SEASONAL DETERMINATIONS OF EXTRACELLULAR HYDROLYTIC ACTIVITIES IN HETEROTROPHIC AND MIXED HETEROTROPHIC AUTOTROPHIC BIOFILMS FROM 2 CONTRASTING RIVERS

The temporal changes in extracellular enzyme activities in freshwater microbial biofilms were examined in two contrasting river sites in Nor th Wales over a 12 month period. Sites were a first order, unshaded ol igotrophic upland stream (Nant Waen) and a fourth order, mildly eutrop hic river with riparian tree cover (River Clywedog). When algal popula tions were low, biofilms of the more eutrophic site supported greater enzyme activities and higher population densities than the oligotrophi c site. Composition, concentration and origin of substrates available to the respective biofilm communities influenced extracellular process ing patterns. Reduction in algal populations depressed total and extra cellular activities in biofilms from the first order site, suggesting that biofilm communities here were maintained by in situ primary produ ction. Biofilms from Nant Waen were often found to contain higher extr acellular activities per cell than the more eutrophic River Clywedog b iofilms, which might represent the enhanced ability of an oligotrophic biofilm to accumulate extracellular enzymes. In contrast, light and d arkgrown River Clywedog biofilms were not enzymatically distinct, infe rring a less important role for biofilm phototrophs. Some evidence was found for increased reliance on allochthonous substrates in the River Clywedog for biofilm maintenance.