  In shallow macrotidal ecosystems with large intertidal mudflats, the
sediment-water coupling plays a crucial role in structuring the pelagic
microbial food web functioning, since inorganic and organic matter and
microbial components (viruses and microbes) of the microphytobenthic biofilm
can be suspended toward the water column. Two experimental bioassays were
conducted in March and July 2008 to investigate the importance of biofilm input
for the pelagic Microbial and Viral Loops. Pelagic inocula (<0.6$\mu$ and
<10$\mu$ filtrates) were diluted either with \textless{}30kDa-ultrafiltered
seawater or with this ultrafiltrate enriched with the respective
size-fractionated benthic biofilm or with \textless{}30kDa-benthic compounds
(BC). The kinetics of heterotrophic nanoflagellates (HNF), bacteria and viruses
were assessed together with bacterial and viral genomic fingerprints, bacterial
enzymatic activities and viral life strategies. The experimental design allowed
us to evaluate the effect of BC modulated by those of benthic size-fractionated
microorganisms (virus+bacteria, +HNF). BC presented (1) in March, a positive
effect on viruses and bacteria weakened by pelagic HNF. Benthic microorganisms
consolidated this negative effect and sustained the viral production together
with a relatively diverse and uneven bacterial assemblage structure; (2) in
July, no direct impact on viruses but a positive effect on bacteria modulated
by HNF, which indirectly enhanced viral multiplication. Both effects were
intensified by benthic microorganisms and bacterial assemblage structure became
more even. HNF indirectly profited from BC more in March than in July. The
Microbial Loop would be stimulated by biofilm during periods of high resources
(March) and the Viral Loop during periods of depleted resources (July).
