Abstract
Seasonal and vertical variations of the main microbial communities (heterotrophic bacteria, autotrophic picoplankton, auto- and heterotrophic nanoflagellates, ciliated protozoa and microalgae) and auto- and heterotrophic activities were estimated in a brown-colored humic and moderately acid lake in central France, the lake of Vassivière. The results demonstrated the dominant role of light in the vertical distribution of autotrophic and mixotrophic microorganisms which are confined to the 0–5 m layer during thermal stratification. The bacterial biomass was high throughout the water column probably because of the great availability of dissolved organic matter. Consequently, the predatory microzooplankton and particularly the various trophic groups of ciliated protozoa, were distributed in the water column according to the vertical distribution of the particular food resources (detritus, bacteria, algae). However, despite the great abundance of algae and bacteria, biomass of flagellated and ciliated protozoa was relatively weak. Most of the phytoplanktonic biomass was filamentous (Diatoms) or colonial (Cyanobacteria) and therefore almost probably difficult to ingest for algivorous microzooplankton. Regarding the low abundance of bacterivorous protozoa, the relation with the special physicochemical properties of this lake is discussed.
Résumé
Les variabilités saisonnière et verticale de l'abondance et de la biomasse des principles communautés de la boucle microbienne (bactéries hétérotrophes, picoplancton autotrophe, protozoaires flagellés auto- et hétérotrophes, protozoaires ciliés, microalgues et microcyanobactéries), et des activités auto- et hétérotrophes, ont été étudiées dans un lac à caractère humique et modérément acide du Massif Central Français, le lac de Vassivière.
Le dénombrement des communautés de microorganismes a été réalisé en microscopie inversée et à épifluorescence après mise en oeuvre des fixations et des colorations adéquates. Les activités photosynthétique, photo- et chemohétérotrophes ont été mesurées à partir de l'assimilation de NaH14CO3 et d'un mélange d'acides aminés tritiés grâce à une technique de double marquage. Les résultats obtenus mettent en évidence le rôle prépondérant du facteur lumineux dans la répartition verticale des microorganismes autotrophes et mixotrophes, dont l'essentiel de la biomasse est confiné dans la zone 0–5 m en période de stratification, alors que la biomasse bactérienne est élevée sur l'ensemble de la colonne d'eau en raison, sans doute, de la grande disponibilité en matiére organique dissoute. Consécutivement, le microzooplancton prédateur, et notamment les différents types trophiques de protozoaires ciliés, se répartit dans la colonne d'eau selon la distribution verticale des ressources nutritives particularies. Cependant, compte-tenu de l'abondance bactérienne et algae, la biomasse des protozoaires flagellés et ciliésest relativement peu importante. Concernant les espèces algivores, l'essentiel de la biomasse phytoplanctonique est de nature filamenteuse (Diatomées) ou coloniale (Cyanobactéries) est est donc sans doute difficilement ingérable pour le microzooplancton. Enfin, les relations entre les caractéristiques physico-chimiques spécifiques de ce lac et le faible développement des protozoaires bactérivores sont discutées.
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Amblard, C., Carrias, JF., Bourdier, G. et al. The microbial loop in a humic lake: seasonal and vertical variations in the structure of the different communities. Hydrobiologia 300, 71–84 (1995). https://doi.org/10.1007/BF00024449
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DOI: https://doi.org/10.1007/BF00024449