Benoît Mialet

Selective Feeding of Bdelloid Rotifers in River Biofilms

In situ pigment contents of biofilm-dwelling bdelloid rotifers of the Garonne River (France) were measured by high performance liquid chromatography (HPLC) and compared with pigment composition of surrounding biofilm microphytobenthic communities. Among pigments that were detected in rotifers, the presence of carotenoids fucoxanthin and myxoxanthophyll showed that the rotifers fed on diatoms and cyanobacteria. Unexpectedly, while diatoms strongly dominated microphytobenthic communities in terms of biomass, HPLC results hinted that rotifers selectively ingested benthic filamentous cyanobacteria. In doing so, rotifers could daily remove a substantial fraction (up to 28%) of this cyanobacterial biomass. The possibility that the rotifers hosted symbiotic myxoxanthophyll-containing cyanobacteria was examined by localisation of chlorophyll fluorescence within rotifers using confocal laser scanning microscopy (CLSM). CLSM results showed an even distribution of quasi–circular fluorescent objects (FO) throughout rotifer bodies, whereas myxoxanthophyll is a biomarker pigment of filamentous cyanobacteria, so the hypothesis was rejected. Our results also suggest that rotifers converted β-carotene (provided by ingested algae) into echinenone, a photoprotective pigment. This study, which is the first one to detail in situ pigment contents of rotifers, clearly shows that the role of cyanobacteria as a food source for meiobenthic invertebrates has been underestimated so far, and deserves urgent consideration.

Contribution of epilithic diatoms to benthic-pelagic coupling in a temperate river

Water residence time in the middle course of rivers is often too short to allow substantial phytoplankton development, and primary production is essentially provided by benthic phototrophic biofilms. However, cells occurring in the water column might derive from biofilm microalgae, and, reciprocally, sedimenting microalgae could represent a continuous source of colonizers for benthic biofilms. A comparative study of biofilm and pelagic microphytic communities (with special focus on diatoms) was carried out over 15 mo in the Garonne River, France. Diatoms dominated both biofilm and pelagic microphytic communities. Typically benthic diatoms were found in high abundance in the water column, and their biomass in the water was correlated with their biomass in the biofilm, indicating the benthic origin of these cells. Variations in river discharge and temperature drove the temporal distribution of benthic and pelagic communities: under high flow mixing (winter) communities showed the greatest similarity, and during low flow (summer)they differed the most. Even during low flow, typical benthic species were observed in the water column, indicating that benthic−pelagic exchanges were not exclusively due to high water flow. Moreover, during low flow periods, planktonic diatoms typically settled within biofilms, presumably because of higher water residence times, and/or upstream reservoir flushing.

Modeling the impacts of multiple environmental stress factors on estuarine copepod populations.

Many studies have focused on natural stress factors that shape the spatial and temporal distribution of calanoid copepods, but bioassays have shown that copepods are also sensitive to a broad range of contaminants. Although both anthropogenic and natural stress factors are obviously at play in natural copepod communities, most studies consider only one or the other. In the present investigation, we modeled the combined impact of both anthropogenic and natural stress factors on copepod populations. The model was applied to estimate Eurytemora affinis densities in the contaminated Scheldt estuary and the relatively uncontaminated Darß-Zingst estuary in relation to temperature, salinity, chlorophyll a, and sediment concentrations of cadmium, copper, and zinc. The results indicated that temperature was largely responsible for seasonal fluctuations of E. affinis densities. Our model results further suggested that exposure to zinc and copper was largely responsible for the reduced population densities in the contaminated estuary. The model provides a consistent framework for integrating and quantifying the impacts of multiple anthropogenic and natural stress factors on copepod populations. It facilitates the extrapolation of laboratory experiments to ecologically relevant end points pertaining to population viability.