Melosira borreri var. subglobosa Grunow, Melosira moniliformis var. subglobosa (Grunow) Hustedt.

Frustule shape is cylindrical to approximately octagonal (Fig. 2A, F). Valve 12.5–26 μm in diameter, mantle height 3.1–8.5 μm. Valve face nearly octagonal with flat tops (Fig. 2B–E, G). Girdle with puncta forming straight and transverse rows 32–56 in 10 μm.

Figure 2. 

SEM (A–D, H) and LM (E–G) images of species found in the transitional zone at Mykolaiv city: A–G Melosira subglobosa (A, F girdle views, B–E, G, valve views) H Melosira varians, partial valve view and girdle view of complete frustule. Scale bars: 10 μm (A–C, E–H); 20 μm (D).

Marine and brackish, benthic-planktonic (tychoplanktonic) species, halophilic, widely distributed. Indicates significant organic pollution (eutrophication), α-mesosaprobiont (Kolpacov et al. 2014). This species prefers water enriched with dissolved organic matter, and is capable to switch from autotrophic to heterotrophic or mixed type of nutrition (Andreeva et al. 2008). Additionally, these diatoms are ubiquitous, and widely distributed in seas and estuaries of temperate zones in habitats with a moderate level with increased level of human impact (Kuzminova and Rudneva 2005; Ryabushko 2009).

Melosira subglobosa is one of the most common species at the sampling sites near the Mykolaiv area in the Southern Bug River (Table 1). For Ukrainian territory, it is known from the coastal zone in the northwestern part of the Black Sea, Crimean seaboard, and some estuaries.

For a worldwide distribution, it was recorded from coastal zones of Europe, and Asia; specifically, from Lake Aral, Baltic, Bering, Black, Caspian, Mediterranean, North and Norway Seas (Tsarenko et al. 2009).

Aulacoseira varians (C. Agardh) Simonsen.

Frustule is cylindrical, valve flat, 15.7–46.6 μm in diameter with numerous small-scale granules, 5.7–15.5 μm high (Fig. 2H).

Common species for freshwaters in streams and lakes, as well as in slightly brackish waters, oligotrophic, eutrophic to dystrophic or polluted environments (Nardelli et al. 2016; Hofmann et al. 2018). Taxon has preferences of alkaline conditions (pH 7‒8.5), with moderate oxygen, regularly found in humid environments, requiring periodically high levels of nitrogen (Soltanpour-Gargari et al. 2011).

Valves were found near Mykolaiv city in the Southern Bug River in our previous sampling study (Table 1; Genkal and Bilous 2015). In Ukraine, it is known from the Southern Bug and lakes in the basins of the rivers Danube, Dnister, Siverskyi Donets, Desna, Prypiat, Dnipro and its reservoirs and estuary.

On the global level, it is a widely distributed taxon, known from Europe (i.e. Berlin, Germany, see Geissler and Kies 2003), Russia (Genkal et al. 2020), Asia (Iran), North America (Canada, USA), Africa (Egypt, RSA); Adriatic, Aral, Azov, Baltic, Barents, Black, Caspian, Kara, Mediterranean, North and Red Seas, and throughout North America (Stoermer and Julius 2003). It is also very common in Brazilian waters (Stoermer and Julius 2003; Tremarin et al. 2009; Nardelli et al. 2016).

Melosira islandica O. Müll., J. Wiss. Bot. 43 (1): 56, pl. 1, figs 3–6. 1906.

Melosira islandica subsp. helvetica O. Müll., M. islandica subsp. vaenernsis A. Cleve.

Frustule cylindrical, valve face flat with randomly located areolae, diameter is 13.2–14.4 μm (Fig. 3A). Curve of the valve with longitudinal areolae (12–14 in 10 μm) and transverse curly rows of areolae. The ringleiste is wide, connective spines are small-sized, sharp-ended, tear-drop-shaped or branched.

Figure 3. 

LM (A,B) and SEM (C–H) images of species found in the transitional zone at Mykolaiv city: A Aulacoseira islandica, valve view B Aulacoseira nivalis, valve view C Aulacoseira subarctica, girdle view of complete frustule D–H Actinocyclus normanii valve view (D pseudonodulus shown with arrow, E, G) and girdle view (H, F). Scale bars: 10 μm.

Aulacoseira islandica is most often occurring as planktonic or tychoplanktonic taxon in water bodies of different types in high latitude or high altitude oligotrophic to mesotrophic large waters (Houk et al. 2010; Stoermer and Julius 2013; Genkal et al. 2020). Sometimes this species may even cause a spring bloom (Stoermer and Julius 2013).

