Introduction

Fluvial ecology disasters, such as mass fish die-offs, are not very common phenomena worldwide, but their frequency varies depending on regional conditions. These incidents occur both in marine and inland waters. Considering freshwater (non-salty), these phenomena affect more often lakes and other reservoirs, but they also happen in rivers. Such disasters have been observed in both natural river courses and artificial water facilities like canals. Notable instances of ecological disasters have been documented in several countries, shedding light on the prevalence of this phenomenon.

In general, mass fish die-offs, most frequently recorded in highly eutrophic reservoirs, are caused by the water movement parameter (Mishra et al. 2022). Mass fish die-offs often occur in coastal areas where the saline water of coastal reservoirs favours the phenomenon of harmful algal blooms (HAB). This phenomenon is considered by many authors to be the most common cause of the mass die-off of freshwater organisms, although it should not concern inland waters. However, high anthropopressure results in high chloride and sodium content in rivers, which, in turn, promotes the development of algae and their toxic blooms (Kim et al. 2021; Woźnica et al. 2023). In the years 1981–2008 in Texas, about 35 million fish died during cyclical fish kill caused by the Prymnesium parvum toxin (Southard et al. 2010; James and De La Cruz 1989). Likewise, in the Red River, the Brazos River, Colorado (Roelke 2011), or in the Wisconsin River (Maakenthun et al. 1945), the HAB phenomenon has caused a mass extinction of aquatic organisms since the second half of the twentieth century. Comparable incidents also occurred in the Chinese rivers: Tianjin, Ningxia, or Shaanxi (Weimin 1983). Mass fish die-offs occurred also in the Florida Channel System between 1984 and 2002 (Hoyer et al. 2009) or in the province of Padua (northern Italy) (Bille 2017). Notable examples of fluvial ecology disasters have been documented in different regions of the world, recently also in Poland.

Similarly to the cases described above, mass organisms die-offs caused by the Prymnesium parvum toxins took place in the flowing water of the Oder River in Poland at the turn of July and August 2022, starting from the lower part of the canalised section down to its mouth to Szczecin Lagoon (Absalon 2023). This event allowed scientists familiar with the water environment of the Oder to undertake a wide-ranging study. These include the research presented in this article. In order to ensure the objectivity of the research and the utilitarian applicability of the conclusions, a description of the Oder's water environment (and the effects of the ecological disaster) is presented below.

In the river's lower stretches, surveys have identified the presence of up to 40 fish species and, in some instances, as many as 52 (Dąbkowski et al. 2017). This includes rare and protected species such as the sichel (Pelecus cultratus) and twaite shad (Alosa fallax). Typically, the fish community encompasses 25–30 species, with dominant species being the common bream (Abramis brama), white bream (Blicca bjoerkna), roach (Rutilus rutilus), perch (Perca fluviatilis), ruffe (Gymnocephalus cernua), and zander (Sander lucioperca) (Szlauer-Łukaszewska et al. 2024). Beyond its fish populations, the Odra valley is a sanctuary for diverse animal groups. This includes waterbirds and aquatic mammals such as the Eurasian beaver (Castor fiber), Eurasian otter (Lutra lutra), and Eurasian water shrew (Neomys fodiens). These species capitalise on the abundant food sources and optimal breeding sites for waterbirds offered by the rivers and floodplains (Marchowski and Neubauer 2019; Marchowski and Ławicki 2023). The freshwater ecosystems are also teeming with a broad spectrum of invertebrates, encompassing insects, mollusks, crustaceans, and other species. These invertebrates are pivotal for nutrient cycling, decomposition, and serve as a food source for higher trophic levels. Native mussel species in the Odra include Anodonta anatina, Unio pictorum, U. tumidus, Pseudanodonta complanata, and Anodonta cygnea (Szlauer-Łukaszewska et al. 2024). Alien species from the genera Dreissena and Corbicula are also present (Szlauer-Łukaszewska et al. 2024).

