Jaromir Seda

Long-term biomanipulation of Rimov Reservoir (Czech Republic)

The history of fish, zooplankton and phytoplankton in RimovReservoir during its 14 years existence is documented. Afterthe filling of the reservoir in autumn 1978, the biomass ofplanktivorous fish increased from near zero to 650 kg ha-1 bythe fourth year of the reservoir‘s existence, after which thefish stock continually decreased under various forms ofbiomanipulative pressure, down to a value of about 100 kg ha-1. Thisdevelopment of fish stock was accompanied by changes in zooplanktonsize structure, but a cascading effect down to reduction ofphytoplankton biomass was not pronounced. The changes ofzooplankton body size structure were: increasing or decreasingaverage size of Daphnia galeata (the main cladoceranherbivore species) and changes in relative proportion ofcladoceran species, but none of the main zooplankton species was replacedor disappeared. The variation in phytoplankton biomass wasexplained by variation in phosphorus concentration and not related tozooplankton structure. It was concluded that the criticalbiomass of planktivorous fish below which the effect of loweringphytoplankton biomass could be observed lies below 100 kg ofplanktivorous fish per hectare. An attempt is made to quantifythe relationship between fish stock density and the size structureof zooplankton potentially able to influence top-down effects onphytoplankton.

Daphnia hybridization along ecological gradients in pelagic environments: the potential for the presence of hybrid zones in plankton.

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 (=hyalina) 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 hybrid-dominated zone in intermediate conditions suggests that local Daphnia hybrid zones may indeed form within reservoirs.

Trophic structure of nine Czech reservoirs regularly stocked with piscivorous fish

For nearly 20 years, most Czech reservoirs supplying drinking water have been under statutory protection which permitted reservoir managers to manipulate fish stocks in order to maintain a sustainable water quality. The most common biomanipulative measure adopted was stocking with piscivorous fish (mostly 5 cm fry) using an annual stocking level of approx. 25 000 fish per reservoir. Nine reservoirs were studied for signs of top-down food web effects, as predicted by the trophic cascade hypothesis based on levels of total phosphorus (TP), chlorophyll a (Chl a), zooplankton biomass (ZB) and zooplankton community structure. In all nine reservoirs, only small Daphnia species were recorded, such as D. galeata and D. cucullata. The proportion of large-bodied daphnids retained on a 0.71 mm sieve was less than 10% of the total crustacean biomass in all reservoirs. The relationship of Chl a level – TP, and of ZB – Chl a, was positive under enhanced piscivory and did not differ statistically from the relationships in other reservoirs with natural fish stocks. This implies that bottom-up forces remained stronger than top-down ones in the studied reservoirs, despite the stocking of piscivorous fish. The failure of this attempt at biomanipulation may be due to an insufficient stocking rate of predatory fish and/or inadequate data on the resident planktivorous fish levels.

Daphnia galeata in the deep hypolimnion: spatial differentiation of a ''typical epilimnetic'' species

Daphnia galeata is traditionally regarded to be a non-migratory species, which lives in warm epilimnetic waters. Depth segregation or vertical migration is usually attributed to other Daphnia species such as D. hyalina or D. longispina. In a two-year study, we found that in a deep, dammed-valley reservoir (Římov Reservoir, Czechia) the majority of the population of D. galeata lives in the warm epilimnetic waters during the summer months, but some specimens of this species could be always found in the deep strata as well. This hypolimnetic subpopulation stays in the cold hypolimnetic water and does not migrate. The abundance of hypolimnetic D.galeata does not exceed one specimen per litre and usually shows seasonal variation (minimal densities in early spring, maximal in late summer). Using allozyme electrophoresis, we found that the subpopulation from the deep hypolimnion was clearly genetically differentiated from the population in the epilimnion. We found significant differences in both allele and multilocus genotype frequencies; the FST values at most sampling dates exceeded 0.05. However, the spatial segregation between the epilimnetic and hypolimnetic subpopulations is not permanent. The reservoir is dimictic and hence, at least twice per year, all vertically segregated parts of the population are mixed together. Our results suggest that the deep hypolimnetic subpopulation is repeatedly re-established in spring by deepwater “colonists”, at least some of which seem to be ecologically specialised for the hypolimnetic conditions, and dominate the hypolimnion by the end of the season. The genetic differentiation is likely the result of both the different depth preferences of various D. galeata clones and different selective pressures in the epilimnion and hypolimnion.

