Characteristics of biofilm assemblages in two 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 two contrasting hydrodynamic and trophic situations. Sources were characterized by slow water velocity, fine sediment particles, high total organic matter (TOM) (both dissolved and particulate estimated by ignition loss), whereas, in the channel, now velocity was higher, granules were coarser, and TOM concentrations were lower. During the growing season, there was significantly higher nitrate concentration and less biodegradable organic carbon in the source water than in the channel water. In sources where hydrodynamic constraints were lower, the heterotroph component of the biofilm was more developed (both total and electron 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 carbon (DOC) and both total bacteria and ETS bacteria. Channel biofilms appeared to be more opportunistic in preferentially using the dissolved organic matter from overlying water, which fluctuates seasonally. The development and activity of microbial assemblages can be relevant descriptors of changes in trophic now; but hydrodynamic factors, water velocity, and grain size can modify the relationships between biofilm characteristics and nutrients.