According to the report provided by the Oder River Emergency Team, the first cases of mass organisms die-offs occurred near the Lipki Weir on 28 July 2022, and then, they were gradually observed in the middle and lower sections of the river. At that time, excessive, mass fish die-offs occurred also in the water of the Gliwice Canal that connects with the Oder River far upstream of the Lipki Weir. It should be noted that fish kill in this water facility (man-made water course) for the first time in 2022 took place on 17 March, while this phenomenon intensified on 14 and 18 July and then between 18 and 20 August and between 28 August and 9 September (Kolada et al. 2022). For this reason, it was deemed appropriate to investigate the relationship between the water of the facility (the Gliwice Canal) and the water in a large natural river (the Oder). The two environments are compared below.

The Gliwice Canal is a water facility, i.e. a man-made water course. The construction of the canal began in 1934, and it was put into service on 8 December 1939 primarily for transport purposes (Bożek 1968; Neumann 1934). It is 40.6 km long and consists of seven lock-separated sections. It also has a flood control function (Gliwice 2015).

In contrast, the Oder River is a natural lowland nature water course. This is a transboundary river flowing in its upper section through the Czech Republic then through Poland and in part through the Federal Republic of Germany. The Oder River flows 742 km in the western part of Poland, and it is the second longest river in the country. The Gliwice Canal joins the Oder River in Kędzierzyn-Koźle (Opole Province), where the river begins its initial flow in the canalied section. Despite adjusting the bed of the river to inland navigation, it has retained its natural character, and the natural environment within the Oder bed is a river environment typical for Central Europe. Apart from the navigation function, the bed of the Oder River and the Gliwice Canal differ in every way, e.g. morphology (concrete bed of the Gliwice Canal vs. the naturally formed bed of the Oder River), water supply, depth, and downslope. However, the primary parameter that distinguishes the bed of the Oder River from the Gliwice Canal bed is the flow rate. The flow rate in the Oder River depends on the natural hydro-meteorological conditions, but it always exceeds the 0 value. The flow rates in the Gliwice Canal are controllable. In closure periods (about 95 days per year) and the lack of navigational activity, no lock operation is performed, so there is no water exchange. In such a situation, individual sections become reservoirs of immovable water (Kostecki et al. 2001). The term immovable water refers to water that does not exchange with its surroundings until this movement is forced (e.g. by opening a lock). In response to the disaster affecting the Gliwice Canal, the authors decided to compare the water quality between two different environments: flowing water and the water in the Gliwice Canal. They also aimed to assess the impact of eutrophic water retained in the canal on the water quality of the Oder River. The influence of water inflow from such facilities is a relatively underexplored topic in international literature, particularly regarding the combined effects of hydro-meteorological conditions on biological environments. Given the simultaneous mass fish die-offs in the Gliwice Canal (a managed water facility) and the Oder River (a natural flowing watercourse) during the summer of 2022, this study addresses the significant issue of the Gliwice Canal's impact on the Oder River’s water quality and the consequent die-off of aquatic organisms.

The main purpose of the study is to assess the Gliwice Canal's impact on the quality of the Oder River water. The studies performed will make it possible to verify the variability of the Oder River water quality above its junction with the Gliwice Canal against its water quality further downstream. On the other hand, by analysing the water quality in individual sections of the Gliwice Canal, authors make attempts to answer the question, of whether the aquatic life die-offs within this water facility were directly related to the mass fish and other organisms die-offs in the Oder River in August 2022.

Materials and methods

Study area

The Gliwice Canal is located in the south of Poland, and it flows through the area of two (voivodeships): Silesian and Opole Province. The 0 + 000 km of the canal is located in Kędzierzyn-Koźle, on 98.1 km of the Oder River. The canal flows in sequence through the commune of Kędzierzyn-Koźle, the southern part of the Ujazd commune, Rudziniec, and the southern part of Pyskowice town, and ends in the southern part of Gliwice city, in the district of Łabędy near the Gliwice Port. The Gliwice Canal is located partly in the catchment area of the Klodnica River, a right tributary of the Oder River.