This species was observed in all sampling sites and is the first confirmed record for the whole basin of the Southern Bug River (Table 1). In the mid-1930s Swirenko (1941) found this species as Melosira islandica var. helvetica O. Müller in the lower part of the River starting from Mygea rapids and some other lower stations along the River bed to the mouth (Swirenko 1941). Aulacoseira islandica occurs in the Dnipro River basin and its reservoirs (Tsarenko et al. 2009).

It is also a common species for Europe (Bulgaria, Iran, Finland, Germany, Lithuania, Romania, Russia, Sweden, Ukraine), Asia (Georgia, Russia, Turkmenistan), North America (Canada, Greenland, USA); Barents, Bering and White Seas (Tsarenko et al. 2009).

Melosira nivalis W. Sm. Syn. Brit. Diat. 2, p. 58, pl. LIII, fig. 336. 1856.

Melosira distans var. nivalis (W. Sm.) O. Kirchner, Aulacoseira distans var. nivalis (W. Sm.) E.Y. Haworth.

Cells cylindrical, valve face flat, 7.5–12.8 μm in diameter, valve mantle 3.8–4.7 μm high. Valve face with rough areolae, located in intersecting diagonal rows (Fig. 3B).

This is a freshwater species, found growing in or near snow and in alpine pools (Houk et al. 2007; English and Potapova 2009) but was also reported for alkaline and acid waters (Kulikovskiy et al. 2016).

At the localities of upstream Mykolaiv, in Mykolaiv City and downstream Mykolaiv Aulacoseira nivalis is the first record for Ukraine (Table 1).

As for other countries, it is known from alpine and northern regions (Krammer and Lange-Bertalot 2000); frequently occurring in western North America (English and Potapova 2009) and as being abundant in lake sediments in Brazil.

Melosira italica subsp. subarctica O. Müll., Jahrb. Wiss. Bot. 43: 78, pl. 2, figs 10–11. 1906.

A. italica subsp. subarctica (O. Müll.) Simonsen.

Frustules are usually low to high-cylindrical, valve face flat. Valve is 16.6 μm in diameter, 4.4 μm high, number of areolae lines on valve bend 15 in 10 μm, in the line 18 areolae in 10 μm. Ringleiste is wide, linking spines are large, sharpened (Fig. 3C).

Aulacoseira subarctica is a planktonic alga in lakes, rivers, reservoirs and temporary water bodies and is confined to higher latitudes. It usually appears in response to moderate increases in nutrients, supposedly to phosphorus concentrations controlling its presence and is disadvantaged by further enrichment (Chris et al. 2003).

Occurred in the Southern Bug River in Mykolaiv city (near Varvarivskyi Bridge) (Table 1). This species is known from the Dnipro River basin (Tsarenko et al. 2009).

Aulacoseira subarctica is widely distributed across Europe (Germany, Great Britain, Holland, Norway, Russia, Ukraine, Scandinavia), Asia (Russia, Japan, China), North America, Australia and New Zealand. The species is rare in the tropics, and positive identifications are lacking for Africa (Chris et al. 2003).

Coscinodiscus normanii W. Greg. in Grev., Quart. J. Microsc. Sci. 7: 80, pl. 6, fig. 3. 1859.

Coscinodiscus curvatulus Grunow, C. fasciculatus A.W.F. Schmidt, C. normannicus W. Greg., A. normanii f. subsalsus (Juhlin-Dannfelt) Hustedt

The frustule is drum-shaped, the valve is flat or slightly concave or convex (Fig. 3D–H). Valve diameter 13.6–26.2 μm, height 3.4–5.2 μm. Areolae are arranged into sectors. The external openings of the process (4–6) are clearly visible on the curved outer surface the mantle (Fig. 3F, H). The pseudonodulus is located above the openings of the rimoportulae, it has a slight depression.

According to Hasle (1977), there are no significant taxonomic differences between A. normanii f. subsalsus and the nominate form (the ranges of valve diameters coincide), but there may be ecological preferences. Krammer and Lange-Bertalot (2000) did not identify forms, but gave so-called morphotypes that do not have a rank in nomenclature and, according to their data, in the A. normanii population from the Rhine region, a continuous range of forms was observed during the life cycle. Kozyrenko et al. (2008) synonymized A. normanii f. subsalsus with the nominate form and we adhere to their point of view.

Cosmopolitan, planktonic and phytobenthic, alkalibiontic and halophytic species, occurring in brackish inland waters influenced by anthropogenic nutrients and salts, waters with moderate to high conductivity (222–918 μS/cm), pH ranges from 7.8–8.6, at a water temperature between 8.0–25.7 °C and may serve as indicator of nutrient-rich habitats and polluted waters (Christie 2014; Vidaković et al. 2016).

