Changes in the epilimnetic bacterial community composition, production, and protist-induced mortality along the longitudinal axis of a highly eutrophic reservoir

We studied changes in the epilimnetic bacterial community composition (BCC) , bacterial biomass and production, and protistan succession and bacterivor y along the longitudinal axis of the canyon-shaped, highly eutrophic Sau Re servoir (NE Spain) during two sampling campaigns, in April and July 1997. L ongitudinal changes in BCC from the river inflow to the dam area of the res ervoir were detected by using oligonucleotide probes targeted to the kingdo m Bacteria, to the alpha, beta, and gamma subclasses (ALFA, BETA, and GAMA) of the class Proteobacteria, and to the Cytophaga/Flavobacterium (CF) clus ter. In general, the inflow of the organically loaded Ter river, with highl y abundant allochthonous bacterial populations, induced a clearly distingui shable longitudinal succession of the structure of the microbial food web. The most dynamic changes in microbial parameters occurred at the plunge poi nt, the mixing area of river water and the reservoir epilimnion. Changes wi thin members of BETA and CF were the most important in determining changes in BCC, bacterial abundance and biomass. Much less relevant changes occurre d within the less abundant ALFA and GAMA bacteria. From the plunge point do wnstream, we described a significant shift in BCC in the form of decreased proportions of BETA and CF. This shift spatially coincided with the highest values of heterotrophic nanoflagellate bacterivory (roughly doubled the ba cterial production). CF numerically dominated throughout the reservoir with out any marked longitudinal changes in their mean cell volume. In contrast, very large cells affiliated to BETA clearly dominated in the allochthonous bacterial biomass brought by the river. BETA showed a marked downstream tr end of decreasing mean cell volume. We conclude that the observed BCC shift and the longitudinal shift in food web structure (bacteria-heterotrophic n anoflagellates-ciliates) resulted from highly complex interactions brought about by several major factors: varying hydrology, the high localized alloc hthonous input of organic matter brought by the river, downstream changing substrate availability, and selective protistan bacterivory.