• Sciences de l'environnement/Milieux et Changements globaux
  

Size fractionation was performed using water from the Great Reef of Toliara (Madagascar) taken from two different habitats (ocean and lagoon) during the dry and wet seasons, to study the growth and mortality rates of bacterioplankton. Experiments were conducted with 1 and 100% of heterotrophic nanoflagellate (HNF) concentrations and virus-free water was obtained by tangential filtration (10 kDa). During the dry season, in both environments, bacterial abundance and production were significantly lower than values recorded during the wet season. Bacterial growth rates without grazers were 0.88 day–1 in the lagoon and 0.58 day–1 in the ocean. However, growth rates were statistically higher without grazers and viruses (1.58 day–1 and 1.27 day–1). An estimate of virus-induced bacterial mortality revealed the important role played by viruses in the lagoon (0.70 day–1) and the ocean (0.69 day–1). During the wet season, bacterial growth rates without grazers were significantly higher in both environments than were values obtained in the dry season. However, the bacterial growth rates were paradoxally lower in the absence of viruses than with viruses in both environments. Our results suggest that changes in nutrient concentrations can play an important role in the balance between viral lysis and HNF grazing in the bacterial mortality. However, virus-mediated bacterial mortality is likely to act simultaneously with nanoflagellates pressure in their effects on bacterial communities.

La dynamique des particules dans les systèmes aquatiques est régit par l’interaction entre la resuspension et la sédimentation. Cette étude est consacrée à la détermination des processus responsables de la resuspension des particules dans le lac de Guiers situé au nord du Sénégal. Des échantillons ont été collectés au cours d’un cycle annuel au niveau de trois stations prises dans la partie centrale du lac. Dans ce site, la resuspension des particules est liée à l’action des vents mais aussi à la crue du fleuve Sénégal. La resuspension sous l’effet des vents se produit pendant l’harmattan, alors qu’en période d’alizés maritimes, c’est la crue du fleuve Sénégal qui est responsable de la remise en suspension des particules. Le taux de particule en suspension est plus important en période d’harmattan qui est marquée par des vitesses du vent plus élevées (moyenne de 3,6 m.s-1 fluctuant entre 1 et 7 m.s-1), mais surtout par un fetch fort (24 km). Cette évolution saisonnière de la resuspension affecte la turbidité de l’eau, les taux de matière en suspension, la biomasse phytoplanctonique, le coefficient d’atténuation lumineuse, le flux des particules et la structure de la colonne d’eau. La resuspension sous l’effet des vagues créées par le vent associé au fetch élevé constitue dans ce système peu profond le principal processus contrôlant la dynamique des particules.

Salinity in estuaries is influenced by a variety of processes including tidal advection and diffusion and river discharge. The effect of hydrodynamic features on salinity was studied in the Senegal River Estuary (SRE). This estuary is of strategic importance for large populations, but it has been greatly affected by human action (regulation of the freshwater inflow, change in the location of the river mouth ...), which has caused major changes in salinity and ecological functioning. To analyse the impacts of these changes and to determine the spatial and temporal variations in salinity in the SRE, we used a combination of three-dimensional modelling and field measurements. The overall salinity depends on freshwater inflow and tidal fluctuations. Salinity variation is mainly driven by ebb and flood near the mouth and by fortnightly cycles upstream. The enlargement of the mouth increases salinity, whereas its shifting location changes the location of the salinity front and creates a slack water zone downstream. Connection and disconnection of the tributaries also affects the salinity. The present study explains how the recent modifications have increased spatial variation and reduced seasonal differences of salinity and provides a tool for managing the water in the estuary.

This study describes the changes in hydrology, zooplankton communities and abundance in the Senegal River Estuary (SRE) before and after the breaching of the sandbar in October 2003. Samples were taken in 2003 at 3 stations located upstream (DI), in mid estuary (HY) and downstream (RM), and in 2005 at the same stations (RM becoming Old River Mouth: ORM), plus the new river mouth (NRM) resulting from the morphological evolution of the SRE. The study showed marked seasonal variations that affected the structure and distribution of zooplankton as well as major changes caused by the sandbar opening: - increased marine influence throughout the whole SRE, - changes in the horizontal gradients, - arrival of euryhaline species and increase in meroplankton, in particular decapod larvae, - transformation of the ORM area into a slackwater area with limited exchanges and the highest zooplankton numbers during high waters.