Actinocyclus normanii is found sporadically in epilithic benthic samples from the the Southern Bug River at the three investigated stations (Table 1). It has previously been recorded for Ukrainian water bodies, especially for the Steppe zone noted in the monograph (Tsarenko et al. 2009) and for the Dnipro-Bug Estuary in particular (Vladimirova 1971; Zhukinskiy et al. 1989). It may travel upstream with highly mineralized waters from the estuary and appear near Mykolaiv City.

Upstream occurrences have been documented for Actinocyclus normanii f. subsalsus for Germany. According to diatom core analyses, this taxon reached the River Havel around 1900 (Schönfelder 1997). It was missing in Berlin (which is situated more than 200 km inland from the North and Baltic Seas) in the 1830s–1850s (Jahn and Kusber, unpubl. data from the Ehrenberg collection at BHUPM) but occurred in recent samples at the beginning of the 20^th century (Kolbe 1925; Geissler and Kies 2003) where it became an established part of the flora (Geissler and Kies 2003; Geissler et al. 2006). It was discussed by Schönfelder (1997) that a prerequisite for naturalisation might be the anthropogenically induced increase of salinity over the minimum value of salt tolerance. In other inland waters, e.g in the Czech Republic, the taxon occurred but did not establish (Fránková-Kozáková et al. 2007).

Additionally, it is a widely distributed species occurring in Europe, North and South America, the islands of the Atlantic Ocean, Africa, Asia, Australia, and New Zealand (Guiry and Guiry 2021). However, the species was considered invasive for Russia (see Kaštovský 2010; Korneva 2014) but was not included in the Handbook of alien species in Europe, outcome of the DAISIE (Delivering Alien Invasive Species Inventories for Europe) project (Handbook of alien species in Europe DAISIE 2009).

Biddulphia laevis Ehrenb. in Ber. Bekanntm. Verh. Königl. Preuss. Akad. Wiss. Berlin: 122. 1843.

Cerataulus laevis Ralfs in A. Pritch., C. polymorpha Van Heurck, Odontella polymorpha Kütz.

The frustule is cylindrical, valves are elliptical-rounded-oval, with diameter 39–61 μm, 15–17 areolae in 10 μm (Fig. 4A–D).

Figure 4. 

SEM (A, B and E–H) and LM (C, D) images of species found in the transitional zone at Mykolaiv city: A–D Pleurosira laevis A, C valve views with arrows shown the ocelli B girdle view E Conticribra weissflogii, valve view with arrows shown central and marginal fultoportulae F Thalassiosira incerta, valve view with arrow shown central fultoportulae G Thalassiosira faurii , valve view H Minidiscus proschkinae, valve view with arrow in the centre shown central fultoportula, and closer to the edge – rimoportula. Scale bars: 10 μm (C–G); 30 μm (A); 50 μm (B); 3 μm (H).

This taxon occurs in brackish and fresh water habitats, commonly found in estuaries of large rivers, also surviving in inland waters with high conductivity, this is a halophilic species. It has considerable abundance in epiphytic, benthic hard waters with relatively high electrical conductivity. Distributed and more abundant in warm-temperate and tropical waters, being mesohalobic, from alkalibiontic to alkaliphilic indicator, β-mesosaprobic, and eutraphentic (El-Awamri 2008).

Valves were found in benthic samples in the Mykolaiv city of the Southern Bug River and downriver (Table 1). For Ukrainian territory it was recorded for estuaries of the Black Sea, also reported for the Southern Bug River (Tsarenko et al. 2009).

Pleurosira laevis is quite cosmopolitan, distributed in the Boreal, near coasts of Europe (Czech Republic, Finland, Germany, Romania, Russia, Sweden, Ukraine), Asia (Korea, Turkey), South America (Brazil), Africa (Egypt), Hawaiian Islands; Azov, Black and Marmora Seas (Tsarenko et al. 2009; Park et al. 2017b). For Europe this taxon is considered an invasive species (Handbook of alien species in Europe DAISIE 2009). An upstream colonisation of Pleurosira laevis with help of different vectors for the River Labe was discussed by Fránková-Kozáková et al. (2007).

Micropodiscus weissflogii Grunow in Van Heurck., 1885.

Eupodiscus weissflogii Grunow, nom. inval., Eupodiscus weissflogii (Grunow) De Toni, Thalassiosira weissflogii (Grunow) G.A. Fryxell & Hasle

The frustule has the form of a drum, valves are almost flat, diameter 24.4–26.6 μm, 8–10 marginal processes in 10 μm, 2–5 central processes (Fig. 4E).

Conticribra weissflogii is a planktonic diatom, from marine and brackish-water environments that also may occur in lacustric and riverine waters. It is reported to occur in a wide range of salinity 2–26‰ (representing oligohalobs to polyhalobs), especially at salinities above 5‰ (Stachura-Suchoples and Kulikovskiy 2014). This taxon tends to increase in population density with rising temperature (Lomas and Glibert 1999) as well as with eutrophication (Zheng et al. 2016). It is also known to grow in waters with relatively high pH, around 8–9.4 (Sala 1997).

