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-Riparian forests are known as important source of allochthonous organic matter entering to water ecosystems via fallen leaves. However, leaf litter, depending on their quality, may create different conditions for benthic microorganisms functioning in littoral zone of water bodies. In order to evaluate the impact of riparian invasive Acer negundo on littoral water zone of the River Neris (Lithuania), we performed physicochemical and microbiological investigations in bottom sediments of three different sites of the river. One sampling site was close by riparian A. negundo, another close by native Alnus glutinosa location and a third zone was near the shore without riparian vegetation. Content of nutrients in the littoral sediments differed between invasive and native trees leaf litter accumulation sites, while not always significantly. The highest microbial densities as well as benthic community respiratory activity (expressed as the rate of organic carbon mineralization) occurred in A. negundo leaves accumulation site. In sediments of this site, the most intensive anaerobic terminal organic carbon mineralization process À sulfate reduction and the highest concentration of hydrogen sulfide were also observed. Differences in the intensity of mineralization processes between sites suggest that the replacement of the riparian native species such as dominant A. glutinosa by invasive A. negundo with higher biodegradability leaves may induce local changes in organic matter processing in the littoral zone of the river. The increase of littoral bioproductivity in the accumulation zone of A. negundo leaf litter can occur due to the inflow of available organic matter and its intensive mineralization.Keywords: leaf litter / benthic microorganisms / river Résumé -Influence de la litière d'Acer negundo invasif sur l'abondance et l'activité microbienne benthique dans la zone littorale d'une rivière en Lituanie. Les forêts riveraines sont connues comme source importante de matière organique allochtone entrant dans les écosystèmes aquatiques via les feuilles tombées. Cependant, la litière, en fonction de sa qualité, peut créer différentes conditions pour que les microorganismes benthiques fonctionnent dans la zone littorale des masses d'eau. Afin d'évaluer l'impact de l'Acer negundo invasif dans la zone d'eau littorale de la rivière Neris (Lituanie), nous avons procédé à des recherches physico-chimiques et microbiologiques dans les sédiments de trois sites différents de la rivière. Un site d'échantillonnage était proche de A. negundo riverain, un autre proche de l'emplacement d'Alnus glutinosa indigène et une troisième zone était près d'une rive sans végétation riveraine. Les contenus en nutriments dans les sédiments du littoral différaient entre les sites d'accumulation de déchets de feuilles d'arbres envahissants et natifs, bien qu'ils ne soient pas toujours significativement différents. Les densités microbiennes les plus élevées ainsi que l'activité respiratoire de la communauté benthique (exprimée en taux de minéral...

Key-words:bacteria, microbial processes, karst lakes Physical-chemical parameters and the vertical distribution of bacteria and organic matter production-destruction processes were studied during midsummer stratification in two karst lakes (Kirkilai and Ramunelis) located in northern Lithuania. The lakes were characterized by high sulfate concentrations (369−1248 mg·L −1 ). The O 2 /H 2 S intersection zone formed at 2−3 m depth. In Lake Kirkilai, the highest bacterial densities (up to 8.7 × 10 6 cell·mL −1 ) occurred at the O 2 /H 2 S intersection zone, whereas in Lake Ramunelis the highest densities were observed in the anoxic hypolimnion (up to 11 × 10 6 cell·mL −1 ). Pigment analysis revealed that green sulfur bacteria dominated in the microaerobic-anaerobic water layers in both lakes. The most intensive development of sulfate-reducing bacteria was observed in the anaerobic layer. Photosynthetic production of organic matter was highest in the upper layer. Rates of sulfate reduction reached 0.23 mg S 2− ·dm 3 ·d −1 in the microaerobic-anaerobic water layer and 1.97 mg S 2− ·dm 3 ·d −1 in sediments. Karst lakes are very sensitive to organic pollution, because under such impact in the presence of high sulfate amounts, sulfate reduction may become very intensive and, consequently, the increase in hydrogen sulfide and development of sulfur cycle bacteria may reduce the variety of other hydrobionts. Article published by EDP SciencesA. Krevs and A. Kucinskiene: Knowl. Managt. Aquatic Ecosyst. (2011) 402, 02 anoxiques (jusqu'à 0,23 mg S 2− ·dm 3 ·jour −1 ) et dans les sédiments (jusqu'à 1,97 mg S 2− ·dm 3 ·jour −1 ). Ces lacs karstiques sont très sensibles à la pollution organique, parce que, dans ce cas, en présence de fortes concentrations en sulfates, la réduction des sulfates peut devenir très intense et, en conséquence, l'augmentation du sulfure d'hydrogène et le développement des bactéries du cycle du soufre peuvent réduire la biodiversité.