MICROBIAL ECOLOGY Microb Ecol (2000) 40:317-329 DOI: 10.1007/s002480000056 92000 Springer-VerlagNew York Inc. The Microbial Food Web in the Recently Flooded Sep Reservoir: Diel Fluctuations in Bacterial Biomass and Metabolic Activity in Relation to Phytoplankton and Flagellate Grazers L.-B. Jugnia, R.D. Tadonl6k4, T. Sime-Ngando, J. Devaux Laboratoire de Biologie des Protistes, UMR CNRS 6023, Universit6 Blaise Pascal (Clermont-Ferrand II), 63177 Aubi6re Cedex, France Received: 16 December 1999; Accepted: 2 June 2000; Online Publication: 29 September 2000 |ABSTRACT The spatial distribution of the bacterial biomass and production and of potential heterotrophic activity (PHA) were measured every 4 h between 23 July (10:00 h) and 25 July (10:00 h) 1997 in a recently flooded oligo-mesotrophic reservoir (the Sep Reservoir, Puy-de-D6me, France), in relation to temperature, the phytoplankton biomass and production, and the abundance of heterotrophic flagellates. The temperature varied slightly with time during the study, but the well-established thermal stratification agreed well with vertical distribution of the biological variables that were measured. Only the bacterial production and the PHA showed significant diel changes (t-test, p < 0.05), with maxima at 18:00 h and minima at 02:00 h. A significant positive relation was found between bacterial abundance and that of heterotrophic flagellates, which, rather than being an association related to the thermal stratification of the water column, was considered to reflect a trophic relation between these two communities. A carbon balance analysis suggested that at least 30% of the C from primary production measured during the sampling period was used by bacteria, and that 42% of this secondary production, or 6% of the primary production, would be used for the development of the heterotrophic flagellates present. We conclude that the bacterioplankton forms, at least occasionally, an important source of carbon for higher trophic levels, and reject the hypothesis that bacterial production in the Sep Reservoir depends exclusively on organic matter of allochthonous origin. Introduction Bacteria are known to efficiently decompose organic matter and regenerate minerals in aquatic ecosystems, and their Correspondence to: T. Sime-Ngando;Fax: +33 4 73 40 76; Emaih Telesphore. SIME-NGANDO@lpb'univ-bpclerm~ abundance represents an index of heterotrophic activity [31, 45]. Their role as a food source for protozoa [21, 57] also makes them an important compartment in the carbon flux within the microbial food web [3]. This food web plays an important role in the regulation of carbon transfer and must be taken into account if more realistic carbon budgets are to