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.

The recognition and discrimination of phytoplankton species is one of the foundations of freshwater biodiversity research and environmental monitoring. This step is frequently a bottleneck in the analytical chain from sampling to data analysis and subsequent environmental status evaluation. Here we present phytoplankton diversity data from 49 lakes including three seasonal surveys assessed by next generation sequencing (NGS) of 16S ribosomal RNA chloroplast and cyanobacterial gene amplicons and also compare part of these datasets with identification based on morphology. Direct comparison of NGS to microscopic data from three time-series showed that NGS was able to capture the seasonality in phytoplankton succession as observed by microscopy. Still, the PCR-based approach was only semi-quantitative, and detailed NGS and microscopy taxa lists had only low taxonomic correspondence. This is probably due to, both, methodological constraints and current discrepancies in taxonomic frameworks. Discrepancies included Euglenophyta and Heterokonta that were scarce in the NGS but frequently detected by microscopy and Cyanobacteria that were in general more abundant and classified with high resolution by NGS. A deep-branching taxonomically unclassified cluster was frequently detected by NGS but could not be linked to any group identified by microscopy. NGS derived phytoplankton composition differed significantly among lakes with different trophic status, showing that our approach can resolve phytoplankton communities at a level relevant for ecosystem management. The high reproducibility and potential for standardization and parallelization makes our NGS approach an excellent candidate for simultaneous monitoring of prokaryotic and eukaryotic phytoplankton in inland waters.

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...

In this overview (introductory article to a special issue including 14 papers), we consider all main types of natural and artificial inland freshwater habitas (fwh). For each type, we identify the main biodiversity patterns and ecological features, human impacts on the system and environmental issues, and discuss ways to use this information to improve stewardship. Examples of selected key biodiversity/ecological features (habitat type): narrow endemics, sensitive (groundwater and GDEs); crenobionts, LIHRes (springs); unidirectional flow, nutrient spiraling (streams); naturally turbid, floodplains, large-bodied species (large rivers); depth-variation in benthic communities (lakes); endemism and diversity (ancient lakes); threatened, sensitive species (oxbow lakes, SWE); diverse, reduced littoral (reservoirs); cold-adapted species (Boreal and Arctic fwh); endemism, depauperate (Antarctic fwh); flood pulse, intermittent wetlands, biggest river basins (tropical fwh); variable hydrologic regime—periods of drying, flash floods (arid-climate fwh). Selected impacts: eutrophication and other pollution, hydrologic modifications, overexploitation, habitat destruction, invasive species, salinization. Climate change is a threat multiplier, and it is important to quantify resistance, resilience, and recovery to assess the strategic role of the different types of freshwater ecosystems and their value for biodiversity conservation. Effective conservation solutions are dependent on an understanding of connectivity between different freshwater ecosystems (including related terrestrial, coastal and marine systems).