L. Forró szerk.: Miscellanea Zoologica Hungarica 10. 1995 (Budapest, 1995)

Gulyás, P.: Rotatoria and Crustacea plankton of the River Danube between Bratislava and Budapest

October 1992. Between the end of October 1992 and early January 1993 only 18 species were recorded between Rajka-Medve. From February on, however, species number showed a continuous increase, and after this period no measurable differences were found. With the exception of 1992, species diversity of the communities was larger than of the Bratislava sampling site. Rare species were the following here: Brachionus quadridentatus cluniorbicularis, B. quadridentatus melheni, B. rubens, Elosa worallii, Keratella hiemalis, K. quadrata frenzeli, K testudo, K. valga, Lecane ludwigii, L. quadridentata, L. ungulata, Lepadella ovális, L. quadricarinata, Lindia torulosa, Paradicranophorus hudsoni, Ptygura melicerta, Trichocerca cylindrica, Trichotria tetractis, T. truncata, Eucyclops macrurus, E. speratus, Alona guttata, Daphnia galeata, Graptoleberis testudinaria, Peracantha truncata. Increase in the number of Crustacea taxa was observed, many of them are not euplanktonic species, but thrive in the littoral zone and macrophyte vegetation. The possible reason for this are the decreasing water velocities - due to smaller flows - providing better habitat for the filter feeder (grazing) species. It is interesting to note the occurrence of Daphnia magna that was drifted away from some Austrian water reservoir. At Medve (1805.5 rkm) the water flow of the Danube is unified. Taxon number here changed between 52-64, representing a rich community. Rare species at this particular location were: Brachionus budapestinensis punctatus, B. diversicornis homoceros, B. plicatilis, Dicranophorus prolectes, Epiphanes macrourus, Keratella tropica, Lindia torulosa, Mytilina bisulcata, Alona costata. Here also, the crustaceans are represented with relatively large numbers, many of them are so-called tichoplanktonic species. Each year dominated Rotatoria species which are characteristic for slowly flowing waters. Species composition of the communities did not show significant difference compared to upstream sampling sites. At Komárom (1768.3 rkm) species diversity increased further, taxon number varied between 54-69. Largest species number was found in 1993. Most of all the number of Rotatoria taxa increased, rare species and accompanying species number has not changed. Dominance of (Brachionus, Filinia, Keratella Polyarthra, Acanthocyclops, Bosmina, Daphnia, Diaphanosoma) genera characteristic to slow-flowing and stagnant waters is evident here. Increase in species diversity of the Zooplankton community is due to the decreasing flow velocities providing better conditions for the Zooplankton species. The richest community was found at Szob (1708.0 rkm). Taxon number varied throughout the three years in the range of 55-69. Species richness is largely due to the effect of smaller tributaries (Garam, Ipoly) only a few kilometers upstream of the sampling site. New rare species are: Asplanchna brightwelli, Brachionus quadridentatus rhenanus, Epiphanes macrourus, Euchlanis triquetra, Lecane ludwigi, L. ungulata, moreover a large number of littoral and benthic species, such as Diacyclops bicuspidatus, Eucyclops macruroides, E. serrulatus, Megacyclops viridis, Alona affinis, A. rectangula, Disparalona rostrata, Iliocryptus agilis, I. sordidus, Macrothryx hirsuticornis, M. laticomis, Simocephalus vetulus presence of which increased the species diversity. Dominant were always planktonic Rotatoria and Crustacea species characteristic to the River Danube. The Nagymaros sampling site (1694.6 rkm) is only 14 river kilometers downstream of the previous one. Taxon number varied between 49-71 during the three years. No larger differences were found between the two sampling points with regards to the occurrence of neither the rare, nor the dominant species. No significant changes were revealed on the basis of the species composition of the communities. The Budapest sampling site was selected on the left-side of the river, upstream of the city (1659.0 rkm). Species number varied between 47-67 in 1992-1994. Dominant species were