We employed high frequency sampling to determine the major factors modulating microbial food-web composition and dynamics through the spring phytoplankton bloom and clear-water phases in a freshwater reservoir. We examined effects of a changing trophic structure of the planktonic community cascading from the level of zooplankton, through phytoplankton composition and exudation rates to the level of growth responses and losses to grazers of phylogenetically narrow bacterial lineages. Specific probes for fluorescence in situ hybridization (FISH) revealed important short-lived peaks of genus-like (Fluviiciola sp. and Limnohabitans spp.) or even taxonomically narrower populations of Betaproteobacteria and Flavobacteria (such as FlavB and Flav2 lineages). Protozoan grazing on bacterioplankton was studied by using fluorescently labeled bacteria and by direct analyses of FISH-probe-targeted bacterial phylotypes in flagellate food vacuoles. Evaluations of selective bacterivory, growth responses, and cell biovolumes of various bacterial groups during the spring bloom indicated that certain bacterial groups such as Limnohabitans can contribute to carbon flow to the grazer food chain up to 10-fold more than similarly abundant small cells from the Ac1 lineage of Actinobacteria. During the clear-water phase, filter-feeding cladocera had dominant effects on bacterioplankton abundance and community dynamics, likely through direct grazing on larger bacteria along with altering major substrate pools (via e.g., the herbivores' sloppy feeding on algae). Fine-temporal resolution data revealed several environmental scenarios, in which the interplay of distinct top-down and bottom-up factors resulted in a competitive advantage of particular bacterial lineages.

Seasonal changes in the abundance and production of epilimnetic bacterioplankton, protistan abundance and bacterivory, and extracellular phytoplankton production (EPP) were studied at 3 sampling stations (DAM, MIDDLE and RIVER) located along the longitudinal axis of the canyon-shaped, meso-eutrophic Ř ímov reservoir (Czech Republic). We found that at the river inflow (RIVER) compared to lacustrine parts of the reservoir (MIDDLE and DAM), different sources of organic carbon and of bacterial mortality control bacterioplankton dynamics and community composition. At the RIVER site, EPP accounted for a negligible part of bacterial carbon demand, thus indicating the prominent role of allochthonous sources of organic substrates in the river inflow. In addition, protistan bacterivory removed there, on average, only 9% of bacterial production. In contrast, at the lacustrine MIDDLE and DAM stations, protistan bacterivory accounted for 47 and 78% of bacterial production, respectively. Moreover, at these stations EPP was an autochthonous source of organic carbon sufficient to meet bacterial carbon demand and EPP was tightly correlated with bacterial carbon demand (DAM, r 2 = 0.589, p < 0.005; MIDDLE, r 2 = 0.716, p < 0.001). At the DAM site, we analyzed changes in EPP in relationship to phytoplankton community dynamics and found that cryptophytes were associated with EPP. Only 2 algal groups, cryptophytes in a spring-early-summer period and diatoms in a summer-fall period, clearly dominated the phytoplankton. Changes in phytoplankton composition were related to changes in bacterial community composition studied by means of group-specific rRNA-targeted oligonucleotide probes. A trend of increased proportions of certain bacterial groups, mainly of the genus-like R-BT065 subcluster of Betaproteobacteria, was detected for the periods of high EPP levels, dominated by cryptophytes. More than 52% of the seasonal variability in the abundance of the R-BT065 cluster was explained by changing EPP levels that indicated a tight taxon-specific algal-bacterial relationship.KEY WORDS: Reservoir · Bacterioplankton composition and production · Protistan bacterivory · Phytoplankton community · Extracellular phytoplankton production · Algal-bacterial relationships · Betaproteobacterial groups Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 51: [249][250][251][252][253][254][255][256][257][258][259][260][261][262] 2008 nounced differences exist with regard to canyonshaped reservoirs, since they are spatially highly heterogeneous systems due to relatively short water retention times and often show pronounced longitudinal heterogeneity (Thornton et al. 1990). In reservoir systems, nutrient and organic matter loads in riverine input combined with morphology and hydraulic retention time of a reservoir are the major factors affecting downstream plankton succession, rates of biological processes and resulting water quality (Comerma et al. 2003, Ma$ín et al. 2003.In common with other aquatic...

