Cyclotella atomus Hust. var. atomus in Arch. Hydrobiol. 15: 143, pl. 9, figs 1-4. 1938.

Morphological description.

Frustule low-cylindrical, central part of the valve is slightly tangentially undulated, 3.6-5.6 μm in diameter, clear boundary between regional and central zones absent, 10-15 striae in 10 μm, and a central process (Fig. 6B, C).

Ecology.

Euplanktonic species, that may exist in marine, brackish or nearshore areas and freshwaters, indicating eutraphentic, α-mesosaprobous conditions and often associated with polluted, warm nutrient-rich water, however particularly tolerating high total phosphorus loads (Denys 1991; Van Dam et al. 1994; Yang et al. 2005; Lowe 2015), halophilic, alkaliphilic, tolerates higher ion concentrations and frequent osmotic stress as well as high temperature conditions and turbulence (Krammer and Lange-Bertalot 2000).

Distribution.

Valves were found at all investigated sites of the Southern Bug during this research (Table 1). For Ukrainian territory, it has been reported for the Dnipro River (Maystrova et al. 2007).

In general, Cyclotella atomus is a cosmopolitan species (Krammer and Lange-Bertalot 2000), widespread in freshwater and marine environments in North America, Europe, and Asia, and has also been recorded from Argentina and South Africa ( Poulíčková 1993; Medioli and Brooks 2003; Tanimura et al. 2004; Yang et al. 2005; Wojtal and Kwandrans 2006; Genkal et al. 2020).