CHARACTERISTICS OF BIOFILM ASSEMBLAGES IN 2 CONTRASTED HYDRODYNAMIC AND TROPHIC CONTEXTS

Characteristics (biomass and activity) of biofilms affected by surface channel water or inflowing lateral source water were investigated on the Rhone River at two seasons (growing and dormant season), and in tw o contrasting hydrodynamic and trophic situations. Sources were charac terized by slow water velocity, fine sediment particles, high total or ganic matter (TOM) (both dissolved and particulate estimated by igniti on loss), whereas, in the channel, now velocity was higher, granules w ere coarser, and TOM concentrations were lower. During the growing sea son, there was significantly higher nitrate concentration and less bio degradable organic carbon in the source water than in the channel wate r. In sources where hydrodynamic constraints were lower, the heterotro ph component of the biofilm was more developed (both total and electro n transport system-active bacteria) than in the channel. In sources, a high positive correlation was established between TOM and both total protein and electron transport: system (ETS) activity: source biofilms consumed TOM of sediments, which was available in each season. In the channel, a strong correlation existed between dissolved organic carbo n (DOC) and both total bacteria and ETS bacteria. Channel biofilms app eared to be more opportunistic in preferentially using the dissolved o rganic matter from overlying water, which fluctuates seasonally. The d evelopment and activity of microbial assemblages can be relevant descr iptors of changes in trophic now; but hydrodynamic factors, water velo city, and grain size can modify the relationships between biofilm char acteristics and nutrients.