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The influence of seasonal variations on the chemical composition and composition of fatty acids in five commercially important freshwater fish species from the Danube: white bream, bream, vimba, zope, and Prussian carp, during May, July and September was determined. Changes in the chemical composition of meat of all examined species had the same tendencies. Water and protein content in the meat decreased, while fat content increased. The most frequent fatty acids in the meat of all the examined fish were the following: 18:1 n-9 (oleic), 16:0 (palmitic), 16:1 (palmitoleic), 18:2 n-6 (linoleic), 20:1 (eicosenoic), 20:5 n-3 eicosapentaenoic acid (EPA), 20:4 n-6 arachidonic acid and 22:6 n-3 docosahexaeonic acid (DHA). The content of saturated fatty acids (SFA) ranged from 25.03% to 32.43% and displayed a tendency to increase during the observed period. The total content of the n-6 group in the meat of Prussian carp was higher than in other species, which was probably a consequence of specific diet. The total content of n-3 fatty acids in the meat of white bream, bream, vimba and zope was the highest in May, and it declined during July-September. We can conclude that the meat of white bream and vimba contains high nutritional values in terms of EPA and DHA content. The n-3/n-6 ratio was also very favourable: 0.9 to 2.0 in the meat of white bream, bream, vimba and zope, with a clear downward tendency in the observed period.

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In 1992 a 120 km long section of the Danube river, part of the border line between Hungary and Slovakia, was diverted into a new riverbed to put into operation the Gabčikovo (Bős-Nagymaros) Hydropower Plant. To follow up the environmental changes a monitoring system in the Szigetköz region, seriously affected by the diversion of the river, was established by the Hungarian Academy of Sciences. Results of the bryological monitoring work, conducted in the river branches are presented in this article. Today, the species composition of the aquatic-riparian bryophyte vegetation living in the various sections of the Szigetköz branch-system is different from that of 1991–1992. The abundance-frequency values of aquatic species have decreased, while the proportions of mesophilous long-lived species and short-lived bryophytes have increased. The changes of water requirement spectra of bryophyte vegetation and the growing importance of certain species groups indicate that the ecological conditions became drier. Apparently, the water supply system operated from 1995 provides insufficient amount of water and is inadequate to stop (and even less so to reverse) the environmental changes that took place in the branch-systems.

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The spatial distribution of bivalves in relation to environmental conditions was studied along a second- and third order stream — medium-sized river (River Ipoly) — large river (River Danube) continuum in the Hungarian Danube River system. Quantitative samples were collected four times in 2007 and a total of 1662 specimens, belonging to 22 bivalve species were identified. Among these species, two are endangered (Pseudanodonta complanata, Unio crassus) and five are invasive (Dreissena polymorpha, D. rostriformis bugensis, Corbicula fluminea, C. fluminalis, Anodonta woodiana) in Hungary. The higher density presented by Pisidium subtruncatum, P. supinum, P. henslowanum and C. fluminea suggests that these species may have a key role in this ecosystem. Three different faunal groups were distinguished but no significant temporal change was detected. The lowest density and diversity with two species (P. casertanum and P. personatum) occurred in streams. The highest density and diversity was found in the River Ipoly, in the side arms of the Danube and in the main arm of the Danube with sand and silt substrate, being dominated by P. subtruncatum and P. henslowanum. Moderate density and species richness were observed in the main arm of the Danube with pebble and stone substrate, being dominated by C. fluminea and S. rivicola. Ten environmental variables were found to have significant influence on the distribution of bivalves, the strongest explanatory factors being substrate types, current velocity and sedimentological characteristics.

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Birk, S., Van Kouwen, L. and Willby, N. 2012. Harmonising the bioassessment of large rivers in the absence of near-natural reference conditions — a case study of the Danube River. Freshwater Biol. 57: 1716

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72 400 409 Stanic, B., Andric, N., Zoric, S., Grubor-Lajsic, G., Kovacevic, R. (2006) Assessing pollution in the Danube River near Novi Sad (Serbia

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. Typology of a great river using fish assemblages: Implications for the bioassessment of the Danube River . River Res. Appl. 33 : 37 – 49 . Fagan , W.F. , 2002

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. Süveges 2005. Origin of shallow groundwater of Csepel Island (south of Budapest, Hungary, Danube River): isotopic and chemical approach. Hydrological Processes (in press). Molnár Zs. and Borhidi. A. 2003. Hungarian

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outskirts of the village of Dunavecse, on the former floodplain of the Danube River. The soils are slightly saline and have a sandy-loamy texture, with increasing average particle size along the depth of the profile. The water table is shallow and saline

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. Poll. 182 . 53 – 62 . O nderka , M. & P ekárová , P. , 2008 . Retrieval of suspended particulate matter concen-trations in the Danube River from Landsat ETM data . Science of

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