This centric taxon appeared at the Mykolaiv site in the Southern Bug River; for Ukraine it was mentioned for the first time in our previous investigation (Table 1; Genkal and Bilous 2015), afterwards it was found in the tributaries of Dnipro in eastern and central parts of the country (Berezovskaya 2019; Kryvosheia and Kapustin 2019).

This is a widely distributed species: Europe, Asia, America (North and South), Africa, Australia and New Zealand; it was even found in Lake Baikal, also in the oceans over the world (Stachura-Suchoples and Kulikovskiy 2014; Genkal et al. 2020).

Coscinodiscus bulla M.H. Hohn & Hellerman.

The frustule is cylindrical, valves approximately flat, 21.4–27.8 μm in diameter, in the middle of a valve 5 processes are located, 4–5 marginal fultoportulae in 10 μm, situated on the valve margin (Fig. 4F). The rimoportula is short-necked with an elongated, compressed narrow lip usually perpendicular to the margin.

The species was recorded as planktonic in water bodies of different types, typical for eutrophic/hypertrophic and highly saprobic marine, brackish, and fresh waters. It is euryhaline and eurythermal, in addition to being known as an alkaliphilic taxon (Okhapkin et al. 2016). Thalassiosira incerta was named an invasive taxon for Russia (Kaštovský 2010; Korneva 2014).

Thalassiosira incerta was recorded near Mykolaiv city in the Southern Bug River (Table 1). For Ukrainian territory, there are few records from reservoirs of the Dnipro River and some estuaries of the Black Sea and coastal waters near Crimea (Tsarenko et al. 2009).

This taxon is quite cosmopolitan, and distributed in the Boreal of Europe (Great Britain, Russia, Ukraine), Asia (Azerbaijan), North America (Canada, USA), Africa (Egypt); Aral, Black and Caspian Seas (Sims 1996; Kuo and Guo 2003; Okhapkin et al. 2016; Genkal et al. 2020).

Coscinodiscus faurii Gasse, 1975. PhD dis. Univ. Paris VI, Vol. II: 24, pl. 32, figs 1, 2. 1975.

The frustule is low-cylindrical, valves are flat, 21.4–25.5 μm in diameter, central processes located on the valve ingroups, 6 marginal fultoportulae in 10 μm (Fig. 4G).

Planktonic and benthic in freshwater reservoirs, lakes, rivers, and fossil. Lack of information for this taxon must be noted, but it is known that this species is very sensitive to salinity (Roubeix et al. 2014).

Thalassiosira faurii (Gasse) Hasle occurred near Mykolaiv city in the Southern Bug River (Table 1). This taxon was registered for the Dnipro River and its reservoirs, mouth of the Danube in the Black Sea (Maystrova et al. 2007; Roubeix et al. 2014).

Worldwide distribution shows this species to occur in some European countries (Hungary, Russia), Asia (Russia), Africa (Ethiopia, Tanzania, Kenia, Kongo) (Tsarenko et al. 2009; Roubeix et al. 2014; Genkal et al. 2020).

Thalassiosira proschkinae I.V. Makarova in Makarova, Genkal and Kuzmin, Bot. Zhurn. 64(7): 922, pl. 1, figs 1–7. 1979.

The frustule is cylindrical, valve flat, diameter 3.8–4.9 μm, areolae polygonal, in quantities 25 in 10 μm, near centre of a valve, the central process and rimoportula are located (Figs 4H, 5A).

This planktonic taxon has been mainly found in estuaries characterized by low salinity and high turbulence, and in seas, but also in freshwaters and may be a halophile indicator (Makarova 1988; Park et al. 2017a; Barinova et al. 2019).

Valves were found in benthic samples in Mykolaiv city of the Southern Bug River and downriver (Table 1). For Ukrainian territory it was recorded for estuaries of the Black Sea and nearshore regions (Tsarenko et al. 2009; Genkal and Terenko 2014).

Minidiscus proschkinae is widely distributed across estuaries and seacoasts of Europe (Germany, Great Britain, Netherlands, Russia, Ukraine), Asia (Azerbaijan, China), Argentina; Azov Sea, Baltic Sea, Caspian Sea (Park et al. 2017a).

Melosira subsalsa A. Cleve in. Arch. Hydrobiol. 7: 509, fig. 1. 1912.

The frustule is cylindrical, valves are flat or slightly convex, diameter 7.8–12.7 μm (Fig. 5B). Frustules are connected with marginal flat-spoon fultoportulae providing a very close connection between valve mantles (Fig. 5C, D).

Figure 5. 

