The fish diversity of the Slovak segment of the Danube (representing the transitional zone from the rhithron to the potamon) was under the control of the inland delta which existed in this region until 1992. Seventy-six species were recorded in the studied river section: 61 species were native, 11 were exotic, 5 or 7 were invaders from the lower sections of the Danube, 3 species (and one form) became extinct, and 7 species were not detected during the past 20 years. The increasing pollution until 1985 decreased the fish species richness, but the improvement of water quality thereafter rose their number. However, the construction of the Gabcíkovo River Barrage System put in operation in October 1992 started to change substantially fish species richness and quantity, and fish catch. The formerly functional inland delta controlled by the pulsating discharge disappeared, and was changed to an artificial system of isolated or only marginally connected habitats. The upstream fish migration was stopped by the Gabcíkovo dam and weirs in Cunovo. Therefore, the fish fauna of the side arms lost contact with fish of the main channel and of the bypass canal downstream. The loss of spawning habitats had fatal effects, especially for the wild carp and the pike, which number rapidly decreased. In general, the number of fish species, their density, and fish catch decreased significantly. To improve the situation, it is proposed: 1) to restore the bilateral connection between the main channel and the side arms, 2) to ensure fish migration between the main channel and the side arms, and 3) to simulate the original flooding of the floodplain.

A long-term study (1985-1999) conducted at backwater and tailwater sites of a large reservoir located in the middle course of the 808 km long Warta River (Poland), showed that not only obligatory riverine species suffer from the effect of the dam. Facultative riverine eel, burbot, wels and pike considerably decreased in number and standing crop, including their apparent absence in some years following the damming. Nevertheless, median densities of perch and roach, two generalists thriving in European regulated rivers, increased from tens and hundreds, respectively, to thousands per hectare. Significant increases in density were also recorded for other four species: zander, ruffe, bream and silver bream. Thus, the reaction of facultative riverine fish, often supposed to be resistant to aquatic habitat deterioration, is diverse with respect to impoundment. Apart from the migratory eel, which have not been caught at the backwater site for the last 8 years, all the remaining significant temporal changes were recorded at the tailwater site. Out of habitat flexibility coefficients relating to refuge, feeding and spawning, only the latter considerably differed between the successful and unsuccessful species after impoundment, which testifies to the critical role of spawning flexibility in a human-modified environment.

The effect of substrate texture on colonization by stream macroinvertebrates was studied in a mountain stream. Experimental patches made of natural substrate, with two levels of cobble and gravel texture (rough and smooth), were subjected to two colonization periods (two hours and one week) within a riffle. Cobble texture affected the number of individuals and taxa colonizing substrate patches, although these relationships depended on time of colonization and some local environmental factors. Texture influence was different for different taxa, and only affected the scraper functional feeding group after one week of colonization, probably in relation to the development of an epilithic layer. These results suggest a significant effect of the surface texture of substrate on colonization by macroinvertebrates, although the effect is complex and needs more careful examination.

Using cluster analysis, ecological profiles of the species, and the (Diamesinae + Orthocladiinae) / Chironominae ratios, prealpine rivers were classified by their chironomid communities based on 338 species: (1) summer-cold mountain rivers (Isar, Salzach, Inn/Mühldorf), (2) summer-warm moraine-stretches (Würm, Alz/moraine rupture, Amper, Altmühl), (3) the summerwarm lowland river Upper Alz (lake outlet and the stretch to Altenmarkt village), (4) the lower section of the river Inn regulated by dams, which is dominated by a littoral community. The river Upper Alz is faunistically differentiated along it’s course. Classification of river stretches is predominantly explained by rhithral and eurytope species. Potamal and littoral species had no effect on classification. The results suggest that water temperature is the most important factor of the chironomid-based river classification, which is in harmony with studies by Rossaro (1992) and Lindegaard & Brodersen (1995). Locally, other morphological parameters as reach slope and substrate may be important factors as well. However, further data are needed to clarify their role in species distribution.

Using the epilithic diatoms as indicators of environmental conditions, 10 lakes in the High Tatra Mountains were characterised with regard to their stage of acidification. The diatom communities were diverse; 210 taxa were found, including 27 species of north and alpine preference. On the basis of cluster analysis, two groups of lakes were distinguished. The first group included shallow, moderately acidified lakes (Dlugi and Zmarzly Gasienicowy lakes), where acidophilous organisms prevailed, with Achnanthes marginulata being the most numerous. The structure of the diatom community in Dlugi lake was stable over 4 years of observations and did not show any progress in acidification. The second group included mostly very deep, non-acidified lakes where circumneutral organisms prevailed, and among them Achnanthes minutissima was dominant in most cases. The diatom community structures mostly confirmed the status of the lakes studied as judged from their water chemistry.

Lake Vela is a shallow lake facing eutrophication, like many other freshwater reservoirs. A reduction in biodiversity has been observed and cyanobacterial blooms are frequent, occasionally leading to large fish kills. An annual sampling cycle was carried out in the present study, with the aim of understanding the present seasonal variation of Lake Vela’s physico-chemistry and cladoceran assemblage. The present results were compared with those obtained in previous studies. Presently, Lake Vela is facing higher nutrient and Chl a levels than in past years. The cladoceran assemblage is now dominated by small-bodied forms, such as Chydoridae and Bosmina longirostris, except in the spring peak of Daphnia longispina. Still, the seasonal pattern of the cladoceran community is more or less coherent with the general model observed in other temperate lakes. The results also suggest that this was a rather unusual year, with a Daphnia summer peak, probably because of a large fish kill that occurred in late spring.

From an ecological viewpoint, diel vertical migration (DVM) of zooplankton is an adapted response to environmental factors. DVM coincides with changes in light intensity, but persists also under constant illumination conditions. This fact suggests the existence of an internal clock. In Daphnia longispina, it could be located in the eye. Indeed, the spatial distribution of the pigments containing in the eye ommatidia exhibits significant changes over time when Daphnia are kept under light/dark (LD) conditions. These variations coincide with displacements of the organisms. Furthermore, it is observed that the blue/dark conditions, which inhibit DVM, also stop changes in the distribution of pigments in the ommatidia. The modifications of the eye observed under LD white light represent either the internal clock or its manifestation.

Pourriotia carcharodonta gen. et sp. nov. is described from freshwater plankton collected at the subantarctic Kerguelen islands. The new taxon is provisionally placed in the Notommatidae. Main diagnostic characters separating the new taxon and the other genera of the family are provided by the morphology of the trophi. Apical rami chambers with toothed inner margin and blunt projection on outer margin. Apical rami teeth and subuncinal teeth with accessory spines. Proximal margin of major uncinal teeth firmly connected to distal margin of apical chambers, forming grasping unit. Fulcrum dumb-bell-shaped. Information is presented on the trophi structure of the genus Pleurata (Notommatidae).