Cyprinid predation on zooplankton along the longitudinal profile of a canyon-shaped reservoir

In the deep, elongated řimov Reservoir (South Bohemia, Czech Republic), the diet composition of the three most abundant cyprinid species, as well as the fish and zooplankton distribution. were investigated at three sites (dam, middle, tributary) located along the longitudinal transect of the reservoir, and during two periods of the year. The studied fish were roach (Rutilus rutilus) and bream (Abramis brama) of two length classes (10-18 cm SL and >18 cm SL), and bleak (Alburnus alburnus) of one length class (10-18 cm SL). The diets of all cyprinids along the entire longitudinal transect constituted almost exclusively crustacean zooplankton. In May, Daphnia galeata was a highly preferred prey item of all fish, except for those of the 10-18 cm length class in the tributary station, which consumed mainly bosminids. In August, bleak and roach of both length classes preyed on D. galeata and Leptodora kindtii. Larger bream (> 18 cm SL) consumed these prey items as well and, moreover, fed significantly on Diaphanosoma brachyurum. Proportions of L. kindtii in the cyprinid diets were particularly conspicuous in the dam and middle sites In the tributary site, cyclopoid copepods were an important prey item in the diets of both length classes of bream, in contrast to their infrequency in the diets of roach and bleak. Densities of epilimnetic zooplankton declined gradually downstream in the reservoir, they were about two-fold higher in the tributary site compared to the dam site in both months, respectively. A relatively higher total fish catch per unit effort was found in the tributary site in comparison to the dam or middle site, and a decreasing mean size of D. galeata in zooplankton was observed from the dam to the tributary site in both months. Such results indicated stronger fish predation pressure in the upstream part of the reservoir during the two periods of the year. Moreover, lower diet overlaps within the cyprinid assemblage of the tributary site than within the cyprinid assemblages of the dam and middle sites suggested intense competition for food at the upstream end of the reservoir.

Rotifer fecundity in relation to components of microbial food web in a eutrophic reservoir

The relationship among rotifers and their potential food sources such as bacteria, heterotrophic nanoflagelates and algae were examined in the eutrophic Římov reservoir. The egg ratio of five rotifer species (Polyarthra spp., Keratella cochlearis, K. quadrata, Kellicottia longispina and Synchaeta spp.) was estimated and related to the concentration of food particles. Generally, the highest egg ratio was found during the spring time, before the clear water phase, although the densities of some species reached their highest numbers in summer. All investigated food sources were found to have a significant relationship to rotifer egg ratio. The fecundity of all species, except Synchaeta, depend on chlorophyll-a. The Monod curve of food-fecundity relationship reflects that K. cochlearis and P. dolichoptera have high affinities to lower concentrations of chl-a. This is in contrast to K. longispina and K. quadrata, which maximized their egg ratio under higher food concentrations. The two components of the microbial food web (bacterial carbon and HNF) were correlated with fecundity but only for K. cochlearis. The seasonal differences in food-fecundity relationships for K. cochlearis andK. longispina indicated that there might be differential food preferences of these two species during the season.

Fine-scale temporal and spatial variation of taxon and clonal structure in the Daphnia longispina hybrid complex in heterogeneous environments

