Diatoms are widespread microorganisms associated with aquatic environments. They can be found in any type of water environment such as canals, rivers, ponds, reservoirs, seas and oceans, as well as in wet soil, on submerged plants and on anthropogenic structures. Furthermore, diatoms can successfully inhabit both marine and freshwater environments, and are the dominant group of aquatic organisms in phytoplankton and periphyton (Furnas 1990; Ács et al. 2004). Even though they constitute only about 1% of Earth's photosynthetic biomass, they are an important group of aquatic organisms. They account for about 45% of the total primary production in the oceans and, in addition, they are of great importance in oxygen production and carbon dioxide removal (Field et al. 1998; Yool & Tyrrell 2003; Ács et al. 2004). Diatoms are considered good indicators of the state of the environment, because the ecological preferences of many diatom species are known. Although their preferences pertain mainly to organic pollutants, pH, hardness, salinity, trophic conditions etc., they also affect their distribution. Both the high productivity and short life cycle of diatoms enable them to detect short-term contaminants or other environmental changes, because of their ability to respond very quickly. Therefore, diatoms are important in surface water monitoring and are considered early warning indicators (Johnston et al. 2006; Bis 2008; Zgrundo et al. 2018). The use of diatoms in biomonitoring is particularly important because, unlike other groups of aquatic organisms, they occur in a wide range of waters from clean waters to those that are highly polluted (Kelly 2002; Bąk et al. 2004; Vilbaste et al. 2004; Bąk & Szlauer-Łukaszewska 2012). Therefore, diatoms appear to be good indicators for monitoring secondary saline waters where anthropogenic factors may limit the occurrence of aquatic fauna and vascular flora (Williams 1987). For example, some fish species can tolerate increased salinity provided it increases slowly. However, a rapid increase, which often occurs in waters impacted by salt mine waters, and an increase in blood salt concentration in individuals of these species within the range of 7000–13 000 mg dm−3 results in osmoregulation disturbances in some freshwater fish species (Bacher & Garnham 1992; James et al. 2003). Moreover, the commonly used method of monitoring fish fauna in Poland, compliant with the EU Water Framework Directive (Directive 2000; Prus et al. 2016), fails in highly saline rivers (D. Halabowski, unpublished observations).
One of the areas where intensive underground coal mining is still carried out is Upper Silesia. Currently, about 35 mines still operate in this area, and their operation requires pumping salt mine water into surface waters, quite often directly into rivers. Despite the fact that more and more mines are being closed, salt water continues to be discharged into surface waters (Strozik 2017). Water resulting from leaching processes of coal mining waste discharged from coal mine dewatering systems is usually characterized by very high values of electrical conductivity, total dissolved solids as well as very high concentrations of chlorides, sulfates, nitrates, phosphates, heavy metals and even radioactive elements (Helios-Rybicka 1996; Jankowski & Rzętała 2000; Harat & Grmela 2008; Cañedo-Argüelles et al. 2013; Lewin et al. 2018; Halabowski & Lewin 2020; Halabowski et al. 2020; Sowa et al. 2020).
Research on diatom assemblages in Upper Silesia and adjacent areas is limited, and most of the published studies are not available to international readers. Only a few of them are indexed in international JCR databases and among those only a few papers discuss the Polish part of Upper Silesia. Some of the papers pertaining to Upper Silesia and adjacent areas deal with the occurrence of diatoms in springs (e.g. Kwandrans 1986; Wojtal 2004; Wojtal & Sobczyk 2012; Wojtal 2013) and in rivers (e.g. Wasylik 1985; Kwandrans 1989, 1993, 1998; Kwandrans et al. 1998, 1999; Kwandrans 2000, 2002; Kawecka & Sanecki 2003; Wojtal & Kwandrans 2006; Cichoń 2016). Recently, the first major paper on the distribution and species composition of diatom assemblages in this region was published by Bąk et al. (2020). However, it does not satisfactorily cover some important aspects and the authors suggest that further research is necessary, especially on secondary saline habitats (Bąk et al. 2020). The conducted research showed that a large proportion of diatom assemblages in highly saline rivers can be represented by brackish and marine diatom species. For example, brackish
The objective of this survey was to present the current distribution and ecological characteristics of the two diatom species –
The research was carried out at seven sampling sites (five rivers) that are located in Southern Poland (Upper Silesia and adjacent areas) in September 2018 (Fig. 1).
