Hybridizing species of the Daphnia longispina complex are key taxa in plankton communities of many European lakes. In several of these lakes, it has been documented that the taxonomic structure of the complex during the 20th century has substantially changed following human-mediated environmental changes, particularly eutrophication and re-oligotrophication and fish stock changes. We characterize these changes in Lago Maggiore (Italy/Switzerland), a southern pre-alpine lake, which also passed through the human-mediated environmental changes. Lago Maggiore is one of best and longest studied European lakes so a large number of historical Daphnia samples from regular monitoring are available. Because local Daphnia do not form dormant egg banks suitable for genetic analysis (being able to overwinter in the water column), we used a combination of morphology and geometric morphometrics (elliptic Fourier analysis) to evaluate the taxonomic and phenotypic changes in the Daphnia longispina complex in Lago Maggiore since the mid-20th century (1948-2012), and attampted to characterize the impact of environmental changes over the respective period on Daphnia body shape and size. Examination of Daphnia phenotype indicates indeed the prevailing presence of D. longispina (hyalina morph) in the 1940s, dominance of...

This thesis focuses on ecological processes among plankton organisms. It especially questions the influence of food quality on growth and survival of freshwater plankton. I focused on the influence of various ratios of nitrogen and phosphorus in available food. I am also interested in finding out the food requirements of chosen consumer Daphnia longispina. Within this broad topic, I specify a question, whether various quality and quantity of food influences the life history parameters of the species Daphnia longispina and whether the absence of common species of Cladoceran Daphnia longispina in Plešné Lake is caused by unsuitable quality of food. I have bred Daphnia longispina in laboratory conditions for two years. I fed it by algae Monoraphidium dybowskii isolated from Plešné Lake and grown in two media P-limited (C:P 1174), P-unlimited (C:P 290) and three concentration (1,2, 4 mg C/L). Cladocerans fed by food from medium that had a C:N:P ratio approximately the same as can be found in Plešné Lake (C:P 1174) lived for a singificantly shorter period of time and during the experiment they did not reproduce even in the case with the largest amount of food (4 mg C/L). This is a possible reason why Daphnia longispina did not return to Plešné Lake after the period of strong acidification, climax of...

The original aim of my diploma thesis was examination of fixed samples of cladocerans from the Daphnia longispina species complex from reservoirs Vír and Vranov for the presence of 4 groups of microparasites: microsporidia, oomycetes, the protozoan Caullerya mesnili, and the yeast Metschnikowia bicuspidata. The next aim was to determine the effect of parasites on Daphnia fecundity, and determination of their spatial and temporal distribution within reservoirs. From the total number 4452 of examined Daphnia females, however, only 56 individuals were infected. This number was not sufficient for the planned analyses. As I recorded for all examined individuals the clutch size, I thus dealt to a large extent with an alternative issue: the temporal and spatial variation of, and the influence of environmental factors on Daphnia fecundity. Both studied reservoirs are characteristic by canyon-shaped profile that allows formation of environmental gradients on the horizontal as well as vertical axis. The results confirm that reservoir identity, season, and location within the reservoir (or gradient of food supply) have all significant effects on fecundity. During my work I have encountered difficulties associated with determining microparazites from fixed zooplankton samples. The appendix section of my thesis...