership among biofilm and plankton in the
photoautotroph communities. Photoautotroph 23S plastid rRNA gene sequence
libraries also clustered strongly by lifestyle (Figure~\ref{fig:pcoa}). Biofilm
libraries were predominantly enriched in \textit{Stramenopile} OTUs whereas
planktonic libraries were enriched in \textit{Haptophyceae},
\textit{Cryptophyta} and \textit{Viridiplantae} OTUs based on OTU positions in
sample ordination space (Figure~\ref{fig:pcoa}, see Ordination Methods). When
photoautotroph OTUs were ordered by differential abundance between lifestyles
(see Figure~\ref{fig:l2fc}), 16 of the 25 most enriched OTUs were enriched in
the biofilm and 9 were enriched in the planktonic samples. Fourteen of these 16
biofilm enriched OTUs were \textit{Stramenopiles} of class
\textit{Bacillarophyta}, the remaining OTUs were classified as members of the
\textit{Chlorophyta} and \textit{Dinophyceae}. The 9 planktonic enriched OTUs
(above) were distributed into the \textit{Viridiplantae} (5 OTUs),
\textit{Cryptophyta} (1 OTUs), \textit{Haptophyceae} (1 OTU),
\textit{Stramenopiles} (1 OTU) and cyanobacteria (1 OTU). The 10 most enriched
photoautotroph OTUs between lifestyles were evenly split between planktonic and
biofilm enriched OTUs. As with the heterotrophs, photoautotroph OTU fold change
between lifestyles are qualitatively consistent with OTU positions in sample
ordination space (see Figures~\ref{fig:l2fc} and \ref{fig:pcoa}).

% Fakesubsubsection:The separation in community
In both the heterotroph and photoautotroph communities low abundance members of
the planktonic communities became highly abundant members of the biofilm
(Figure~\ref{fig:rank_abund}). The separation in community membership among
biofilm and planktonic communities is supported statistically by the Adonis
test \citep{Anderson_2001} for both the heterotroph and photoautotroph
libraries (p-value 0.003 and 0.002, respectively). The lifestyle category
represents 18\% and 45\% for pairwise sample distance variance in heterotroph
and photoautotroph libraries, respectively. The Adonis test result is also
consistent with lifestyle (biofilm versus planktonic) clustering along the
first principal component for the photoautotroph libraries but not for the
heterotroph libraries (Figure~\ref{fig:pcoa}).

\subsubsection{Heterotroph community membership changes with C amendments}
Although community membership was predominately driven by lifestyle, we also
investigated how resource amendments affected community membership and
structure. Because the abiotic (e.g. DOC) and all biomass indicators (e.g.
heterotroph abundance, Chl \textit{a}) were only significantly different in the
highest resource C:P treatment we compared resource C:P $=$ 500 (high C) to all
other mesocosms (i.e. control, C:P $=$ 10, and C:P $=$ 100 -- low C). The high
and low carbon amended mesocosms had statistically different heterotroph
communities (Adonis p-value 0.018) but not photoautotroph communities (Adonis
p-value 0.59). Nine heterotroph OTUs were enriched in the high C treatment
relative to low C. Four of the 9 high C enriched OTUs were annotated as
\textit{Alteromonadales}, 3 as \textit{Campylobacterales} and 1 each into
\textit{Vibrionales} and \textit{Pseudomonadales}. The most enriched OTU in low
C mesocosms was annotated as belonging to the "HTCC2188" candidate order and
shared 99\% identity with a 16S sequence annotated as "marine gamma
proteobacterium HTCC2089" (accession AY386332). We only observed differences at
the community level between high and low C amendments in the heterotroph
communities and therefore did not assess differential abundance of OTUs between
high and low C treatments in photoautotroph communities.  
