Cladocerans are a considerable food source for such planktivorous fish as salmonids. In Latvia 26 lakes are categorized as priority fish lakes for salmonids and therefore studies on cladocerans as salmonid fish prey are of particular interest. For the study of salmonids' food base, i.e. zooplankton diversity, and the genetical diversity of Daphnia cucullata in four Latvian lakes zooplankton samples were taken in the summers of 2010 and 2011. The most dominant species of Cladocera were Diaphanosoma brachyurum, Daphnia cucullata, Bosmina crassicornis, Bosmina longispina, and Bosmina longirostris. The highest diversity and biomass of cladocerans were observed from June to late July. This can be explained by seasonality. The common cladoceran species such as Daphnia cucullata may be used as a good model organism for ecological genetics research. We found D. cucullata in all the investigated lakes. The genetic structure and plasticity in D. cucullata were studied using nuclear random amplified polymorphic DNA (RAPD). Carl Roth random primers PCR were used for the amplification of DNA samples. The DNA markers were polymorphic segments with band sizes from 500 to 3000 bp. The number of polymorphic DNA bands of D. cucullata in the four investigated lakes was different.
Vertical structure of zooplankton communities from five deep Latvian lakes during the late summer and early autumn season was analysed. Changes in species composition, abundance, and dominant species assemblages were assessed in relation to temperature, oxygen, pH, chlorophyll a, oxidation reduction potential, and conductivity. Where possible, comparison with historical data was made.No significant changes of ecological conditions and annual dynamics of zooplankton diversity and abundance during the last 50 years were observed. However, a decrease of oxygen concentration occurred in deep lakes, caused probably by the pressure of human induced eutrophication. Also, slight differences were observed in the species composition, which most likely mark changes in the trophic status.A total of 24 to 38 Rotifera and Cladocera species were found during investigations from all lakes. The dominant species were Conochilus hippocrepis, Keratella cochlearis, Polyarthra vulgaris, Filinia longiseta, Kellicottia longispina, Daphnia (Daphnia) cucullata, Diaphanosoma brachyurum, Bosmina (Eubosmina) crassicornis, and B. (Eubosmina) longispina.Zooplankton communities can be defined as distinct groups both at species and higher levels on the basis of seasonal stratification in deep lakes.Temperature, oxidation reduction potential, and chlorophyll a concentration were found to be statistically the most influential environmental factors in the vertical distribution of zooplankton. Also sampling date and depth showed strong trends concerning zooplankton abundance and species composition. Vertical segregation between samples occurred within water layers of 0-10 m, 10-20 m, and below 20 m depth. The samples collected at the beginning of the sampling season assembled into a separate group.
Lake Garais is the second deepest lake in Latvia. Its maximum depth is 56.0 meters, but its average depth is 16.5 meters. Lake Geranimovas-Ilzas is the fifth deepest lake in Latvia. Its maximum depth is 46.0 meters, but its average depth is 9.8 meters [13]. During the research of Lake Gernimovas-Ilzas and Lake Garais three zooplankton groups have been found, i.e. Rotatoria, Cladocera and Copepoda. Having analysed the quality composition of species the authors have concluded that the pollution level of these lakes is low, because there are such olygosoprobic zooplankton species as Keratella cochlearis, Keratella quadrata, Filinia longiseta, that die from lack of oxygen in few hours.
The research has been done on qualitative and quantitative composition of zooplankton in the Daugava between Suroza and Dunava. The investigation was carried out in July – August 2008. The collection of samples was done using the net with 65µ mesh. For analysing the abundance of zooplankton method APHA 10200 was applied. Samples from 21 places were analysed assembling from the right side, left side and midchannel of the river. Species Rotatoria, Cladocera and Copepoda were found and zooplankton communities and diversity of species in the Daugava left side, right side and midchannel were compared taking into account the factors affecting them.
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