In the physical and geographical terms, the Gliwice Canal is located within the Extra Alpine Central Europe megaregion. Its eastern part lies in the area of the subprovince Polish Highlands, while its western part belongs to the subprovince of the Central European Plain. The western part of the Gliwice Canal consisting of sections VI, V, IV, and III flows through the macroregion Silesian Highland, while its western part consisting of sections II, I, and 0 through the macroregion of Silesian Lowland. The Gliwice Canal flows through the area of three mesoregions—sections 0, I, and II, are within the Racibórz Basin, sections III and IV, the prevailing part of section V, are located within the Bojszów Depression, while the smaller eastern part of sections V and VI is located within the Katowice Highland (Richling 2018). The total length of the Gliwice Canal is 40.6 km. The Gliwice Canal overcomes a height difference of 43.6 m because of six locks (Table 1).

Table 1 The locks of the Gliwice Canal
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These locks divide the Gliwice Canal into seven sections (Fig. 1). The canal is supplied in water by the Kłodnica and Drama Rivers, and water reservoirs Dzierżno Duże and Dzierżno Małe (RZGW Gliwice 2015). Water samples from the Gliwice Canal were taken at nine sites on 16 April 2023 (Fig. 1).

Fig. 1
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Study area

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Chemical analysis

The measurements of pH, electrolytic conductivity, dissolved oxygen (DO) levels, and temperature were taken in the field. The pH parameter was measured using a CP-401 pH metre supplied by Elmetron. The electrolytic conductivity was measured using a water-tight conductometer CC-401 supplied by Elmetron. The dissolved oxygen level in the canal water was measured using an oxygenmeter Oxi 330 supplied by WTW. The concentrations of sulphates (SO42−), phosphates (PO43−), nitrate nitrogen (N(NO3−)), chlorides (Cl−), sodium (Na+), potassium (K+), magnesium (Mg2+), and calcium (Ca2+) were measured in the laboratory of Institute of Earth Sciences, Faculty of Natural Science, University of Silesia.

The concentration of sulphates and phosphates was measured by using the HACH LANGE DR 3900 spectrophotometer. The level of phosphates in samples was determined by using a colourimetric test method, while the concentration of sulphates was determined by using a turbidimetric test method. The concentration of chlorides and nitrate nitrogen in test samples was analysed using the potentiometric method by using an ion-selective electrode. The level of sodium and potassium was measured by using flame emission spectrophotometry using the REMED FPM 871 EM photometer. The concentration of calcium and the general hardness of water samples were measured by using a titration method with a digital burette BRAND 25 mL. The concentration of magnesium was calculated based on calcium content and the general hardness of water samples.

The results from the remaining periods were made publicly available by the Chief Inspectorate for Environmental Protection. Data had been downloaded regularly from the website: https://www.gov.pl/web/odra/badania-odry (recent access on 30/04/2023).

Unveiling relationships in data—correlation and principal component analysis

Statistical analyses of data were used to calculate research results. Correlation analysis is a statistical technique used to uncover and quantify relationships between two and more variables within a dataset. It assesses the degree to which changes in one variable correspond to changes in another. This analytical method is employed in various fields to explore connections, dependencies, and associations between variables. Correlation analysis provides valuable insights into the strength and direction of relationships.

Furthermore, to check the accuracy of the correlations obtained, principal component analysis (PCA) was used. The PCA is a typical display method that allows us to estimate the internal relations in a dataset. There are various variants of PCA but basically, their common feature is that they show a linear combination of the original columns in a dataset responsible for the description of the variables characterising the observation objects. The linear combinations represent a type of abstract measurement (factors and principal components) being better descriptors of the data pattern than the original (chemical or physical) measurements. Usually, the new abstract variables are called latent factors, and they differ from the original ones called manifest variables. It is a common finding that just a few of the latent variables account for a large part of the dataset variation. Thus, the data structure in a reduced space can be studied (Kozak et al. 2016).

Results

Measured environmental parameters

The following physicochemical parameters were taken into account during the analysis:

  • Temperature

  • Electrolytic conductivity

  • pH

  • Oxygen

  • Sulphates

  • Chlorides

  • Sodium

Measurements performed by authors included also the level of phosphates, magnesium, and calcium. The measurements sites, together with the difference in measurement frequency, are presented in the tables below (Tables 2 and 3).

