The structure of phytoplankton communities was examined seasonally across five rivers with a focus on small cells and their relative importance. Picophytoplankton (0.2-2 μm), previously considered insignificant in rivers, reached densities as high as those observed in lakes and oceans (~ 10e4-10e5 cells/mL). Their relative importance was not a function of trophic state with the highest contribution to algal biomass found in the most eutrophic river. Body size distributions were analyzed from both chlorophyll-a size fractions and taxonomic enumerations; no significant effect of river or season was detected, suggesting that phytoplankton size distribution is not a useful metric of change in rivers. Unlike lake ecosystems, the rivers were uniformly dominated by small cells (< 20 μm). Taxonomic analyses of the seasonal succession did not reveal a common periodicity of particular divisions (e.g. diatoms). However, strong dominance was more typical of eutrophic rivers even though taxa richness was similar.

The structure of phytoplankton communities was examined seasonally across five rivers with a focus on small cells and their relative importance. Picophytoplankton (0.2-2 μm), previously considered insignificant in rivers, reached densities as high as those observed in lakes and oceans (~ 10e4-10e5 cells/mL). Their relative importance was not a function of trophic state with the highest contribution to algal biomass found in the most eutrophic river. Body size distributions were analyzed from both chlorophyll-a size fractions and taxonomic enumerations; no significant effect of river or season was detected, suggesting that phytoplankton size distribution is not a useful metric of change in rivers. Unlike lake ecosystems, the rivers were uniformly dominated by small cells (< 20 μm). Taxonomic analyses of the seasonal succession did not reveal a common periodicity of particular divisions (e.g. diatoms). However, strong dominance was more typical of eutrophic rivers even though taxa richness was similar.

The structure of phytoplankton communities was examined seasonally across five rivers with a focus on small cells and their relative importance. Picophytoplankton (0.2-2 μm), previously considered insignificant in rivers, reached densities as high as those observed in lakes and oceans (~ 10e4-10e5 cells/mL). Their relative importance was not a function of trophic state with the highest contribution to algal biomass found in the most eutrophic river. Body size distributions were analyzed from both chlorophyll-a size fractions and taxonomic enumerations; no significant effect of river or season was detected, suggesting that phytoplankton size distribution is not a useful metric of change in rivers. Unlike lake ecosystems, the rivers were uniformly dominated by small cells (< 20 μm). Taxonomic analyses of the seasonal succession did not reveal a common periodicity of particular divisions (e.g. diatoms). However, strong dominance was more typical of eutrophic rivers even though taxa richness was similar.

Artificial ponds are excellent for the study of phytoplankton ecology. They permit repeatable initial conditions and sufficient replication of independent experimental units in complex experiments to test hypotheses about the control of structure and function in natural communities. There were 20 experimental ponds constructed in Kokoř nsko Protected Landscape Area, from which samples were taken and analyzed. This study was part of the EU BIOPOOL. During the first two years after the filling of the ponds the colonization and the seasonal development of phytoplankton communities in the ponds were examined. The phytoplankton species composition was determined, then species richness and relative abundance of individual taxa were quantified. Species composition data were compared with the environmental parameters measured (temperature, oxygen concentration, pH, transparency, conductivity, depth, chlorophyll-a concentration, and zooplankton). In the spring 2007, the ponds were periodic, and followed a similar trend over time (February- April). There were dominated by the following taxonomic groups: Dinophyta, Chrysophyta, Cryptophyta, and Euglenophyta. After installing foil on the bottom of the ponds, the ponds were of permanent character. In terms of environmental parameters, the individual ponds...

Spatial distribution and seasonal succesion of phytoplankton along the longitudinal axis of a eutrophic Římov reservoir was investigated in 2007. Inflow, transitional and lacustrine zones were distinquished in the reservoir according to physical, chemical and biological parameters. Using a functional group concept, typical phytoplankton assemblages were found.

Bacterioplankton are keystone organisms in marine ecosystems. They are important for element cycles, by transforming dissolved organic carbon and other nutrients. Bacterioplankton community composition and productivity rates change in surface waters over spatial and temporal scales. Yet, many underlying biological processes determining when, why and how bacterioplankton react to changes in environmental conditions are poorly understood. Here, I used experiments with model bacteria and natural assemblages as well as field studies to determine molecular, physiological and ecological responses allowing marine bacteria to adapt to their environment. Experiments with the flavobacterium Dokdonia sp. MED134 aimed to determine how the metabolism of bacteria is influenced by light and different organic matter. Under light exposure, Dokdonia sp. MED134 expressed proteorhodopsin and adjusted its metabolism to use resources more efficiently when growing with lower-quality organic matter. Similar expression patterns were found in oceanic datasets, implying a global importance of photoheterotrophic metabolisms for the ecology of bacterioplankton. Further, I investigated how the composition and physiology of bacterial assemblages are affected by elevated CO2 concentrations and inorganic nutrients. In a large-scale experiment, bacterioplankton could keep productivity and community structure unaltered by adapting the gene expression under CO2 stress. To maintain pH homeostasis, bacteria induced higher expression of genes related to respiration, membrane transport and light acquisition under low-nutrient conditions. Under high-nutrient conditions with phytoplankton blooms, such regulatory mechanisms were not necessary. These findings indicate that open ocean systems are more vulnerable to ocean acidification than coastal waters. Lastly, I used field studies to resolve how bacterioplankton is influenced by environmental changes, and how this leads to seasonal succession of marine bacteria. Using high frequency sampling over three years, we uncovered notable variability both between and within years in several biological features that rapidly changed over short time scales. These included potential phytoplankton-bacteria linkages, substrate uptake rates, and shifts in bacterial community structure. Thus, high resolution time series can provide important insights into the mechanisms controlling microbial communities. Overall, this thesis highlights the advantages of combining molecular and traditional oceanographic methodological approaches to study ecosystems at high resolution for improving our understanding of the physiology and ecology of microbial communities and, ultimately, how they influence biogeochemical processes.