Ecological implications of mass growth of benthic cyanobacteria in rivers

The environmental and biological factors related to the wax and wane of benthic cyanobacteria were analysed in a river system (Llobregat River, NE Spain) with high nutrient content and low discharge. The cyanobacterial masses grew from early January to the end of May in shallow areas and extended progressively towards the riffle zones. Significant fractions of the mats became progressively unattached and free-floating, dispersing downstream and showing different structural and physiological characteristics. Chlorophyll a was slightly higher for the free-floating (34.6 +/- 19 mug cm(-2)) than for the attached compartments (24.9 +/- 19.5 mug cm(-2)). The exoenzymatic beta-glucosidase, phosphatase (APA) and aminopeptidase (AMA) activities were higher in the free-floating than in the attached mats. The low APA:AMA ratio indicated that nitrogen limitation could affect both the attached and the free-floating mats. This situation was coincident with a peak in geosmin production, suggesting that nitrogen limitation could trigger the increase of geosmin in the cyanobacterial mats. The decay dynamics of the free-floating mats was revealed by higher AMA and P-glucosidase activities, which could be activated by the release of proteinaceous and polysaccharidic compounds of degrading cells. The degradation processes occurring within the mat could favour geosmin release into the water. Moreover, the large number of meiofauna in the free-floating mat could contribute both to the degradation process and to the release and diffusion of the geosmin in the river.