BackgroundCyclical parthenogenetic water fleas of the genus Daphnia have become a prominent model organism in ecology and evolution. In the past, analyses of their population structure have been limited by the prevailing use of allozyme markers, which in general do not allow for the distinction of individual clones. In this study, we used 10 microsatellite markers to track changes in the taxonomic and clonal composition of Daphnia populations, and traced the abundance of the most common clones in two European reservoirs. One of the localities was inhabited by a single species of the Daphnia longispina complex (D. galeata), the other by two parental species (D. galeata and D. longispina) and their interspecific hybrids. The study took place during the transition from summer stratification to autumn mixing, representing a period of major environmental change within lake habitats.ResultsIn both reservoirs, we observed temporal (generation-to-generation) and spatial (along the heterogeneous reservoir environment) changes in Daphnia community structure. In the single-species reservoir, the clonal diversity of D. galeata increased with time, as a few dominant clones were replaced by a higher number of less common clones. A loss in selective advantage for the dominant clones may have been due to gradual changes in the environment, or due to selection acting in a negative frequency-dependent manner. In the multispecies reservoir, there were no apparent temporal trends in clonal diversity but we observed significantly lower clonal diversity in the interspecific hybrids than in the coexisting parental species, supporting the existence of reproductive barriers between the parental genomes.ConclusionsOur study, tracing clonal lineages of Daphnia in time and space by the fine-resolution markers, contributes to the understanding of how clonal reproduction impacts community structure in cyclically parthenogenetic organisms.

Fine-scale genetic analysis of Daphnia host populations infected by two virulent parasites – strong fluctuations in clonal structure at small temporal and spatial scales

Numerous theoretical studies suggest that parasites impose a strong selection pressure on their host, driving genetic changes within host populations. Yet evidence of this process in the wild is scarce. In the present study we surveyed, using high resolution microsatellite markers, the genetic structure of cyclically parthenogenetic Daphnia hosts within two different Daphnia communities belonging to the Daphnia longispina hybrid complex. One community, consisting of a single host species, was infected with the protozoan parasite Caullerya mesnili. The second community consisted of two parental Daphnia spp. and their hybrids, and was infected with the yeast parasite Metschnikowia. Significant differences in the clonal composition between random and infected sub-samples of Daphnia were detected on several occasions within both communities, indicating that host genotypes differ in resistance to both parasites. In addition, one parental species in the multi-taxon community was consistently under-infected, compared with the other taxa. Overall, our field data confirm that infection patterns are strongly affected by host genetic composition in various Daphnia-microparasite systems. Thus, parasite-driven selection operates in natural Daphnia populations and microparasites influence the clonal structure of host populations.

A low prevalence of mycobacteria in freshwater fish from water reservoirs, ponds and farms.

A survey of the occurrence of mycobacteria was conducted from 717 freshwater fish (25 species) in two water reservoirs, five ponds and two farms in the Czech Republic. A total of 2182 tissue samples from these fish were examined using the conventional culture method. Thirteen mycobacterial isolates were obtained from 12 (1.7%) fish belonging to nine species. Isolates were identified using sequence analysis of the 16SrRNA gene as: Mycobacterium algericum, M. fortuitum, M. gordonae, M. insubricum, M. kumamotonense, M. nonchromogenicum, two isolates of M. peregrinum, M. terrae and M. triplex. Mycobacteria were isolated more frequently from fish skin and gills than from internal organs or muscles.

The stabilizing effect of resting egg banks of the Daphnia longispina species complex for longitudinal taxon heterogeneity in long and narrow reservoirs

We compared the spatial distribution of taxa from the Daphnia longispina complex (D. longispina, D. galeata, D. cucullata, and their hybrids) in the active water column community and in resting egg banks in five long narrow reservoirs in the Czech Republic (Central Europe). In each reservoir, we sampled at both ends of the longitudinal gradient: in the inflow region and at the dam. Ephippia abundance in the sediments significantly increased in the downstream direction, reflecting differences in the sedimentation regime and Daphnia population size. Similarly to the active zooplankton community, in which D. cucullata and D. longispina tended to occur at opposite ends of the reservoirs, Daphnia species and interspecific hybrids in resting eggs revealed a spatially diversified pattern; however, we observed some differences in taxon distributions between sediments and water columns. High relative abundances of hybrid genotypes (up to 16% of resting eggs, and 74% of Daphnia in the water column) confirm that interspecific hybridization is frequent in these reservoirs, and some hybrids are successful in competition with the parental taxa. We assume that the spatial heterogeneity of Daphnia taxonomic composition in reservoirs, being affected by the seasonal selection of taxa within the mixed reservoir species pool, is substantially strengthened by the presence of spatially heterogeneous egg banks.