SEM images of species found in the transitional zone at Mykolaiv city (A, B, E–H valve views, C, D girdle views): A Minidiscus proschkinae with arrows showing rimoportula and marginal fultoportula B–D Skeletonema subsalsum with arrows showing marginal fultoportulae E Stephanodiscus hantzschii with arrow shown spines F Stephanodiscus makarovae with arrows showing marginal and central fultoportulae G, H Stephanodiscus minutulus with arrows showing spines and central fultoportula. Scale bars: 3 μm (A); 4 μm (B, F, H); 5 μm (C, D, G); 10 μm (E).

Planktonic taxon, preferring low salinities, usually occurring in salinities up to 15‰, recorded mainly for the brackish waters, however, is known from rivers, lakes, inland seas, coastal waters, and marshes, and often associated with eutraphentic conditions, temperate taxon, alkaliphilic (Hasle and Evensen 1975; Sarno et al. 2005; Hofmann et al. 2018). In addition, in the DAISIE database Skeletonema subsalsum is considered to be an invasive species for Russia’s and Ireland’s water bodies (Kaštovský 2010; Korneva 2014) but is not listed in the Handbook of alien species in Europe (2009).

This taxon was found at the site of Mykolaiv city in the Southern Bug River (Table 1). Skeletonema subsalsum is widely distributed in Ukrainian waters, known from the Dnipro River in its freshwater reservoirs, in estuaries connected to the Black Sea and from coastal waters of different river basins as well as Crimea coast.

It is a cosmopolitan species, known from the Boreal, Europe (Finland, Germany, Ireland, Italy, the Netherlands, Romania, Russia, Sweden, Ukraine), North America (Canada, USA); Baltic, Black and Caspian Seas. (Gibson et al. 1993; Krammer and Lange-Bertalot 2000; Tsarenko et al. 2009; Genkal et al. 2020).

Stephanodiscus hantzschianus Grunow, S. hantzschii var. delicatula A. Cleve, S. hantzschii var. zachariasii (Brun) Fricke, S. zachariasii Brun.

Cyclotella operculata sensu Hantzsch in Rabenhorst, Fl. Alg. Eur.: N 1104. 1861.

The frustule is low-cylindrical, valves flat 13.6–21.4 μm in diameter, striae multiseriate with 6–7 in 10 μm, central processes are absent, spines large and pointy, growing from each rib (Fig. 5E).

Planktonic in lakes and rivers, indifferent, alkaliphilic, α-mesosaprobic, eutraphentic serving as an indicator of eutrophication in rivers, reservoirs, lakes worldwide mostly because of phosphorus loads (Håkansson and Stoermer 1984; Van Dam et al. 1994; Burge and Edlund 2016; Hofmann et al. 2018).

Stephanodiscus hantzschii was identified for the Southern Bug River in Mykolaiv city and downstream (Table 1). It is widespread taxon in Ukrainian water bodies: the rivers Danube, Dnister, Southern Bug, Siverskyi Donets, Dnipro and its reservoirs, coastal waters.

It is a cosmopolitan species, known from Europe (Belarus, Bulgaria, Finland, France, Germany, Moldova, Norway, Romania Russia, Ukraine), Asia (Armenia, China, Georgia, Japan, Korea, Mongolia, Russia, Tadjikistan, Uzbekistan), North America (Canada, USA); Aral, Azov, Baltic, Black and Caspian Seas (Krammer and Lange-Bertalot 2000; Tsarenko et al. 2009; Genkal et al. 2020).

Frustule disciform, valve with slightly convex or concave centre, frequently flat, diameter 5.9–8.3 μm, striae are double, rarely triple, in numbers of 14–16 in 10 μm. One central process is present. Spines pointy, small-scale, growing from each costa (Fig. 5F).

Planktonic in rivers, lakes and reservoirs, freshwater, but mainly in mesotrophic-eutrophic water bodies. In addition, reported occurrence of this species in high numbers in highly mineralized waters (Genkal 2007; Genkal et al. 2009; Tsarenko et al. 2009).

Stephanodiscus makarovae was observed downstream from Mykolaiv city in the Southern Bug River (Table 1). It was observed in the Dnipro River and its reservoirs, the Danube River and in the coastal area of the Black Sea (Genkal et al. 2009; Tsarenko et al. 2009; Genkal and Terenko 2014).

As for general distribution, this taxon has only a few records around the world – Europe (Russia, Ukraine), Asia (Armenia, Russia), Africa (Egypt) (Kulikovskiy et al. 2016; Genkal et al. 2020).

According to Houk et al. (2014), S. makarovae (Genkal 2007) was erroneously included in the synonym C. delicatus (Genkal) Casper & Scheffler, since, according to the diagnosis, the marginal fultoportulae of S. makarove have 2 satellite pores, and for C. delicatus 3. The difference in the number of satellite pores at the marginal fultoportulae in centric diatoms is a good diagnostic feature. For S. makarovae, the valve relief also varies from flat to slightly convex or concave, and there are also other morphological differences (see same publication Genkal 2007). As for the transfer of S. makarovae to the genus Cyclostephanos, this is a debatable issue and molecular genetic studies are needed.

