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.

El estudio de las comunidades de cianobacterias en la cuenca del río Muga permite profundizar en la búsqueda de factores que favorezcan o inhiban la acumulación intracelular de microcistinas y su liberación al medio externo en condiciones oligotróficas o con leves signos de eutrofia, al tiempo que permiten aportar nuevos conocimientos que ayuden a mejorar el entendimiento de los mecanismos y procesos que participan en la dinámica de las comunidades de cianobacterias bentónicas y las microcistinas asociadas en ecosistemas fluviales mediterráneos. Se ha realizado un estudio de la estructura y variabilidad de las comunidades de cianobacterias durante un ciclo anual completo mediante muestreos mensuales y muestreos con menor periodicidad durante un segundo ciclo posterior. Dichos muestreos han permitido observar diferentes patrones espacio-temporales a lo largo de todo el período de estudio en los 4 puntos de muestreo seleccionados en la cuenca del río Muga. Estos 4 tramos incluyen 1 punto regulado:aguas abajo del embalse de Boadella, y 3 sin regular:2 cabeceras y un punto aguas abajo de la población de Albanyà. Se han identificado un total de 34 especies de cianobacterias bentónicas en las 4 zonas estudiadas, predominando siempre especies de aguas limpias o poco alteradas. Dos especies, Rivularia biasolettiana (zonas más limpias y expuestas) y Phormidium retzii (sonas esciáfilas o con alta velocidad de corriente), resultan dominantes en gran parte de ellas. Se han reconocido un total de 6 microcistinas acumuladas intracelularmente en la comunidad bentónica de cianobacterias, identificando 3 de ellas: MC-RR, MC-LR y MC-YR. La concentración de microcistinas intracelulares ha disminuído a lo largo del eje principal del río Muga, determinada por diferentes factores físicos, químicos y biológicos. A nivel general, la acumulación total de microcistinas intracelulares ha estado asociada a un aumento de la temperatura del agua, un descenso en la diversidad de la comunidad de cianobacterias y pequeños incrementos de la concentración de fósforo disuelto en el agua. En cuanto a la liberación de microcistinas, ha estado vinculada a distintas perturbaciones soportadas por la comunidad cianobacteriana, tanto asociadas a factores físicos naturales como antrópicos. / The study of cyanobacteria communities in the Muga River basin enables further search for factors that promote or inhibit the intracellular accumulation of microcystins and their external release in oligotrophic or sligthly eutrophic conditions. At the same time, it provides new knowlodge to improve the understanding of the mechanisms and proccesses involved in the community dynamics of benthic cyanobacteria and associated microcystins in Mediterranean River ecosystems. A two-year study of the structure and variability of cyanobacterial communities was carried out: monthly sampling during the first annual cycle and lower frequency sampling during the second cycle. This allows observing the different spatial-temporal patterns over the study period in the 4 selected sampling points in the Muga River basin. These 4 points comprise 1 regulated point downstream of the Boadella reservoir, and 3 unregulated points, namely 2 headers and a thrid one downstream of Albanyà village. A total of 34 species of benthic cyanobacteria were identified in the 4 studied areas. The predominant species were linked to clear waters or slightly altered areas. 2 species were dominant in most of them: Rivularia biasolettiana (in cleaner and exposed areas) and Phormidium retzii (in unexposed areas or with high flow velocity).In the benthic community of cyanobacteria, a total of 6 intracellular microcystins were detected. 3 of them were identified: MC-RR, MC-LR and MC-YR. The accumulation of intracellular microcystins decrease along the stream as determined by different physical, chemical and biological factors. At a general level, the total accumulation of intracellular microcystins was associated with an increase in water temperature, a decrease of benthic cyanobacterial diversity and small increases in the concentration of dissolved phosphorus in the water. On the other hand, microcystins release was linked to the effects of different physical disturbances on the cyanobacterial community, wether of natural or anthropical origin.