Trophic significance of bacterial extracellular polysaccharide and dissolved organic matter in a blackwater river

Dynamics of bacteria and algae, and extracellular polysaccharide content were examined in the epixylic biofilm of the Ogeechee River, a blackwater river in southeastern Georgia, U.S.A. The ability of black fly larvae to assimilate extracellular polysaccharide, and to grow on a diet of abiotically flocculated DOM was studied. These studies were designed to assess the trophic availability of noncellular components of biofilm and amorphous detrital particles which comprise the bulk of seston and gut contents of filter-feeding and collector-gathering invertebrates in the Ogeechee River.; Comparisons of chlorophyll a, bacterial density, frequencies of dividing cells, ash-free dry mass 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). There were no significant differences in bacterial cell densities, frequencies of dividing cells, AFDM or extracellular polysaccharide content between light and dark treatments. Ash content and bacterial biomass was similar to seston. Of total epixylic organic carbon 7.2% was estimated to be extracellular polysaccharide, and 0.8% was bacterial carbon.; Black fly larvae (Simulium) assimilated with high efficiency (80-90%) a extracellular polysaccharide (EPS) extracted from a Ogeechee River Pseudomonad. Assimilation was traced using {dollar}partialsp{lcub}13{rcub}{dollar}C-enriched EPS. EPS with protein removed was assimilated with lesser efficiency (80%) than EPS without proteins removed (90%). Larvae incorporated 7 and 15 {dollar}mu{dollar}g EPS C larva{dollar}sp{lcub}-1{rcub}{dollar} day{dollar}sp{lcub}-1{rcub}{dollar} of crude and purified EPS, respectively. These results, the first demonstration of EPS assimilation by a freshwater invertebrate, suggest the importance of bacterial EPS as a food resource in lotic systems.; Growth rates of black fly larvae fed natural seston, natural seston enriched with bacteria, and particles produced by the abiotic flocculation of DOM were compared in a laboratory study. There was no significant difference in final AFDM of larvae fed flocculated particles, seston, or seston enriched with bacteria. Growth rates of larvae fed seston and flocculated particles were both 0.044 day{dollar}sp{lcub}-1{rcub}{dollar}. Larvae fed bacteria-enriched seston grew at a rate of 0.064 day{dollar}sp{lcub}-1{rcub}{dollar}. Estimated bacterial biomass available to larvae fed flocculated particles (.00001% of total C) provided only an insignificant portion of carbon in larval growth.