Changes in bacterial community composition and microbial activities along the longitudinal axis of two canyon-shaped reservoirs with different inflow loading

Abstract

Changes in microbial activities and bacterial community composition (BCC) were studied using fluorescence in situ hybridization (FISH) along the longitudinal axis of two canyon-shaped reservoirs with contrasting retention times and nutrient and organic matter loads. The reservoirs showed different specific longitudinal changes in plankton communities and BCC shifts that were related mainly to allochthonous input and longitudinal patterns of primary production and nutrient availability. The studied branch of the Orlik reservoir is entered by a large river and so had a short retention time (23 days) and a large allochthonous input. Primary production, bacterial abundance, bacterial production and protozoan bacterivory showed a consistent trend, decreasing gradually from the river inflow downstream. Overall, shifts in BCC were minor and statistically insignificant. The relatively large input of the river-borne allochthonous plankton communities and the short retention time of the reservoir probably limited the effect of the autochthonous in-reservoirs processes. In contrast, the Řimov reservoir has a long retention time (∼100 days) and had a limited input of allochthonous organic matter to its lacustrine area. The autochthonous processes of primary and bacterial production were tightly linked and they showed peaks in the middle part of the studied stretch. This together with a trend of downstream decreasing nutrient concentrations and increasing protistan grazing pressure seem to support the development of specific BCC in the lacustrine part of the reservoir that significantly differed from that in river inflow. Apart from the marked differences between the limnological variables of reservoirs we found some commonalties in BCC – in both systems the phylogenetic groups of β-Proteobacteria and Cythophaga/Flavobacterium clearly dominated bacterioplankton assemblages. %