MARTA COMERMA 1 , JUAN CARLOS GARCÍA 1 , JOAN ARMENGOL 1 , MARIA ROMERO 1 and KAREL ŠIMEK 2,3 1 Departament d’Ecologia, Facultat de Biologia, Universitat de Barcelona, Avgda. Diagonal 645, E-08028-Barcelona, Spain, E-mail: mcomerma@porthos.bio.ub.es 2 Hydrobiological Institute of the Academy of Sciences of the Czech Republic, and 3 the University of South Bohemia, Faculty of Biological Sciences, Na sádkách 7, CZ-37005 České Budějovice, Czech Republic. Planktonic Food Web Structure along the Sau Reservoir (Spain) in Summer 1997 key words: longitudinal gradients, reservoir, bacterial production, protistan bacterivory, Halteria grandinella Abstract We studied the planktonic food web in eutrophic Sau Reservoir (Catalonia, NE Spain). Along the longitudinal axis from the Ter River downstream to the dam, we characterized a microbial succession of food web dominance of bacteria-HNF-ciliates. The Ter River transports a large load of organic mate- rial into the reservoir, with a bacterial density of ~ 9 · 10 6 large cells per ml. While at the first lacus- trine station of the Reservoir HNF were the dominant bacterial consumers, at the others, an oligotrich ciliate, Halteria grandinella, was the main protozoan bacterivore. Most of the bacterial production in the reservoir epilimnion was consumed by grazing. The spatial succession of the reservoir microbial food webs was followed downstream by maximum densities of their potential predators among zoo- plankters – rotifers, and early developmental stages of copepods. 1. Introduction The microbial food web was initially described in oligotrophic ecosystems as a source or a sink for carbon which potentially mediated energy flow to higher trophic levels (POME- ROY, 1974). The pelagic microbial loop consists of bacteria, phagotrophic flagellates, cilia- tes, and other protists and is primarily fuelled by organic carbon released from phytoplank- ton exudates (AZAM et al., 1983). Bacteria make efficient use of the dissolved organic mat- ter (excretions from pelagic organisms), before being consumed by heterotrophic protozo- ans. The heterotrophic protozoans (mostly flagellates) are consumed by metazoan zoo- plankton, thereby channelling the energy from the microbial loop into the “classic” food chain (LAMPERT and SOMMER, 1997). A high biomass of picoplankton, especially that of bac- teria, can sequester a marked proportion of nutrients (N, P) and limit the overall efficiency and production of a system. The principal role of microbial consumers, namely of pha- gotrophic protozoa, is the liberation of the nutrients bound in the picoplankton biomass (CARON and GOLDMAN, 1990), thus mediating their availability to primary producers. In eutrophic systems the “new” production levels are high due to large allochthonous nutrient inputs (WEISSE and STOCKNER, 1993). Under such circumstances, the significance of micro- bial food webs is even so important in eutrophic and hypereutrophic lakes, as they frequently contribute > 50 % to the annual carbon production and nutrient cycling (WEISSE and STOCK- NER, 1993). Several studies exist on microbial food webs in lakes (e.g. BERMAN, 1990; RIEMANN and CHRISTOFFERSEN, 1993) and in the sea (e.g. AZAM et al., 1983). However, these communi- Internat. Rev. Hydrobiol. 86 2001 2 195–209