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The aim of this study was to reveal the spatial and temporal variation in biomass, density, and size structure of the invasive clam, Corbicula fluminea in the Danube River catchment and to explore the environmental factors determining these patterns. Samples were taken seasonally during the years 2007 and 2008 at 15 sites located along a river continuum from a second order stream to the Danube River. C. fluminea was the most abundant species in the main arm of the Danube; however, it was found in great density in the side arms, too. In the Ipoly River it only occured close to the conjuction, and it was not present at all in the second and third order streams. Regression models were developed to predict dry mass (DW, g) from linear body dimension [shell length (L), mm]. For the description of length-dry mass relationship, the power function (DW = a * Lb) was applied. The highest density, biomass, and mean shell length were 178 ind. m−2, 24.9697 g DW m−2, and 21.99 mm, respectively. ANOVA detected significant spatial variation in biomass; however, significant temporal variation was not recorded between the years, only among the seasons. Multiple linear regressions were used to find the relationship between the biomass and the environmental parameters. The coarse and very fine sediment fractions and the high water temperature provided suitable habitat for C. fluminea. Although statistically significant correlation was not detected between the chlorophyll-a content of water and the biomass, the highest biomass values were recorded at sites with high chlorophyll-a value.

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., The project was financially supported by the Hungarian Scientific Research Fund under the contract No. OTKA T/046180. Special thanks to the DanubeIpoly National Park for the help in field work., info:eu-repo/semantics/publishedVersion

In Europe, at least two Corbicula morphs (generally referred as C. fluminea and C. fluminalis) are known. However, their taxonomic and systematic position is controversial. In order to ascertain their taxonomy and systematics, we studied their characteristics in River Danube. Morphometric analysis confirmed the existence of two distinguishable morphs. Based on mtCOI sequence, morph-1 is identical to C. fluminea, while morph-2 is an undefined species. According to the median joining analysis, morph-1 belongs to the C. fluminea clade, whereas morph-2 forms a distinct clade, which is known only from invasive populations. Differences were recorded in population structure and reproductive biology, too. It is probably correct to treat the two morphs as distinct taxa, but as long as the African and western Asian parts of the genus’ range are poorly studied, taxonomic position of morph-2 remains unclear. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Climate change is one of the most crucial ecological problems of our age with great influence. Seasonal dynamics of aquatic communities are — among others — regulated by the climate, especially by temperature. In this case study we attempted the simulation modelling of the seasonal dynamics of a copepod species, Cyclops vicinus , which ranks among the zooplankton community, based on a quantitative database containing ten years of data from the Danube's God area. We set up a simulation model predicting the abundance of Cyclops vicinus by considering only temperature as it affects the abundance of population. The model was adapted to eight years of daily temperature data observed between 1981 and 1994 and was tested successfully with the additional data of two further years. The model was run with the data series of climate change scenarios specified for the period around 2070- 2100. On the other hand we looked for the geographically analogous areas with the God region which are mostly similar to the future climate of the God area. By means of the above-mentioned points we can get a view how the climate of the region will change by the end of the 21 st century, and the way the seasonal dynamics of a chosen planktonic crustacean species may follow this change. According to our results the area of God will be similar to the northern region of Greece. The maximum abundance of the examined species occurs a month to one and a half months earlier, moreover larger variances are expected between years in respect of the abundance. The deviations are expected in the direction of smaller or significantly larger abundance not observed earlier.

On the basis of data series from a faunistic survey of 62 localities in the riparian zone of the main channel of the Hungarian Danube and the adjacent two wetlands (Szigetkoz and Gemenc) in 1998-1999 an attempt was made to evaluate any differences among the separate water bodies (wetland areas, main channel and/or within the main channel) of the Hungarian Danube with regard to size and composition of macroinvertebrate communities. Representants of 19 higher taxonomic groups were present in the samples. Gastropoda, Chironomidae, Amphipoda, Hirudinea, Ephemeroptera and Trichoptera were present at ≥50 % of the sampling sites (Fig. 3). Platyhelminthes, Polychaeta and Decapoda occurred only at one site. In total, 233 taxa (183 species, 45 genera and 5 families/subfamilies) were found. There were only two species (Dikerogammarus villosus: 66 % and Lymnea peregra: 63 %) which were collected at ≥50 % of the sampling sites, and 173 taxa (74 %) occurred only at six or less sampling sites (Fig. 4). The number of laxa present at one but absent from the other water bodies was significant. The spatial dispersion of taxa proved to be contagious or aggregated, while the taxonomic groups were dispersed randomly over the whole area studied. The spatial pattern of the riparian macroinvertebrate communities is greatly influenced by the taxonomic level of identification. Differences which become clear on the basis of taxa composition remain hidden using higher taxonomic levels. Based on the riparian macroinvertebrate communities at the end of the nineties of the previous century, marked differences could be detected between the different parts of the Danube main channel and the water bodies of the two great wetlands (Szigetkoz and Gemenc). There were differences not only in the composition of the macroinvertebrate communities, but also in the community parameters.

