The role of natural biofilms affecting the water quality in rivers has been the main theme in this study. Firstly, the study developed the capacity of biofilms in retention and/or production of DOC. Secondly, the study also approached the production of the geosmin metabolite by benthic cyanobacterial mats. In the two developed aspects, the structure and function of the biofilms showed their relevance in evaluating the capacity of biofilms on the amelioration of the water quality. The importance of the biofilms on ecological research in running waters has been focus in their capacity of adsorption and transformation of nutrient, and therefore, in the self-purification capacity of streams and rivers. Biofilms are also major sites of carbon cycling in streams and rivers, affecting the water DOC dynamics. Eutrophication of streams and rivers have been also linked with an increase of algal biomass. As a consequence, the massive growth of cyanobacterial mats can cause the production of toxic and nuisance metabolites, like geosmin, a secondary metabolite which causes earthy and musty taste and odor in the water.

The first conclusion of this study was that biofilms may play an important role in the retention of water DOC. We elucidated whether biofilms act as sinks or sources of fluvial DOC, depending on their structure and biomass accumulation. Metabolism (extracellular enzymatic activity) and structure (algae, bacteria, C/N content) of light-growth and dark-growth systems were analyzed over a year. Biofilms from the light-growth regime presented monthly variation in DOC uptake/release rates, but the annual average presented higher DOC uptake than dark-growth biofilms. However, the latter had a constant DOC consumption along the year, permitting a maintenance of low water DOC levels. The biofilm structure and the relative contribution of autotrophs and heterotrophs had a relevant implication in the carbon recycling, since metabolic activities were affected by variations in this structure.

Geosmin production was related with the massive growth of benthic cyanobacteria (formed mainly by Oscillatoria limosa) in littoral zones, where water velocity was low, warmer temperatures and high nutrient concentration and low N/P ratio. The masses were further detached and drift downstream, being the responsible for the dispersion of geosmin along the river. The high biomass accumulation per surface unit, may cause a nutrient depletion inside the mat, e.g. nitrogen limitation. Structural and functional differences were found in the different fractions of the cyanobacterial mat (attached vs. free-floating). Free-floating mats presented higher biomass and exoenzymatic activities. Otherwise, the low phosphatase/aminopeptidase ratio found in both compartments indicated a nitrogen limitation inside the mats. Microstructural analyses with oxygen and redox microsensors have been useful to understand the function of the different cyanobacterial patches inside the mat. During the dark, Oscillatoria micro-patches aggregated and accumulated in thick masses where some anoxia conditions were found, giving a very low redox potential. This low diffusion could be associated with resource depletion, limiting the nutrient availability and defining the appropriate conditions for the geosmin production.