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Here we present the results of a mayfly study conducted on a hydromorphologically altered lowland river in Croatia, the Bednja River. Twenty-two mayfly species were recorded, Baetis fuscatus (Linnaeus, 1761) and Serratella ignita (Poda, 1761) being the most widespread and Paraleptophlebia submarginata (Stephens, 1836) and Cloeon dipterum (Linnaeus, 1761) the rarest species. A significant increase in mayfly species richness was observed along the river course. Nevertheless, cluster analysis showed that their assemblages were not grouped according to the study position along the Bednja River. The lowest species richness and diversity were recorded for the spring area. Surprisingly, the highest species richness was found at a site with moderate hydromorphological alterations, and the highest diversity at a site with severe hydromorphological alterations. This could be because the river is connected to numerous lateral streams and a variety of microhabitats are available. New distribution records in Croatia are presented for some rare species in Croatian freshwater habitats, such as Heptagenia longicauda (Stephens, 1836). Ephemera lineata Eaton, 1870 was recorded for the first time in the Pannonian lowland ecoregion (ER 11) in Croatia. Caenis cf. pseudorivulorum Keffermüller, 1960 has not yet been recorded in Croatian freshwater habitats, but since our identifications are based on juvenile specimens, its occurrence is still to be confirmed.

The impact of water quality, landuse, hydromorphology on benthic macroinvertebrate assemblages of the Bednja River was studied. 20 longitudinally distributed study sites were sampled using the AQEM multihabitat sampling method resulting in the collection and identification of 193,638 benthic macroinvertebrate individuals belonging to 259 taxa. Diversity and assemblage composition greatly varied between the study sites and microhabitats. There was no grouping of communities based on river type. . Two separate assessment systems for hydromorphological modification were tested on the same river reach. RHS scores explained more variation in assemblages than EN 15843:2010 scores. The two assessment systems gave differing results regarding the extent of hydromorphological modification, but never the less had highly correlated subscores. Several water quality and nutrient parameters followed a longitudinal gradient but hydromorphological modification did not, showing that degradation is present along the entire course of the river. Benthic invertebrate diversity and functional metrics generally poorly correlated with hydromorphological modification scores. Assemblages on technolithal differed from but those on natural lithal substrate. The high diversity supported by technolithal is explained by the presence of periphyton. The correlation of hydromorphological modification with predators is due to Hydrachnidia which are associated with technolithal. The response gradient shows an improvement in Ecological Status with increased HQA scores. Istražen je utjecaj kakvoće voda, korištenja zemljišta i hidromorfologije na zajednicu makrozoobentosa rijeke Bednje. Uzorkovano je 20 longitudinalno raspoređenih postaja korištenjem AQEM “multihabitat” metode čime je sakupljeno i identificirano 193,638 jedinki i 259 svojti makrozoobentosa. Raznolikost i sastav zajednice uvelike se razlikovala između postaja i mikrostaništa. Bray Curtis indeks sličnosti nije grupirao zajednice prema dvaju riječna tipa rijeke Bednje. Dvije različite metode za ocjenu hidromorfološkog stanja primijenjene su na istim riječnim odsječcima. RHS ocjene su objasnile više varijacije u zajednicama od EN 15843:2010 ocjena. Iako su dvije metode ishodile različite rezultate vezano za stupanj hidromorfološke degradacije, njihove ocjene svejedno visoko međusobno koreliraju. Više parametra kakvoće voda i hranjivih tvari pratilo je longitudinalni gradijent što se nije pokazalo i kod hidromorfološke degradacije koja je prisutna duž toka cijele rijeke. Raznolikost i funkcionalne metrike su uglavnom loše korelirale sa hidromorfološkim stanjem. Zajednice tehnolitala razlikovale su se od zajednica prirodnog litala. Visoka raznolikost koju podržava tehnolital obrazložena je prisutnošću perifitona. Korelacija hidromorfološke degradacije sa predatorima jer radi skupine Hydrachnidia koja je povezana s tehnolitalom. Gradijent promjena ukazuje na poboljšanje ekološkog stanja s povećanjem HQA ocjena.