THE IMPACT OF STORM‐FLOW ON RIVER BIOFILM ARCHITECTURE1

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Abstract
The impact of storm‐flow on river biofilm architecture was investigated 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 (approx. 235 cm·s−1+ river sediment) in a laboratory flume. Line transect methodology was used to quantify information from TEM ultrathin sections of LG material. In the LG ambient‐flow biofilm, bacteria were more abundant directly adjacent to the substratum and were noticeably denser 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. Significant horizontal and vertical nearest‐neighbor associations were noted for both bacteria and cyanobacteria. Cells of Cocconeis were common adjacent to the substratum, providing shelter to, and often elevated upon, an “organic pad” of bacteria, cyanobacteria, and densely staining 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 the substratum and were sometimes more dispersed higher in the biofilm than in ambient‐flow samples. We suggest that storm‐flow hydrodynamic forces 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, unless beneath Cocconeis. The DG biofilm consisted almost entirely of bacteria. Storm‐flow only removed surface growth from DG biofilms, and SEM revealed peritrich stalk abrasion and “blow‐down.” Pre‐disturbance biofilm architecture appears to influence the form of destruction. We suggest that the “microcosms” of Cocconeis and their underlying cells not only serve as an inoculum to recolonize the surface when conditions permit but enhance immigration by interrupting flow patterns across the surface.