The upper reaches of the Centuria and Wiercica rivers are located in protected areas, namely the “Źródła Centurii” (Centuria springs) natural monument and the “Parkowe” nature reserve, respectively. The abiotic types of the studied rivers were defined according to the Directive (2000) and together with the geographic coordinates and altitude of the rivers are listed in Table 1.
General characteristics of the studied rivers (sampling sites)
Geographic coordinates/Characteristic features | the Bolina River, upper reaches | the Bolina River, lower reaches | the Centuria River, upper reaches | the Centuria River, lower reaches | the Mitręga River, lower reaches | the Mleczna River, lower reaches | the Wiercica River, upper reaches |
---|---|---|---|---|---|---|---|
Coordinate N | 50°13′47.6″ | 50°14′44.5″ | 50°24′52.7″ | 50°21′55.2″ | 50°26′04.2″ | 50°07′01.1″ | 50°41′18.0″ |
Coordinate E | 19°05′08.5″ | 19°06′04.7″ | 19°29′11.4″ | 19°29′40.9″ | 19°17′57.4″ | 19°04′29.2″ | 19°24′42.0″ |
Altitude (m a.s.l.) | 262 | 257 | 343 | 311 | 300 | 236 | 309 |
Abiotic type | (type 5) mid-altitude siliceous streams with a fine particulate substrate | (type 6) mid-altitude calcareous streams with a fine particulate substrate on loess | (type 17) lowland sandy streams |
Water samples for physical and chemical analyses were collected from each sampling site at the same time as biological samples. Physical and chemical parameters of water, i.e. electrical conductivity, total dissolved solids, oxygen concentration, temperature and pH, were measured in the field using HI-9811-5 Hanna Instruments and CO-401 Elmetron portable meters. Other parameters of water (concentrations of nitrates, nitrites, ammonium, phosphates, chlorides, sulfates, iron, alkalinity, calcium, magnesium and total hardness) were analyzed in the laboratory according to the standard methods of Hermanowicz et al. (1999). Morphometric features of the riverbeds and velocity were measured according to Hauer & Lamberti (2007).
Diatoms for taxonomic analyses were collected from each sampling site by cutting submerged macrophytes and scraping diatoms from a surface area of 10 cm2 with a soft brush (Zgrundo et al. 2018). Permanent slides for light microscopy were prepared following the standard protocol according to Battarbee (1986) and Bodén (1991). The slides were examined using Zeiss Axio Scope A1 and Nikon Eclipse E600 light microscopes. SEM micrographs were created using a Hitachi SU 8010 SEM at the Podkarpacie Innovative Research Center of the Environment (PIRCE) at the University of Rzeszów, Rzeszów, Poland. Measurements and photographic documentation were made using AxioVision Rel. 4.8 software. For all analyzed samples, a minimum of 400 valves were identified to the species or variety level and their relative abundance was determined. Based on the identified diatom taxa and the number of valves in the studied samples, the ecological condition of the rivers was assessed using a multimetric diatom index (IO) calculated using Excel forms commissioned by the Chief Inspectorate of Environmental Protection in Poland (Zgrundo et al. 2018). The EU WFD uses abiotic parameters to classify streams and rivers into types. For rivers, a fixed typology, i.e. “System A” typology, is defined in the Directive according to ecoregions and size based on catchment area, catchment geology and altitude. Ecoregions are based on the fauna living in European inland waters. Therefore, based on the “System A” typology, homogenous water bodies were selected for the sampling sites in accordance with the above requirements of the Directive (2000), including ecoregions. According to the Regulation (2019) and based on the obtained IO values, the ecological condition classes are discussed using the following class limit values:
For type 5 of rivers – mid-altitude siliceous streams with a fine particulate substrate: very good condition (> 0.69), good condition (≥ 0.50), moderate condition (≥ 0.30), poor condition (≥ 0.15), bad condition (< 0.15);
For type 6 of rivers – mid-altitude calcareous streams with a fine particulate substrate on loess: very good condition (> 0.69), good condition (≥ 0.48), moderate condition (≥ 0.30), poor condition (≥ 0.15), bad condition (< 0.15);
For type 17 of rivers – lowland sandy streams: very good condition (> 0.54), good condition (≥ 0.39), moderate condition (≥ 0.30), poor condition (≥ 0.15), bad condition (< 0.15).