The relative homogeneity of pelagic environments has been regarded as the reason for the absence of hybrid zones for hybridizing planktonic Daphnia (Crustacea: Cladocera); occasional dominance of interspecific hybrids over parental species was explained by their temporal superiority in fluctuating environments. However, water bodies with spatially varying environmental conditions might facilitate the formation of hybrid zones in plankton. We studied the distribution of species and hybrids of the Daphnia longispina complex in 11 canyon-shaped reservoirs, localities characterized by horizontal environmental gradients (particularly of food supply and size-selective predation); we also analysed patterns of carapace size and fecundity among coexisting taxa. Spatial distribution of taxa agreed with their ecological characteristics; those showing different affinities along longitudinal reservoir profiles differed in size according to the presumed fish predation gradient. Only hybrids of Daphnia galeata with Daphnia cucullata and D. longispina (Zhyalina) were recorded. The latter two species preferred opposite ends of gradients, such spatial segregation probably explaining the absence of their hybrids. Distributional patterns were relatively stable in two consecutive summers, apart from a substantial decline of D. galeata!cucullata in the second year. The observed pattern of a hybriddominated zone in intermediate conditions suggests that local Daphnia hybrid zones may indeed form within reservoirs.

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The distribution and diet of age 0+ fish were studied in the deep canyon-shaped Římov Reservoir (Czech Republic), which is characterized by a longitudinal trophic gradient. During late summer of two years, 0+ fish were sampled from inshore and offshore habitats along the longitudinal reservoir axis. Offshore catches of 0+ fish from the surface layer were dominated by roach (Rutilus rutilus), bream (Abramis brama) and perch (Perca fluviatilis), whereas in the deeper open water perch predominated. Inshore catches of 0+ fish were constituted mainly by perch and roach. The proportions of roach in the inshore catches were highest at the upper and most eutrophic part of the reservoir, whereas the proportions of perch in the inshore catches were higher at the downstream areas. Total catches of both inshore and offshore 0+ fish increased upstream in the reservoir. Offshore 0+ perch were of consistently smaller size than inshore 0+ perch. Inshore 0+ perch had significantly smaller size at the upstream reservoir part than at the downstream, more lacustrine regions. The diet of both inshore and offshore 0+ fish consisted predominantly of crustacean zooplankton. Perch diet was generally dominated both by cladocerans and copepods, whereas roach diet consisted chiefly of cladocerans.

and 2005, an extensive study of the fish community was carried out in the largest water supply reservoir in the Czech Republic and Central Europe, the canyon-shaped Ž elivka Reservoir, using a fleet of Nordic multimesh gillnets. Fishes were sampled at eight locations along the longitudinal profile of the reservoir and at five benthic depth layers covering depths from the surface down to 18 m (benthic gillnet 1Á5 m high), and at three pelagic depth layers down to the depth of 5 m above the bottom (pelagic gillnets 4Á5 m high). Catches of both juvenile (age 0þ year) and adult (fishes >1 year) fishes were highest in the upper layers of the water column (i.e. in the epilimnion down to 5 m, and down to 10 m in the benthic habitats). Along the tributary-dam axis in the pelagic habitats, both juvenile and adult fishes preferred the upper part of the reservoir, where the maximum number of species and also the greatest abundance of zooplankton were found. In the benthic habitats, fishes selected location according to factors other than trophic status. More juvenile fishes were recorded in the benthic habitats than in the pelagic habitats. Depth had the largest explanatory power for predicting fish community composition, followed by the affiliation with benthic and pelagic habitats, and location on the longitudinal axis of the reservoir. The fish community was represented mainly by cyprinids and consisted of two distinct groups of species, with bleak Alburnus alburnus, rudd Scardinius erythrophthalmus and asp Aspius aspius dominating the offshore group while perch Perca fluviatilis and ruffe Gymnocephalus cernuus were affiliated with the inshore group of the adult fish community. Roach Rutilus rutilus, bream Abramis brama and pikeperch Sander lucioperca occurred in important proportions in both the inshore and the offshore zones. All species, with the exceptions of adult perch (1þ year and older), 0þ year perch and 0þ year roach, preferred the most eutrophic tributary part of the reservoir. The fish community was relatively stable between the 2 years sampled.

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