At present, the invasive round and bighead gobies are the most abundant benthic fish species in the near shore zone of the Middle Danube. We compared their diet seasonally in natural and artificial habitats and contrasted it with the food supply. The composition of the macroinvertebrate community was determined mainly by seasonal changes, whereas habitat type had smaller effect. Round gobies followed these changes flexibly. They consumed mainly chironomids in the spring, whilst amphipods and molluscs in the summer and autumn. Bighead gobies relied on amphipods in each season and in both habitats, and consumed fish, too, including round goby (intraguild predation). Diet overlap was determined by the morphological differences of the species allowing a varying degree of differentiation according to the seasonally changing food supply. IntroductionOne of the most spectacular changes in the fauna of Central and Eastern European large river systems in the last decades has been the ongoing spread and proliferation of PontoCaspian fauna elements, primarily macroinvertebrates (crustaceans, molluscs) and gobiid fishes (BIJ DE VAATE et al., 2002;COPP et al., 2005). These aquatic invaders have been reported to cause serious changes in the structure and functioning of their newly invaded habitat including the displacement of their native counterparts or the reconstruction of the food web (RICCIARDI, 2001;MINCHIN et al., 2002;VAN RIEL et al., 2006). Round goby (Apollonia melanostoma (PALLAS, 1814), formerly Neogobius melanostomus) for example, the most successful invasive goby species caused substantial changes in the composition and abundance of benthic macroinvertebrates (BARTON et al., 2005), declines of native fish species (FRENCH and JUDE, 2001;CORKUM et al., 2004) and increased energy transfer to predators in the well studied Laurentian Great Lakes. Gobiid fishes * Corresponding author 610 P. BORZA et al.

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. Abbreviations: PCA-Principal Components Analysis, CCA-Canonical Correspondence Analysis, TBOM-total benthic organic matter, CBOM-benthic organic matter content in coarse fraction of sediment, FBOM-benthic organic matter content in fine fraction of sediment, VBOM-benthic organic matter content in very fine fraction of sediment, UBOM-benthic organic matter content in ultra fine fraction of sediment Nomenclature: Checklist of the European Continental Mollusca (Falkner et al. 2001) and the catalogue of Fehér and Gubányi (2001) reflecting the Hungarian situation.

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 * L b ) 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.