Colonization dynamics of an encrusting cyanobacterial mat in a hardwater river (Eaulne, France)

Many hardwater rivers of the Seine basin (France) have gradually become aff ected by extensive stromatolite (sensu Monty 1973) growth during the last 3 0 years. In some cases, calcareous buildup development is fast enough to ce ment large sectors of the stream bed. Although static aspects (mineralogy, morphology, microflora) are well known, few attempts have been made so far to understand the dynamics and regulating factors of buildup development. The formation of an encrusting algal mat was studied from 1996 to 1998 in a hardwater river of Seine-Maritime (France). The structural organization of all stromatolitic buildups was highly homogeneous in all sites. Encrusted microflora settled only on stable, solid substrates and were largely domina ted by the cyanobacterial genus Phormidium. Annual dynamics of calcitic str ucture development closely followed the cyanobacterial microflora growth ph ases. The main determining factor of encrusting microflora extension appeared to be river-bed stability. Current velocity seems to play a fundamental part o n at least two scales. On the "site scale" (100 m(2)), the discharge variat ions determine the spatial distribution of the fine-grained, unstable depos its that cannot host encrusting microflora. On the "substrate scale" (0.1 t o 1 m(2)), the maximum cyanobacterial biomass increases with current veloci ty. The maximal extension rate measured in the present study showed that a stable, cobble riverfloor could be cemented by cyanobacterially driven calc areous deposits in 6 to 7 months.