Table  2 Location of the measurements sites
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Table  3 Number of valid observations for each parameter
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Temperature

The time series of the temperatures registered in Utrata and Lipki Weir cross-sections (the Oder River) and in Gliwice Marina, Pyskowice, Ujazd, Rudzieniec (Gliwice Canal—“GC”) are presented in Fig. 2. It can be noticed that the temperatures at all measuring sites (obviously) show a very similar seasonal pattern. This is confirmed by the time series displayed in the form of boxplots with monthly windows (Fig. 3).

Fig. 2
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Temperature time series

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Fig. 3
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Temperature time series in the form of monthly boxplots

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The seasonal water temperature variability in the Oder River and the Gliwice Canal does not show any anomalies as compared to the seasonal variability from a multiannual period. In the Gliwice Canal (section III—Ujazd) temperatures in February and March unusually increased. On the other hand, along with the increase in water temperature of the Gliwice Canal in March and April 2023, the cases of fish die-offs were observed (significantly fewer than in summer). Figure 2 clearly shows that the water temperature during the ecological disaster was 20 °C, whereas in the Oder River, it reached 25 °C, while in the Gliwice Canal was nearly 30 °C. The temperatures recorded in the summer months of 2022 (August and September) at all measurement sites on the Gliwice Canal were higher than the temperatures of water in the Oder River (Fig. 3). This dependency clearly shows that immovable water in a water facility (man-made waterway) heats up much faster than water flowing in a natural river bed. The higher the temperature the more intense the photosynthesis, which causes higher biological activity resulting in i.a. algal blooms (Fig. 4).

Fig. 4
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Conductivity time series

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Conductivity

Testing specific conductivity allows us to detect the changes in water chemical composition (chemistry), especially in periods of high salinity. The conductivity time series differ significantly between the Oder River and the Gliwice Canal. There are significant differences between the cross-sections of the Gliwice Canal. The Gliwice Marina and Pyskowice measurement sites show similar values in time, also the sites in Ujazd and Rudziniec are similar to each other. However, in autumn/winter (October to 22 January), the values between Gliwice Marina and Pyskowice measurement sites differ significantly from the values observed in Ujazd and Rudziniec. In Rudziniec, a seasonal conductivity pattern may have occurred, but comparing them would require a greater amount of data. These differences clearly show that the quality of water flowing in a specific canal section does not affect the quality of water in the neighbouring section. On the other hand, the level of salinity in individual sections (as a parameter that directly influences the value of conductivity) depends on the intensification of anthropogenic pressure in a specific section (e.g. wastewater discharge).

In Fig. 5, the differences in seasonal patterns have become even more visible. At the Lipki Weir, Utrata (both in the Oder River), and at Gliwice Marina and in Pyskowice, the conductivity levels tend to rise during August, November, and December. However, in Ujazd and Rudziniec, the situation is different. The levels of conductivity in the Oder River and the Gliwice Canal differ significantly. Values observed in the Oder are lower than in the Gliwice Canal. However, it is impossible to notice that downstream the junction of the Gliwice Canal with the Oder, the values of the electrolytic conductivity increased as compared to the upstream measurement site. Therefore, it indicates that the water flowing into the Oder from the Gliwice Canal does not change significantly the salinity of water in the Oder River.

Fig. 5
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Conductivity time series in the form of monthly boxplots

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The plot provided in Fig. 6 presents the conductivity values measured by authors on 16 April 2023 (red dots) compared with the closest (in time and space) measurements taken by the Chief Inspectorate of Environmental Protection (GIOŚ) (blue dots). The values in the Oder River are compatible. The values observed in the Gliwice Canal are similar to those observed in the time-series plot (Fig. 4). This plot (Fig. 6) confirms that the conductivity levels in the Gliwice Canal are higher than in the Oder River.

Fig. 6
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Conductivity comparison in the Gliwice Canal and neighbouring cross-sections in the Oder River

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The difference in the value of the conductivity parameter between the Oder (a natural river) and the Gliwice Canal indicates the natural ability of flowing water to dissolve and reduce salts, mainly introduced by human activity in the region under research.

pH

The average pH values in the Oder River are lower than in the Gliwice Canal. Also, pH values in the Gliwice Canal show a wider range of variability than the values found in the Oder River (Figs. 7 and 8) (pH time series differ significantly between the Gliwice Canal and the Oder River). In all the pH time series, a period (October–November) can be noticed when the pH value variability is lower. However, such a situation may result also from a lower frequency of measurements (see Table 3).

