Development and composition of the epixylic biofilm in a blackwater river

SUMMARY 1. Comparisons of chlorophyll a, bacterial density, frequencies of dividing cells, ash‐free dry mass (AFDM) and extracellular polysaccharide content were made for biofilm developing on wood (Salix) submerged in replicated stream‐side flumes exposed to either ambient light (light treatment) or covered to exclude light (dark treatment). Biofilm was sampled on days 3, 6, 9 and 14 during experimental periods occurring irrMay, September, November and December. 2. There were no significant differences in bacterial cell densities, frequencies of dividing cells, AFDM or extracellular poiysaccharide content between light and dark treatments. Ash content and bacterial biomass was similar to seston, suggesting the importance of seston as a source of material accumulating in the biofilm. 3. Of total epixylic organic carbon 7.2% was estimated to be extracellular polysaccharide, and 0.8% was bacterial carbon. At least nine times more carbon was contained in extracellular polysaccharide than in bacterial biomass. 4. In the epixylon of the Ogeechee River, bacterial dynamics appear to be controlled by factors other than the availability of algal substrates.