In 1998 and 2001 collections were carried out at 83 sampling sites in different water bodies of the Hungarian Danube section. In the period of 1996-2003 during the hydrobiological monitoring of the Szigetkoz area 15 sites, which are affected especially by the hydrological changes due to the diversion of the Danube, were studied. Regarding the sampling sites altogether, 16 small-sized mussel species of 3 families were found, which presents 73 % of the nation-wide fauna. Among them two species (Corhicula fluminea, Dreissena polymorpha polymorpha) are invasive mussels and four (Pisidium amnicum, Pisidium milium, Sphaerium rivicola, Sphaerium solidum) are rare species in Hungary. The radical alteration in the hydro-morphology of the Szigetkoz area caused by the diversion of the Danube to the operation channel of Bos/Gabcikovo hydroelectric power plant was followed by the impoverishment of the mussel fauna. The composition of mussel fauna had been considerably changed between 1996 and 2003. In the period before 2000 the small-sized mussel fauna contained 11 species, but in the period after 2000 the fauna increased with 2 fingernail clams (Sphaerium rivicola, Sphaerium solidum) and one pea clam (Pisidium casertanum var. ponderosum). Beside the increase in the number of species a process to uniformity could be detected, since the ratio of species occurring only in one water body gradually decreased. The number of species occurring in more than one water body increased until the beginning of 2000. The previous flood prevention classification of the Szigetkoz area could be found less and less typical in the spatial pattern of the mussel fauna. In the last years of the investigated period the water regime of the active alluvial floodplain followed the natural water fluctuation of the Danube. The water dynamics of the active alluvial floodplain differs both from the abandoned main channel and the protected area due to the changed mode of water supply. The decrease of similarity between the water bodies could represent a tendency towards the situation before altering the hydrological exchange conditions in the wetland.

The organic matter of the sediment is prerequisite for the detritivorous community in the river ecosystem. To quantify this organic matter content and its changes in time and space, investigations were performed at two typical sites of the Hungarian Danube, which differ in their local hydrological characteristics due to the deposition and erosion processes. The distribution of the benthic particulate matter and the organic matter content of that were analysed in the core samples of the river side and the near shore line in three layers (0-5, 5-10 and 10-15 cm) and in four fractions: C = coarse (2360-710 μm), F = fine (710-250 μm), V = very fine (250-63 μm), and U = ultra fine (

In 2003, 26 Heteroptera species (16 aquatic and 10 semiaquatic) were collected from 53 sampling sites in the Szigetkoz region in Hungary. Hyocoris cimicoides and Gerris argentatus were the most frequent aquatic and semiaquatic species, respectively. Large differences both in the species number and species composition were observed among the different flood-prevention areas, arm systems and habitats. Strong negative correlation was found between the average number of aquatic species and current velocity, and a strong positive correlation between the average number of aquatic and semiaquatic species and macrophyte density. From a faunistic point of view, the marsh at Arak is the most interesting and valuable site.

In a field microcosm experiment, we tested the hypothesis that adult collembola, depending on their density, may enhance or decrease mycorrhizal development and growth of maize ( Zea mays L.) or red fescue ( Festuca rubra L.). Collembola decreased mycorrhizal spore number in the soil. A significant decrease in the spore number, compared to the treatment without collembola, was recorded and ranged from 0.2 to 1.6 individuals g −1 soil in both experiments. Highest AM fungi colonization was found at 0.2 and 0.4 collembola g −1 soil density in the maize and red fescue treatments, respectively. This finding may explain why about 0.1 individuals g −1 soil was reported as optimum for plant growth by some authors.