Cyclotella minutula Kütz. Kieselschal. Bacill.: 50, pl. 2, fig. 3. 1844.

S. astraea var. minutula (Kütz.) Grunow, S. minutulus (Kütz.) Round, S. parvus Stoermer & Håk., S. perforatus Genkal & Kuzmin, S. rotula var. minutulus (Kütz.) R. Ross & P.A. Sims.

The frustule is disciform, valves flat or with slightly convex or concave centre, 8.8–9.1 μm in diameter, striae double to triple, numbering 10 in 10 μm (Fig. 5G, H). A central process is present. The spines are short, growing from each rib (Figs 5G, H, 6A).

Figure 6. 

SEM (A–F, H) and LM images (G) of species found in the transitional zone at Mykolaiv city, valve views: A Stephanodiscus minutulus with arrows shown marginal and central fultoportulae B, C Cyclotella atomus var. atomus with arrows shown marginal and central fultoportulae D–F Cyclotella atomus var. gracilis with arrows shown marginal and central fultoportulae on D picture, only central fultoportula on E and central fultoportula and alveolae on (F) G Cyclotella choctawhatcheeana with arrow shown central fultoportulae H Cyclotella cryptica with arrows shown marginal and central fultoportulae. Scale bars: 3 μm (B, C); 4 μm (D, F); 5 μm (A, E, H); 10 μm (G).

It is a planktonic taxon, described as eutraphentic (Hofmann et al. 2018). Stephanodiscus minutulus reaches greatest abundance in productive nearshore regions, in the mouths of large rivers and coastal embayments. This taxon is an indicator of increased TP concentrations, alkaliphilic, mesosaprobic indicator (Stoermer and Yang 1969; Bradbury et al. 2002; Reavie and Kireta 2015).

Stephanodiscus minutulus occurred at the Mykolaiv city and downstream the Southern Bug river sites (Table 1). This taxon is distributed over the entire territory of Ukraine, common for such rivers as the Dnipro with its reservoirs and tributaries, the rivers Dnister, Danube, Siverskyi Donets, estuaries of main rivers (Tsarenko et al. 2009).

Concerning worldwide distribution, Stephanodiscus minutulus is a widespread taxon, recorded for Europe (i.e. Estonia, Finland, France, Germany, Great Britain, Hungary, Ireland, Moldova, Norway, Russia, Ukraine), Asia (Armenia, Georgia, Iran, Israel, Russia, Japan), North America (Canada, Greenland, USA) (Tsarenko et al. 2009; Kiss et al. 2012; Kulikovskiy et al. 2013; Genkal et al. 2020).

Houk et al. (2014) considered Stephanodiscus minutulus to be different from Stephanodiscus parvus (and Stoermer and Håkansson 1984) and noted the main difference between S. minutulus and S. parvus the convex-concave valve relief in contrast to flat valves, respectively. However, many authors have shown that in S. minutulus the valve relief varies from convex-concave to flat, and therefore S. parvus was treated as a synonym (see Genkal 2010), and we adhere to this point of view.

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).

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).

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).

The frustule is low-cylindrical, central part of valve is slightly tangentially undulated, valves 4.6–7 μm in diameter, and a clear boundary between regional and central zones is present, 15–20 wedge-shaped striae in 10 μm, with central process (Fig. 6D, E).

Planktonic in rivers, lakes, freshwater, eutraphentic (Hofmann et al. 2018). It is regarded as an euryhaline species (Kiss et al. 2012).

Cyclotella atomus var. gracilis is here first reported for the studied area, and was found at all investigated sites during this study (Table 1). In turn, its existence was reported in Dnipro waters (Maystrova et al. 2007), as well as for the Danube River (Genkal and Ivanov 1990).

This species is cosmopolitan, i.a. it was recorded for European waters (Kiss et al. 2012; Genkal et al. 2020).

Cyclotella hakanssoniae Wendker, Nova Hedwigia 52: 360. 1991.

Frustule low-cylindrical, central part of valve tangentially undulated, valves 9.1–12.3 μm in diameter, 12–14 striae in 10 μm, 1–4 central fultoportulae, 6 marginal fultoportulae in 10 μm (Fig. 6G).