Conductivity, total dissolved solids, chlorides and total hardness were extremely high at the sampling sites located in Upper Silesia and affected by underground salt mine water, i.e. in the upper and lower reaches of the Bolina River and the lower reaches of the Mleczna River (sampling sites S1, S2 and S6). Such results are a consequence of the discharge of mine water containing high concentrations of salts from Carboniferous rocks by underground hard coal mines (Strozik 2017). This is in contrast to other sampling sites located in the Kraków-Częstochowa Upland, where rivers originating from calcium carbonate rocks do not have similarly elevated conductivity (Kondracki 2011). Flow velocity of the studied rivers ranged from 0.023 to 0.722 m s−1. Organic pollution of water was also relatively high as evidenced by the values of biochemical oxygen demand (BOD) and the concentration of sulfates (sampling sites S1, S2 and S6). The concentrations of ammonium, nitrites and nitrates were up to 1.81, 2.702 and 21.26 mg dm−3 at sampling sites S2 and S6, respectively (Table 2). Although water parameters at the sampling sites located in areas adjacent to Upper Silesia are determined primarily by the geology of a river, they are also affected by varying degrees of anthropopressure resulting from hard coal mining, agriculture, urbanization and domestic sewage. The studied rivers impacted by saline coal mine drainage water, located in a highly urbanized area, were characterized by increased conductivity (9290–14 630 μS cm−1), increased total hardness (850–3700 mg CaCO3 dm−3), increased concentration of sulfates (212–660 mg dm−3) and chlorides (3960–16 180 mg dm−3) in water as well as other related water parameters in contrast to other sampling sites where water parameters are generally affected by geology. The studied rivers flowing in the upland are characterized by conductivity values of 280–400 μS cm−1, total hardness of 145–320 mg CaCO3 dm−3 and concentrations of sulfates and chlorides in water of 12–42 mg dm−3 and 10–29 mg dm−3, respectively (Table 2).
Physical and chemical parameters of water, velocity, morphometric features of the studied rivers (sampling sites).