Fig. 7
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pH time series

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Fig. 8
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pH time series in the form of monthly boxplots

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Table 1 displays information on the frequency of measurements. It can be noticed that measurements in Rudziniec were taken less frequently than at other measurement sites of interest.

In general, pH values observed in both cross-sections of the Oder River are very similar. In the Gliwice Canal, the differences between pH values registered in different cross-sections are higher. However, pH observed at Gliwice Marina, in Pyskowice and Ujazd are more similar to each other, than to values observed in Rudziniec.

In Fig. 9, pH values taken from the official data are compared with the values measured by the authors. Although the values differ, they can be considered very similar. The right-hand side of the figure displays the pH values measured along the Gliwice Canal. The results obtained by the authors confirm the aforementioned conclusions stating that the pH values observed in the Gliwice Canal are higher than in the Oder River.

Fig. 9
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pH comparison in the Gliwice Canal and neighbouring cross-sections in the Oder River

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With reference to Fig. 9, it should be noted that the pH of the Oder water is higher downstream of the junction with the Gliwice Canal, which is confirmed by both authors’ measurements and the ones taken by the Chief Inspectorate for Environmental Protection (GIOŚ).

Oxygen

The plot below (Fig. 10) presents the time series of the oxygen measurements in the Oder River and the Gliwice Canal. The values observed at measuring sites in the Oder River are similar to each other. A similar situation can be noticed in the Gliwice Canal. However, the oxygen levels in the Gliwice Canal between August and October are significant. The values in all cross-sections of GC (Gliwice Canal) (except for Rudziniec—no data available) vary between very extreme values (5–23 mg/L). Between October and mid-January, the oxygen levels vary less. However, the variability of the dissolved oxygen (DO) intensifies in the second half of January. A similar situation can be observed for the Oder River, but the oxygen levels and their variability are not as intensive as for the Gliwice Canal. At the end of the observation period, the oxygen levels in the Gliwice Canal increased significantly.

Fig. 10
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Oxygen time series

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Measurements were taken at different periods. A huge variability noticed during the period of ecological disaster could be explained by the impact of golden algae, much stronger in the Gliwice Canal (where fish die-offs happened) than in the Upper Oder (where no fish die-offs happened). This strong oxygen content variability in the Gliwice Canal is also correlated with pH variability in August 2022.

The boxplots (Fig. 11) displaying the oxygen levels present the aforementioned conclusions in a different form. It can be concluded that in the first 3 months of the measurements, oxygen levels in the Oder on average are lower than in the Gliwice Canal. It can also be found that the oxygen levels in the Oder River are more stable than in the Gliwice Canal. Seasonal patterns can be observed both in the Oder River and the Gliwice Canal. The pattern at both cross-sections of the Oder River is similar. The patterns in Gliwice Marina and Pyskowice are similar, whereas oxygen concentration patterns in Ujazd and Rudziniec are not similar to each other, nor are Gliwice Marina and Pyskowice.

Fig. 11
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Oxygen time series in the form of monthly boxplots

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The plot below (Fig. 12) presents oxygen levels at measuring sites in the Oder River and along the Gliwice Canal. As previously, the results from measurements taken a day earlier by the authors are similar to values coming from the official data source (GIOŚ). The oxygen levels in section 0 of the Gliwice Canal are similar to those registered in the Oder River. The values observed by the research team in the Gliwice Canal confirm its peculiar characteristics spotted earlier in the time series of oxygen levels (highest value exceeding 27.5 mg/L).

Fig. 12
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Comparison of oxygen concentrations in the Gliwice Canal and neighbouring cross-sections of the Oder River

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Likewise, for changes in pH value, the oxygen level in the Oder water increases downstream the Gliwice Canal, which is confirmed by both own measurements and the measurements taken by the Chief Inspectorate for Environmental Protection (GIOŚ) (Fig. 12). However, the increase is not significant.