Cyclotella choctawhatcheeana is a small centric diatom from the plankton of water bodies tolerating a wide temperature range. Originally this species was described as a marine species in the northern Gulf Coast of Florida; it is also recorded from several localities in Florida Bay and its global distribution is discussed (Prasad et al. 1990). For Germany, it was described from the River Schlei close to the Baltic Sea (Wendker 1991). Nowadays, it may be classified as an invasive species in brackish waters (Kiss et al. 2012). This species may grow in different seasons and with high and low nutrient availability (Oliva et al. 2008). In turn, some authors note that the existence of this species has a positive linear relationship with nutrient concentration (Jaanus et al. 2009).

It was recorded for the first time in Ukraine in our previous investigation near Mykolaiv city of the Southern Bug River and this study confirms its existence in Mykovaiv city and at the downriver sites (Table 1).

Cyclotella choctawhatcheeana was recorded as a cosmopolitan species. Its presence has been confirmed in different localities around the world in brackish waters and rivers connected with saline lakes (Prasad et al. 1990). It was found as a component of the phytoplankton in the saline Mexican lake Alchichica (Oliva et al. 2008), in the Baltic Sea, with salinity between 3 and 11‰ (Wendker 1991; Håkansson et al. 1993), and the Salton Sea, with a salinity in excess of 40‰ (Lange and Tiffany 2002). Additionally, it is known from saline lakes in North America and Africa (Carvalho et al. 1995), reservoirs in Russia (Genkal et al. 2020).

Frustule cylindrical, the medium part of a valve slightly tangentially undulated, or flat, valve diameter is 6.4–6.7 μm, a clear boundary between edge and central zone is absent, striae wedge-shaped, 8 in 10 μm, single central fultoportula (Figs 6H, 7A).

Figure 7. 

SEM (A–E, G, H) and LM images (F) of species found in the transitional zone at Mykolaiv city, valve views: A Cyclotella cryptica B Cyclotella marina C, D Cyclotella meduanae E, F Cyclotella meneghiniana G Cyclostephanos dubius H Cyclostephanos invisitatus, with arrows shown on A, D, H marginal and central fultoportulae, B marginal fultoportulae, F central fultoportulae. Scale bars: 2 μm (B, D); 4 μm (A); 5 μm (C, E); 10 μm (F–H).

Cyclotella cryptica is a planktonic species, known from marine and brackish environments, may be found in high chloride concentrations. It occurs at maximum abundance around 20 °C (Liu and Hellebust 1976; Makarewicz 1987; Mills et al. 1993). Cyclotella cryptica is a saprophilic species (Barinova et al. 2019), requires NO₃ as its source of nitrogen and Ni ions in order to grow autotrophically, however is capable of heterotrophic growth in bottom water or mud enriched in glucose and known to grow mesotrophically (Oliveira and Antia 1984; Saros and Fritz 2000).

Valves were identified at the site in the south of Mykolaiv city in the Southern Bug River (Table 1).

It is a widespread species, recorded for Europe, North America, and Asia (Mills et al. 1993; Guiry and Guiry 2021).

Cyclotella atomus var. marina Tanimura, Nagumo & Kato in, Bull. Natn. Sci. Mus., Tokyo, Ser. C. 30: 6–7, figs 3–15. 2004.

Frustule low-cylindrical, valve diameter is 3.2–3.9 μm, a clear boundary between edge and central zone is absent, striae wedge-shaped, 18–20 in 10 μm, central process is absent (Fig. 7B).

According to literature data, C. marina has a high ecological relevance, with a preference in brackish waters, also inhabiting marine environments even with salinity ranges around 30‰; smaller numbers were recorded for freshwaters under 10‰ (Tanimura et al. 2004; Chung et al. 2010; Aké-Castillo et al. 2012). There are known cases where C. marina exists in shallow waters with freshwater discharges. At the same time, the appearance of this taxon is connected to high nutrient concentrations. In addition, cases of blooming of this species are known in the south-eastern Gulf of Mexico (Aké-Castillo et al. 2012).

C. marina is found in epilithic benthic samples at all investigated sites during this research for the Southern Bug River (Table 1). For Ukrainian territory it is also reported from Khmelnytskyy NPS, (Genkal et al. 2012) and in phytoplankton in Odessa Bay of the Black Sea (Genkal and Terenko 2014).

It is also a common species for Europe, North America, and Asia (Aké-Castillo et al. 2012; Genkal et al. 2012; Guiry and Guiry 2021).

Frustule low-cylindrical, central part of valve is slightly tangentially undulated or flat, valves 5.8–9.4 μm in diameter, striae wedge-shaped, 6–9 in 10 μm, central process is absent (Fig. 7C, D).

Cyclotella meduanae was recorded ecologically as a planktonic taxon from different types of water bodies (lakes, reservoirs, rivers), freshwater and brackish, of different halobity, eutraphentic (Kiss et al. 2012; Genkal 2014; Hofmann et al. 2018).

