The impact of storm-flow on river biofilm architecture was investigate
d using transmission (TEM) and scanning (SEM) electron microscopy. TEM
resin substrata were colonized under light-grown (LG) or dark-grown (
DG) conditions for 33 weeks in the Clywedog River, North Wales, prior
to exposure to ambient-flow (approx. 60 cm.s(-1)) or storm-flow (appro
x. 235 cm.s(-1) + river sediment) in a laboratory flume, Line transect
methodology was used to quantify information from TEM ultrathin secti
ons of LG material. In the LG ambient-flow biofilm, bacteria were more
abundant directly adjacent to the substratum and were noticeably dens
er directly under the adnate diatom Cocconeis. Higher in the biofilm,
the bacteria were loosely dispersed in the matrix between other cells.
Cyanobacteria occurred most frequently, as single cells but were also
found in large ''palisade'' formations adjacent to the substratum. Si
gnificant horizontal and vertical nearest-neighbor associations were n
oted for both bacteria and cyanobacteria. Cells of Cocconeis were comm
on adjacent to the substratum providing shelter to, and often elevated
upon, an ''organic pad'' of bacteria, cyanobacteria, and densely stai
ning exopolysaccharide. Cyanobacteria and Cocconeis were resistant to
removal by storm-flow, but Cocconeis frustules were sometimes damaged.
Bacteria in the LG storm-flow samples were less common adjacent to th
e substratum and were sometimes more dispersed higher in the biofilm t
han in ambient-flow samples. We suggest that storm-flow hydrodynamic f
orces may redistribute bacteria adjacent to the substratum into higher
areas of the biofilm. In addition, bacteria and the exopolysaccharide
matrix were sometimes removed down to the substratum by storm-flow, u
nless beneath Cocconeis. The DG biofilm consisted almost entirely of b
acteria. Storm-flow only removed surface growth from DG biofilms, and
SEM revealed peritrich stalk abrasion and ''blow-down.'' Pre-disturban
ce biofilm architecture appears to influence the form of destruction.
We suggest that the ''microcosms'' of Cocconeis and their underlying c
ells not only serve as an inoculum to recolonize the surface when cond
itions permit but enhance immigration by interrupting flow patterns ac
ross the surface.