| Abstract: | In some hardwater rivers of the Seine basin (France), the development ofan encrusting cyanobacterial microflora may gradually lead to a cementation ofriver bed sediments, to a reduction of spawning ground areas and to a loss ofbenthic floral and faunal diversity. Current methods for build-up management arebased on mechanical dredging of the stream bed. In the short term, the habitatdiversity and microfaunal diversity are improved, but in 1–3years the cyanobacterial growth resumes and the bed sediments arecemented again. The early growth stages of stromatolitic structures on newlyavailable substrata were studied for three years in the river Eaulne(Upper-Normandy, France) in relation with microfloral development, sedimentaryprocesses and water quality. Seasonal current velocity variations appeared tocontrol the construction process (colonization dynamics and dependency ofstromatolite growth to mat extension) through substratum availability andnutrient renewal. Under the conditions allowing an optimal microfloraldevelopment, the carbonate precipitation rates were inversely correlated withsubstratum grain size, i.e. the maximal encrusting biomass was hosted bygravels. Homogeneous substrata showed the highest short-term potentialities forhigh sediment cementation rate, whereas heterogeneous, mixed grain size had thehighest potentialities for long-term colonization by the encrusting microflora. |