This taxon is sporadically found in epilithic benthic samples from the Southern Bug River at two investigated stations (in Mykolaiv City and downstream Mykolaiv) (Table 1). For other Ukrainian waters it has been reported for the Dnipro River (Maystrova et al. 2007), the Danube River (Genkal and Ivanov 1990).

Globally, this taxon is distributed in the Boreal zone, Europe, Asia and North America (Genkal 2014; Genkal et al. 2020).

C. kuetzingiana Thwaites, C. rectangula Bréb. ex Rabenh.

Frustule cylindrical, valves with a tangentially undulated central part, 10.4–33.3 μm in diameter, striae wedge-shaped 5–8 in 10 μm. Central processes (usually from 1 to 9) and spines are present at the mantle of the valve (Fig. 7E, F).

Cyclotella meneghiniana was recorded as tychoplanktonic, in coastal and estuarine locations with water of varied chemistry (Trigueros and Orive 2000). Its optimal development occurred at temperatures in the range of 20.1–20.6 °C (Stoermer and Ladewski 1976) but it was eurythermal (Gasse 1986). This is a mesopolysaprobic, and eutraphentic taxon, particularly common for shallow, nutrient rich waters, favoured by moderately alkaline conditions (Håkansson 1993; Van Dam et al. 1994).

Valves were found at all investigated sites of Southern Bug during this research (Table 1). For Ukrainian territory it has been reported for the Dnipro Estuary, the Southern Bug as well, but near Vinnitsya (300 km upriver form Mykolayiv), the rivers Siverskyi Donets, Dnister, Danube, Dnipro, Desna, Prypiat, Teteriv, Oskol, small rivers in Odessa region and other rivers (Tsarenko et al. 2009).

Concerning global distribution, Cyclotella meneghiniana is considered a widespread taxon (Houk et al. 2010); it was also recorded for Berlin, Germany (Geissler and Kies 2003).

Stephanodiscus dubius Hust., Krypt.-Fl. Deutschl., 2.Aufl., 7 (1): 367, fig. 192. 1928.

Cyclotella dubia Fricke in A.W.F. Schmidt

The frustule is disciform, valve face concentrically undulate, 13.6–21 μm in diameter, striae multiseriate with 9–12 areolae in 10 μm, ribs continue on the curve of a valve, spines grow from each rib (Fig. 7G).

Cyclostephanos dubius is considered to be a pelagic taxon, planktonic component of both fresh and brackish lakes (Cholnoky 1968). Often indicating meso- to eutrophic conditions, sometimes recorded for hypertrophic lakes (Van Dam et al. 1994; Kirilova et al. 2010). The species is common in flowing and stagnant water in coastal area, oligosaprobic, alkalibiontic, halophilous “0.0–5 g/dm³” (Hustedt 1957); pH value above 7.0 (van der Werff and Huls 1957–1974; Foged 1973).

This species was found above Mykolaiv city in the Southern Bug River bed (Table 1). From Ukrainian territory, it is known from the Dnipro River, the Dnipro-Bug Estuary (Topachevsky and Oksiyuk 1960; Vladimirova 1971) and the Danube River (Tsarenko et al. 2009).

Cyclostephanos dubius is a cosmopolitan species, recorded for Berlin, Germany (Geissler and Kies 2003) and further sites in Europe (Hungary, Dania, Estonia, Moldova, Russia, Ukraine), Asia (Georgia, Russia, Uzbekistan), North America (Canada, USA), Africa (Egypt); Baltic and White Seas (Tsarenko et al. 2009; Kiss et al. 2012; Genkal et al. 2020).

Stephanodiscus invisitatus M.H. Hohn & Hellerman, Trans. Am. Microscop. Soc. 82 (3): 325. 1963.

Frustule disciform, valve face flat, 9.4–14.5 μm in diameter, multiseriate striae 10–14 in 10 μm, ribs are continuing on curve of the valve, spines grow from every rib (Fig. 7H).

Cyclostephanos invisitatus was recorded as planktonic species from rivers, ponds, lakes, reservoirs and seas, freshwater, brackish and marine waters. Also known from waters of eutraphentic conditions, moderate and higher trophy and moderate alkalinity (Krammer and Lange-Bertalot 2000; Siver et al. 2005; Kirilova et al. 2010; Hofmann et al. 2018).

C. invisitatus was found 5 km downstream Mykolaiv city (Table 1), and was recorded for this River earlier (Bilous et al. 2012; Bilous et al. 2014; Belous 2016). In turn, it is common for the Dnipro River and there are some findings in the Danube River (Tsarenko et al. 2009).

Probably a cosmopolitan species, known from Europe (Germany, Hungary, Poland, Russia), Asia (Armenia, Azerbaijan, Russia), Northern America (USA), Africa (Egypt); Caspian Sea (Wojtal and Kwandrans 2006; Tsarenko et al. 2009; Kiss et al. 2012; Genkal et al. 2020).