Sulphates

The levels of sulphates in the Oder River and the Gliwice Canal differ significantly. In the Oder River, the levels of sulphates are very stable, and they usually do not exceed 200 mg/L. In the Gliwice Canal, a similar situation took place in Ujazd and Rudziniec from October to December. In general, the variability of sulphate concentration in the Gliwice Canal is very high, as compared to the Oder River. The sulphate levels detected at Gliwice Marina and in Pyskowice are generally higher than in Ujazd and Rudziniec. However, at the end of the observation period, the values become more similar, yet still the differences reach 200 mg/L (Fig. 13).

Fig. 13
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Sulphate level time series

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Monthly boxplots displayed in Fig. 14 show that sulphate levels vary more in the Gliwice Canal than in the Oder River. Sulphate levels in the Oder are significantly lower than in the Gliwice Canal. In both watercourses, the occurrence of seasonal sulphate level variability is possible. However, it shows different characteristics in each watercourse. Also, the seasonal pattern differs between the Gliwice Canal cross-sections. The Gliwice Marina and Pyskowice measurement sites show similar patterns, whereas Ujazd and Rudziniec sites are not similar to each other nor were previously mentioned measurement sites.

Fig. 14
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Sulphate level time series in the form of monthly boxplots

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The plot shown in Fig. 15 confirms that sulphate levels in the Gliwice Canal are higher than in the Oder River. However, the observed values do not exceed 225 mg/L, which is rather not high as compared to values observed in the time series (up to 800 mg/L). As previously, the values measured by the authors confirm the values presented by the official data source.

Fig. 15
figure 15

Comparison of sulphate levels in the Gliwice Canal and neighbouring cross-sections of the Oder River

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Downstream the junction between the Oder and the Gliwice Canal, the sulphate level also increases (Fig. 15). The increase in another parameter (besides pH and oxygen level) shows the impact of the Gliwice Canal water on the salinity of the Oder River.

Chlorides

The levels of chlorides in the Oder River are lower than in the Gliwice Canal. Also, the variability of chloride levels in the Oder is lower than in the Gliwice Canal. The levels of chlorides in both cross-sections under consideration in the Oder are very similar throughout the observation time span (Fig. 16). The exceptions can be noticed in August, September, December, and November. However, the differences do not seem very large. For the Gliwice Canal, the chloride levels are considerably different. From August to October at Gliwice Marina and in Pyskowice, the observed values are very similar; however later, the difference between them becomes more and more significant. Chlorides in Ujazd and Rudzieniec are lower than at Marina and in Pyskowice sites. Starting from February 2023, the chloride levels become more similar. The values observed in Ujazd become higher, while the values detected at Gliwice Marina become lower.

Fig. 16
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Chlorides time series

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The monthly boxplots of chloride levels reveal seasonal patterns similar to those observed for different parameters. The patterns observed in Utrata and Lipki Weir are very similar. Similar patterns can be also observed at Gliwice Marina and in Pyskowice, whereas patterns observed in Ujazd and Rudziniec differ (Fig. 17).

Fig. 17
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Chloride time series in the form of monthly boxplots

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The chlorides levels in the Oder River measured by the authors the day before the official data was recorded differ by about 200 mg/L (Fig. 18). The chloride levels in the Gliwice Canal are higher than in the Oder (ranging from about 600 to 900 mg/L).

Fig. 18
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Comparison of chlorides in the Gliwice Canal and neighbouring cross-sections of the Oder River

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Authors suppose that the difference primarily results from the significantly limited ability of the canal water to self-clean because of the low water exchange (water movement is forced only by the operation of locks). In contrast, the flowing water of the Oder River intensifies dissolving the anthropogenic pollution (Carstea et al. 2014). In addition, the high level of chlorides in the Gliwice Canal can result from the water discharge from the Kłodnica River supplied with rich in chlorides waters coming from underground mine drainage systems (Matysik 2018; Lach et al. 2004).

Sodium

The sodium concentration time series and seasonal patterns are very similar to the ones observed previously. The values observed at the Oder River measuring sites are similar, whereas the values observed at the Gliwice Canal measuring sites differ. The differences are most significant between October and January. Before and after that period, the observed values are closer to each other. Yet still, their variability is clearly visible. Rudziniec and Ujazd measurement sites show similar sodium levels in time. This is also the case for the Gliwice Marina and Pyskowice sites (Figs. 19 and 20).

