-This study presents investigations of the periphytic diatoms on artifi cial (glass slides) and natural substrates in the karstic, limnocrene spring of Bunica situated in the south of Bosnia and Herzegovina. Investigations were performed in summer 2010. Samples were collected every seven days for eight weeks. Physical and chemical characteristics of water, temperature, oxygen saturation, dissolved oxygen, electric conductivity and nutrients as well as fl ow velocity at sample site, were measured simultaneously with each sampling. Physical and chemical characteristics showed low temperature oscillations, good aeration and oligotrophic conditions. In general, greater diatom diversity was noted on natural substrate. A total of 104 diatom species were found on natural substrate and 82 on glass slides. The best represented genera on both types of substrate were Gomphonema and Navicula (each with eight species), Nitzschia (with six species), and Cocconeis (with fi ve species). Achnanthidium exiguum, Achnanthidium minutissimum, Amphora pediculus, Cymbopleura amphicephala and Surirella minuta were recorded in all samples of natural substrate and Gomphonema minutum in artifi cial substrate samples.

Centric diatom taxa in the Cyclotella ocellata and C. comensis complexes show high morphological variability and often apparently continuous morphological transitions. In this study we investigated natural assemblages of the C. ocellata/C. comensis complex from Hungarian and Croatian lakes and from Turkish streams using morphological and molecular methods. The studied assemblages contained cells with morphologies resembling C. ocellata as well as other, closely related, species: C. comensis, C. pseudocomensis, C. costei, and C. trichnoidea. The goal of our paper was to assess whether the observed morphological differences were due to intraspecific variability or suggest the existence of several, putatively distinct species.Ten morphometric characters were measured, which, either individually, or in pairs, did not differentiate the nominal taxa in our assemblages. However, multivariate discriminant analysis has revealed a group including C. ocellata and C. trichonidea morphologies could be separated from another containing C. comensis, C. pseudocomensis and C. costei.A nuclear (18S rDNA) and a chloroplast (rbcL) gene were amplified and partially sequenced from environmental DNA or from isolated cells. The sequences showed little variability among the assemblages and nominal species. Although general congruence of molecular and morphometric separation supports the species level separation of C. ocellata/trichonidea from the probably conspecific C. comensis/pseudocomensis/costei, sequence divergences between the groups are in the same range as within them, so that a onspecificity of all four taxa cannot be unequivocally excluded.

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