The effect of copper additions (Cu ranging from 0 to 30 μM) on the photosynthesis of three different microalgal biofilms was studied to identify the factors that cause sensitivity differences between benthic and pelagic algae. The response of biofilms which colonized artificial substrata in the River Meuse was compared with those of two laboratory-grown monospecific biofilms, one consisting of the diatom Synedra ulna, and the other composed of a filament-forming cyanobacterium, Oscillatoria sp. The photosynthetic yield ΦII (quantum efficiency of photosystem II) was studied with PAM (Pulse Amplitude Modulated) fluorimetry. S. ulna biofilms appeared to be the most sensitive to Cu, followed by the cyanobacteria, while natural biofilms, dominated by supposedly very sensitive diatom species such as Melosira varians and Diatoma vulgare, were the most resistant to Cu. In the highly productive biofilms, pH is suggested to play a role in lowering toxicity by helping the precipitation of cupric ions. Cu accumulation by the biofilms during the exposure period followed a linear relationship with Cu concentration, saturation not being observed; natural biofilms had an accumulation factor of 1–2·5 × 103 relative to the concentrations in the water, while the diatoms growing unattached to the substratum had a higher concentration factor, up to 4·9 × 103. It was concluded that the physical structure of the biofilm (package of cells and thickness), and not the species composition, was the main factor regulating the sensitivity of the biofilm to Cu toxicity during short-term exposures.

La structure et l'évolution temporelle des communautés zooplanctoniques de deux bras morts de la Meuse («noues de Tailfer»et «noue de Waulsort») ont été suivies durant les périodes printanières et estivales 1993 et 1996 et comparées à celles du coursprincipal du fleuve. La composition des communautés zooplanctoniques se caractérise par une abondance plus importante decrustacés à Waulsort que dans la Meuse, alors que la structure et la densité du zooplancton dans les noues de Tailfer sont prochesde celles du cours principal. Dans les noues se remarque une forte hétérogénéité spatiale de la répartition zooplanctonique, nettementinfluencée par les communications avec le fleuve et les facteurs hydrologiques.

This paper presents a comprehensive study of the impact of damming on the spawning migrations of Barbus barbus in the canalized River Meuse (Southern Belgium). A Denil fish pass on the Ampsin-Neuville dam was controlled 251 times in 1989-1993. The most striking feature is the almost complete absence of barbel in 1990-1993 (15 fishes in 4 years). Most captures of barbel in the fish pass in 1989 were clumped within a few days (mid-May) and related with spawning migrations. Stepwise multiple regression analyses revealed that attractivity was the major condition set allowing barbel to migrate successfully through the pass, while feasibility parameters (water velocity) were found not to be relevant. The variables involved in the attractivity condition set refer indirectly to the influence of water catchment by a hydroelectric plant and to the relative importance of the flow in the pass. The study concludes that this additional condition set significantly interferes with the natural environmental stimuli triggering spawning migrations in barbel and questions the effectiveness of the thermally-related reproductive strategy of the species in an environment with restricted longitudinal connectivity. This statement is discussed in parallel with the recent evolution of barbel populations in canalized rivers.