Parameter | Unit | S1 | S2 | S3 | S4 | S5 | S6 | S7 |
---|---|---|---|---|---|---|---|---|
Riverbed width | m | 7.10 | 4.30 | 4.07 | 3.60 | 2.70 | 8.10 | 7.00 |
Riverbed depth | cm | 27.0 | 21.0 | 14.0 | 58.5 | 30.0 | 105.0 | 26.5 |
Flow velocity | m s−1 | 0.055 | 0.451 | 0.105 | 0.722 | 0.023 | 0.170 | 0.143 |
Temperature | °C | 19.3 | 19.4 | 10.4 | 11.8 | 17.5 | 16.5 | 11.0 |
pH | 8.3 | 8.2 | 8.7 | 8.6 | 6.9 | 7.8 | 6.8 | |
Conductivity | μS cm−1 | 14 630 | 35 700 | 280 | 320 | 400 | 9290 | 330 |
Total dissolved solids | mg dm−3 | 7320 | 17 840 | 140 | 150 | 200 | 4640 | 160 |
Chlorides | 6160 | 16 180 | 10 | 12 | 29 | 3960 | 14 | |
Dissolved oxygen | 4.54 | 5.29 | 4.07 | 4.35 | 3.24 | 2.24 | 4.21 | |
Sulfates | 660 | 480 | 40 | 41 | 42 | 212 | 12 | |
Iron | 0.36 | 0.91 | 0.06 | 0.25 | 0.86 | 0.30 | 0.01 | |
Ammonium | 0.58 | 1.83 | 0.00 | 0.12 | 0.38 | 1.03 | 0.01 | |
Nitrites | 0.532 | 2.687 | 0.000 | 0.143 | 0.069 | 2.702 | 0.054 | |
Nitrates | 0.00 | 0.00 | 2.66 | 6.65 | 1.33 | 21.26 | 11.08 | |
Total nitrogen | 1.9 | 4.1 | 2.3 | 2.5 | 5.9 | 2.5 | 4.1 | |
Phosphates | 0.06 | 0.06 | 0.01 | 0.04 | 0.25 | 0.44 | 0.11 | |
Total phosphorus | 0.210 | 0.240 | 0.130 | 0.100 | 0.065 | 0.380 | < 0.050 | |
Biochemical Oxygen Demand (BOD) | 7 | 23 | < 3 | < 3 | < 3 | 4 | < 3 | |
Total Organic Carbon (TOC) | 2.5 | 3.5 | < 2.0 | < 2.0 | 9.8 | 8.7 | < 2.0 | |
Calcium | 412 | 696 | 57 | 60 | 70 | 190 | 78 | |
Magnesium | 75.57 | 476.64 | 0.73 | 0.12 | 35.36 | 91.25 | 19.56 | |
Total hardness | mg CaCO3 dm−3 | 1340 | 3700 | 145 | 150 | 320 | 850 | 275 |
Alkalinity | 360 | 280 | 95 | 130 | 150 | 235 | 150 |
Abbreviations: S1 – the upper reaches of the Bolina River, S2 – the lower reaches of the Bolina River, S3 – the upper reaches of the Centuria River, S4 – the lower reaches of the Centuria River, S5 – the lower reaches of the Mitręga River, S6 – the lower reaches of the Mleczna River, S7 – the upper reaches of the Wiercica River
A total of 141 diatom taxa from 54 genera were identified in the surveyed rivers. The highest number of diatom taxa was recorded for two genera –
Number of diatom taxa, values of the diatom index (IO) for the studied rivers (sampling sites)
Characteristic | the Bolina River, upper reaches | the Bolina River, lower reaches | the Centuria River, upper reaches | the Centuria River, lower reaches | the Mitręga River, lower reaches | the Mleczna River, lower reaches | the Wiercica River, upper reaches |
---|---|---|---|---|---|---|---|
Number of taxa | 39 | 45 | 43 | 48 | 15 | 19 | 24 |
Diatom index (IO) | 0.254 | 0.294 | 0.582 | 0.557 | 0.285 | 0.236 | 0.632 |
The lowest values of the IO index were recorded in the rivers impacted by salt mine water (the Bolina and Mleczna rivers). A similar value of the IO index was recorded for the Mitręga River, whose TOC value was the highest of all the rivers studied (Tables 2 and 3). However, about twice as high values of the IO index were recorded for other rivers. In addition, only the upper reaches of the Wiercica River (S7) were characterized by very good ecological condition. Both S3 and S4 were characterized by good ecological condition, while the remaining sampling sites (S1, S2, S5 and S6) – by poor ecological condition (Table 3).
Percentage of taxa (%) in diatom assemblages at the sampling sites (relative abundance >5% at least at one site).
