EPILITHIC BACTERIAL RESPONSES TO VARIATIONS IN ALGAL BIOMASS AND LABILE DISSOLVED ORGANIC-CARBON DURING BIOFILM COLONIZATION

This study experimentally examines potential shifts in epilithic bacte rial biomass and productivity in response to variations in epilithic a lgal biomass and labile dissolved organic carbon (DOG) during stream b iofilm colonization. I predicted that epilithic bacteria would respond positively to allochthonous DOC early in biofilm colonization and res pond positively to increased algal biomass late in biofilm colonizatio n. Using once-through, experimental-stream channels, a 2 x 2 factorial design was employed in which light (shaded vs. non-shaded) and labile DOC (glucose-amended vs. ambient) were manipulated. Ceramic tiles wer e used as substrates for biofilm colonization and were sampled at diff erent colonization stages. Shading significantly reduced chlorophyll a , live-algal biovolume, and ash-free dry mass throughout colonization. Bacterial biomass increased significantly during biofilm colonization , but was not significantly different among treatments. Incorporation of [H-3]thymidine into bacterial DNA, which was measured as a surrogat e for bacterial productivity, was significantly greater in the glucose -amended channels throughout colonization, but it increased in the uns haded, ambient treatment in late colonization as well. These results s uggest that labile DOC in the water column can potentially function as a control for epilithic bacteria throughout biofilm colonization, whe reas epilithic algae can stimulate bacteria late in biofilm colonizati on in productive stream ecosystems.