a b s t r a c tControlling factors for the occurrence of littoral macrophyte species were investigated in Lipno Reservoir (area: 46.5 km 2 ; z mean : 6.6 m; z max : 22 m; water residence time: 0.6 yr), a dam impoundment with moderately fluctuating water level (amplitude up to 4 m) which is used for hydropower and downstream flow augmentation. The options of supporting littoral macrophyte growth for improving the reservoir's ecological potential were evaluated according to the European Water Framework Directive's definitions. Macrophytes were examined at 115 sites, each 50 m long, along the whole reservoir perimeter at regular 1 km intervals. Sites were surveyed for their phytocoenology, shore morphometry, pedological characteristics of substrate, transparency of water, and anthropogenic impacts. Most of the sites were characterised by a wide eulittoral zone with a tenuous cover of terrestrial hydrophilic, amphibious, and/or emergent macrophytes and a low proportion of silt, clay, and organic fractions in the substrate. Infralittoral submerged and floating-leaved macrophytes were restricted to a few sites within the mouths of the main and side tributaries where transparency was maintained by clear inflow water independent of reservoir pool fluctuations. Water transparency was low (1-2 m) compared to annual changes of water level (2 to >3 m). Multivariate statistical analysis (DCA, CCA) confirmed relationships between the occurrence of macrophyte species and morphological and substrate characteristics of the shore. Improvement of macrophyte cover and establishment of vital infralittoral zones requires improvements in water transparency and changes in reservoir operation, i.e., limiting the range of water level fluctuation to less than ca. 1 m.

Lakes and reservoirs that are used for water supply and/or flow regulations have usually poorly developed littoral macrophyte communities, which impairs ecological potential in terms of the EU Water Framework Directive. The aim of our study was to reveal controlling factors for the growth of littoral macrophytes in a storage reservoir with fluctuating water level (Lipno Reservoir, Czech Republic). Macrophytes occurred in this reservoir only in the eulittoral zone i.e., the shoreline region between the highest and the lowest seasonal water levels. Three eulittoral sub-zones could be distinguished: the upper eulittoral with a stable community of perennial species with high cover, the middle eulittoral with relatively high richness of emergent and amphibious species present at low cover values, and the lower eulittoral devoid of permanent vegetation. Cover and species composition in particular sub-zones were primarily influenced by the duration and timing of flooding, followed by nutrient limitation and strongly reducing conditions in the flooded organic sediment. Our results stress the ecological importance of eulittoral zone in reservoirs with fluctuating water levels where macrophyte growth can be supported by targeted management of water level, thus helping reservoir managers in improving the ecological potential of this type of water bodies. RÉSUMÉ Réponse des macrophytes littoraux aux fluctuations du niveau d'eau dans un réservoir Mots-clés :Les lacs et réservoirs qui sont utilisés pour l'approvisionnement en eau et/ou les régulations d'écoulement ont généralement des communautés littorales de macrophytes peu développées, ce qui altère le potentiel écologique selon la directive européenne cadre sur l'eau. Le but de notre étude était de révéler les facteurs de contrôle de la croissance des macrophytes littoraux dans un réservoir avec un niveau d'eau fluctuant (Lipno, République tchèque). Les macrophytes ne sont pré-sents dans ce réservoir que dans la zone eulittorale i.e., la région du littoral entre les plus hauts et les plus bas niveaux d'eau saisonniers. Trois sous-zones eulittorales peuvent être distinguées : la eulittorale supérieure avec une communauté stable d'espèces pérennes avec un fort recouvrement, la eulittorale médiane avec Article published by EDP Sciences M. Krolová et al.: Knowl. Managt. Aquatic Ecosyst. (2013) 408, 07 une richesse relativement élevée d'espèces émergées et amphibies présentant des valeurs de couverture faible, et la eulittorale inférieure de végétation permanente. Couverture et composition des espèces, dans chaque sous-zone, sont principalement influencées par la durée et le moment des inondations, suivis par la limitation en nutriments et des conditions fortement réductrices dans les sédiments organiques inondés. Nos résultats soulignent l'importance écologique de la zone eulittorale dans des réservoirs à niveaux d'eau fluctuants, où la croissance des macrophytes peut être prise en compte par une gestion ciblée du niveau d'eau, ce qui aide les gestionnaires du réservoir à ...

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