COMPARISON OF EPILITHIC AND EPIXYLIC BIOFILM DEVELOPMENT IN A BOREAL RIVER

1. We assessed substratum effects on lotic biofilm development by placing glass and white pine sampling units in a fourth-order boreal river, and analysing, at 6-week intervals, upper-surface biofilms for ATP, chlorophyll, ergosterol, and the activities of nine exoenzymes. 2. All parameters, except chlorophyll standing stock (range 80-320-mu-g dm-2) and beta-xylosidase activity (range 0.4-4.8-mu-mol h-1 dm-2), were significantly greater for epixylic biofilms than for epilithic ones, but the magnitude of the increases varied from 2 to 5 fold, showing that, even under similar hydrodynamic conditions, epilithic and epixylic biofilms are structurally and functionally distinct. For example, ergosterol concentrations ranged from undetectable to 0.93-mu-g dm-2 for epilithon and from 11-49-mu-g dm-2 for epixylon; corresponding ranges for ATP were 1.6-3.7 (epilithon) and 4.2-7.7-mu-g dm-2 (epixylon), for acid phosphatase activity: 2.3-4.9 and 20-41-mu-mol h-1 dm-2, and for alkaline phosphatase activity: 1.9-8.1 and 29-150-mu-mol h-1 dm-2, respectively. 3. The more extensive epixylic development was attributed to utilization of the wood substratum as a supplemental carbon source and to a higher density of microbial attachment sites.