The Krka River is located at Balkan Peninsula in the Dinaric region of Croatia and it is a typical groundwater-fed karst river characterized by an unusual phenomenon-tufa. Tufa designates porous CaCO3 deposits forming under specific physical and chemical conditions and hosting very diverse freshwater biota. Karst springs are considered one of the most diverse habitats for diatoms and are usually inhabited by specific diatom assemblages, which include the Krka spring zone. However, such habitats are particularly impacted by human activities, so previous studies have recently been conducted to raise awareness of the importance of karstic spring zones, especially because they are inhabited by specific and rare species. During the research at Krka River, samples were collected in the spring zone in May 2017, and the observations revealed the existence of a one Cymbopleura species characterized by slightly convex to almost linear valves, which are slightly protracted, broadly rounded and weakly dorsally bent at the apices. Based on detailed morphological analyses on light and scanning electron microscopy it appears that this population resembles two previously described taxa: C. rupicola var. minor Krammer and C. rupicola var. korana Krammer. Here we provide a more detailed description of the mentioned population under the new name Cymbopleura amicula stat. nov. et nom. nov. due to the significant differences from C. rupicola, together with a comparison with similar taxa in Cymbopleura.

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The genus Amphora sensu lato was considered highly heterogeneous for a long time. Modern microscopy and molecular analyses revealed significant differences in cell morphology and structure, thus suggesting polyphyly of the genus. New findings resulted in separation and description of several amphoroid genera. Recently, the genus Tetramphora Mereschkowsky was re-established based on ultrastructural and molecular analyses. The genus is widely spread in marine and brackish habitats. Recent observations of contemporary flora and cores from Lake Vransko (Croatia) revealed a new species of Tetramphora. The new species is characterized by having large semi-elliptical valves with arched dorsal margin and biarcuate ventral margin, length of 49.0–78.0 µm and width between 7.5–12.0 µm. The raphe is discontinuous, with proximal raphe ends externally covered with large siliceous flap. Striae are uniseriate, composed of small round to elongated areolae. Based on this unique combination of characters visible on light and scanning electron microscope, a new species Tetramphora croatica sp. nov. is described.

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In the light of ever-growing issue of climate changes, a special focus has been directed towards freshwater periphytic algal communities. Diatoms, a commonly dominant group of protists in periphytic assemblages, are frequently used for ecological water-quality assessments of rivers.Diatom-based indices for ecological status assessment are based on a weighted-averaging model using relative abundances and the environmental optima of taxa. The aim of this study was to assess the ecological status of the Krka River based on taxonomic analyses of its periphytic communities. Sampling of periphytic and environmental variables was carried out in September 2017 and September 2018 on nine stations along the Krka River course, with 3 microhabitats at each station. Taxonomic analysis of samples in 2017 revealed a total of 120 diatom species, whilst in 2018 a total of 226 diatom species was noted. Ecological status of the Krka River, evaluated using TDI HR (Croatian Trophic Diatom Index) and the corresponding ecological quality ratio (EQR), was classified as high in most of the watercourse of the Krka River.