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The influx of solar radiation to the Baltic Sea and its penetration into its waters is described on the basis of selected results of optical and bio-optical studies in the Baltic published by various authors during the past ca 50 years. The variability in the natural irradiance of this sea is illustrated on time scales from short-term fluctuations occurring during a single day to differences in mean monthly values over a period of many years. Data on variability of the proportions between UV, * The paper was invited by the Chairman of the Polish National SCOR. The work was carried out within the framework of IO PAS's statutory research and also as part of the Satellite Monitoring of the Baltic Sea Environment project -SatBałtyk, co-founded by the European Union through European Regional Development Fund contract No. POIG 01.01.02-22-011/09.The complete text of the paper is available at http://www.iopan.gda.pl/oceanologia/ 534 J. Dera, B. Woźniak VIS and IR energy in the light reaching the sea surface are also discussed. Longterm monthly mean values of the incident solar radiation flux at the surface of the Baltic Proper are given; they were obtained from meteorological and solar radiation measurements and model approximations. The transmittances of these mean monthly radiation fluxes across the surface of the Baltic are given, as are the typical energy and spectral characteristics of the underwater irradiance, its attenuation with depth in the sea and the associated euphotic zone depths, as well as typical ranges of variability of these characteristics in different Baltic basins. Some of these characteristics are illustrated by typical empirical data. These mean values are not fully representative, however, because with the sole use of classical in situ measurement methods from on board research vessels in the Baltic, it has not been possible to gather a sufficiently representative set of empirical data that would adequately reflect the variability of the optical characteristics of all the basins of this sea. The article goes on to introduce the statistical model of vertical distributions of chlorophyll a concentration in the Baltic and the bio-optical model of Baltic Case 2 waters, the use of which contribute very significantly to this description of the optical characteristics and will enable this data set to be hugely expanded to include all the Baltic basins. This opportunity is presented by the optical parameterization of Baltic Case 2 waters, i.e. by the mathematical formulas of the model linking the coefficient of attenuation of downward irradiance with the surface chlorophyll a concentration, as well as the method developed for the efficient and systematic satellite remote sensing of the chlorophyll a concentration over the entire Baltic Sea area.

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.

-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...