Taxa | S1 | S2 | S3 | S4 | S5 | S6 | S7 |
---|---|---|---|---|---|---|---|
1.0 | 0.5 | 16.9 | 16.4 | - | 0.9 | 0.9 | |
- | - | 1.4 | 3.7 | - | - | 5.5 | |
- | 0.5 | - | 1.5 | - | - | 9.1 | |
- | - | 0.7 | 5.2 | - | - | 2.7 | |
- | - | 0.7 | 18.7 | - | - | 0.9 | |
- | - | - | 5.2 | - | - | - | |
- | - | 9.2 | - | - | - | - | |
0.5 | 5.4 | - | - | - | - | - | |
- | 10.3 | - | - | - | - | - | |
1.5 | 7.6 | - | - | 10.2 | 15.1 | - | |
- | - | 2.1 | 1.5 | - | - | 9.1 | |
- | - | 19.7 | - | - | - | - | |
3.4 | 8.7 | - | - | - | - | - | |
28.2 | 6.0 | - | 0.8 | 75.2 | 74.9 | - | |
0.5 | 8.2 | - | - | - | - | - | |
- | 5.4 | - | - | 4.4 | 0.5 | - | |
- | 6.0 | - | - | - | - | - | |
10.2 | 6.5 | - | - | 0.7 | - | - | |
- | - | 0.7 | 0.8 | - | - | 5.5 | |
- | - | - | - | - | - | 6.4 | |
17.0 | 0.5 | - | - | - | - | - | |
- | - | 5.6 | 3.0 | - | - | - | |
- | 0.5 | 2.8 | 1.5 | - | - | 33.6 |
Abbreviations: S1 – the upper reaches of the Bolina River, S2 – the lower reaches of the Bolina River, S3 – the upper reaches of the Centuria River, S4 – the lower reaches of the Centuria River, S5 – the lower reaches of the Mitręga River, S6 – the lower reaches of the Mleczna River, S7 – the upper reaches of the Wiercica River
The dimensions of
The rivers flowing through Upper Silesia, which is one of the most industrialized and urbanized regions in Europe, are exposed to strong anthropopressure. The strongest pressure is associated with discharge of salt mine water from underground hard coal mines into the rivers. In addition, in northern regions, including Poland, salt is used for de-icing of roads, which also gets into aquatic environments. This phenomenon leads to the formation of saline lotic and lentic habitats with salinities sometimes similar to those in the Baltic Sea or the North Sea. Both climate change and the growing human population will exacerbate the problem of global salinization of inland waters (Neubauer & Craft 2009). Because of climate change, less de-icing salt is needed each winter in northern areas (such as Poland). As a result, only anthropogenically salinated rivers (e.g. by underground hard coal mines) are a permanent environment with such properties. Examples of such secondary saline lotic waters are the Bolina and Mleczna rivers, which flow through the study area.
Anthropogenic salinity of inland waters leads to the disappearance of freshwater diatom taxa and their replacement by brackish and marine species, which is contributed by, among other things, osmotic stress caused by excess salt in water (e.g. Fritz et al. 1991; Saros & Fritz 2000; Čecháková et al. 2014; Herbert et al. 2015; Bąk et al. 2020). Our results confirm these observations. We recorded a similar number of taxa in the rivers of one abiotic type (type 5 – mid-altitude siliceous streams with a fine particulate substrate) in both the Bolina and Centuria rivers. However, the Bolina River is exposed to strong pressure of salt mine water, while the Centuria River receives no saline water inflow. When comparing rivers of the same abiotic type (type 6 – mid-altitude calcareous streams with a fine particulate substrate on loess), we recorded more taxa in the Mleczna River (under the pressure of salt mine water) than in the Mitręga River (with no inflow of salt water). This phenomenon can be explained by the high proportion of eurytopic species as well as salt-tolerant and marine species. However, it is difficult to conclude which of the phenomena we observe in the studied secondary saline rivers – an invasion of marine species, or periodic occurrence of these organisms under conditions unfavorable for freshwater species (Bąk et al. 2020). On the other hand, the dominance of
A recent paper by Bąk et al. (2020) has demonstrated the necessity to include
Considering the current occurrence of