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Man-made freshwater habitats have been widely used to successfully modify the environment for human benefit, including purposes of irrigation, flood control or electricity generation. Over the past decade, our knowledge about the mayfly fauna and its ecology in South-East Europe has been growing, yet the data related to lentic assemblages remain very scarce. In this study, littoral mayfly assemblages were investigated at artificial and heavily modified lentic habitats in the Dinaric Western Balkan (n=21) and Pannonian Lowland (n=15) ecoregions. Mayfly nymphs were sampled during the summer months of 2016 or 2017. At each sampling site, ten samples were collected using a benthos hand net. A total of 21 mayfly species were recorded, though species richness per site was rather low (i.e., between zero and seven species). Cloeon dipterum (Linnaeus, 1761) was the most frequent species recorded, while Caenis horaria Linnaeus, 1758 was the most abundant. This study showed that the Croatian mayfly fauna is still growing, with the first record of Siphlonurus aestivalis Eaton, 1903 for the country. The assemblage structure was mainly dominated by lower reaches and lentic (potamal and littoral) elements and detritivores (gatherers/collectors and active filter feeders). Orthophosphates and chemical oxygen demand had the highest influence on mayfly assemblages, reflecting a higher level of pollution at sites in the Pannonian Lowland ecoregion, and consequently markedly lower mayfly abundances. As mayflies are widely used as bioindicators of freshwater ecosystems, detailed information about their assemblages in heavily modified and artificial habitats could contribute to future conservation activities of freshwater habitats and their communities. Additionally, these results could be applied in creating a monitoring system for artificial lakes according to the requirements of the European Water Framework Directive.