Fig. 19
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Sodium time series

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Fig. 20
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Sodium time series in the form of monthly boxplots

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The sodium levels in the Oder measured by the authors are very close to the values provided by the Chief Inspectorate of Environmental Protection (GIOŚ). Sodium levels measured by the authors in the Gliwice Canal are higher than in the Oder, which confirms the official data (Fig. 21).

Fig. 21
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Comparison of sodium levels in the Gliwice Canal and neighbouring cross-sections of the Oder River

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The concentration of sodium is much lower downstream of the junction between the Oder and the Gliwice Canal (Fig. 21). The seasonal sodium level variability in individual sections of the Gliwice Canal correlates with the seasonal variability of chloride levels. The values of those parameters likewise conductivity are much higher in the Gliwice Canal than in the Oder and confirm higher salinity in the water facility (canal), but the water from the Gliwice Canal does not affect the salinity level in the Oder River, which is clearly visible in plots (Figs. 21, 18, and 12).

Phosphates, magnesium, and calcium

Data for the Oder River are not available. No measurement results for these parameters provided by the Chief Inspectorate of Environmental Protection (GIOŚ) are available below (Figs. 22, 23, and 24).

Fig. 22
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Comparison of phosphate levels in the Gliwice Canal and neighbouring cross-sections of the Oder River

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Fig. 23
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Comparison of magnesium levels in the Gliwice Canal and neighbouring cross-sections of the Oder River

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Fig. 24
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Comparison of calcium levels in the Gliwice Canal and neighbouring cross-sections of the Oder River

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The monitoring conducted by the Chief Inspectorate of Environmental Protection (GIOŚ) at sites under analysis is the so-called intervention monitoring that resulted from the ecological disaster that occurred. That is why the first data come from August 2023, while for selected parameters or measurement stations, the monitoring is not conducted at regular intervals. The data presented herein come from the data published on the website developed specially to monitor the Oder River and the Gliwice Canal (https://www.gov.pl/web/odra/badania-odry). The remaining parameters were analysed only based on the authors’ own tests; therefore, the above-mentioned parameters cannot be compared.

Statistical analysis

Statistical analysis assumes paramount significance in the realm of environmental data due to its pivotal role in furnishing a rigorous, evidence-driven framework for comprehending intricate environmental systems. Environmental datasets frequently exhibit a propensity for variability and uncertainty, rendering statistical methodologies indispensable for discerning trends, appraising the influence of environmental variables, and facilitating judicious decision-making. Such analyses facilitate the discernment of underlying patterns, the assessment of causal relationships, and the precise quantification of environmental risks, all of which are imperatives in the pursuit of effective environmental management and conservation.

Correlations between parameters

Correlation analysis allows us to spot important dependencies between parameters under consideration. The analysis below utilises a linear correlation only.

The pair plot (Fig. 25) was created for combined data from all (significant) measuring sites under consideration, grouped with respect to the watercourse of origin. Although such an approach does not distinguish particular measuring sites, it provides a general insight into the relationships between the variables (measured parameters) in watercourses. Moreover, the scatter plots displayed in Fig. 25 reveal that values observed in the Oder River and the Gliwice Canal are different and form small clusters in different areas of the plots. Such a situation, for example, can be easily spotted in plots made for x = temp and y = conductivity or x = pH and y = conductivity. Also, histograms (at the bottom), boxplots (at the right), and kernel density plots (KDE—on diagonal) confirm that the distributions of all parameters except for temperatures differ significantly in the Oder River and the Gliwice Canal.

Fig. 25
figure 25

The plot of correlation pairs of the GIOŚ data: Red colour denotes the Oder River, and blue colour denotes the Gliwice Canal

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Clear correlations can be stated between the following parameters:

  • Conductivity and sulphates.

  • Conductivity and chlorides.

  • Conductivity and sodium.

  • Sulphates and chlorides.

  • Sulphates and sodium.

  • Chlorides and sodium.

These correlations result from the fact that the conductivity value indicates the salinity level while chlorides, sulphates, and sodium are salts that affect the level of salinity (Woźnica et al. 2023).

There is also a correlation between the temperature and oxygen concentration that can be noticed, however only for the Oder River, as such a relationship does not occur for the Gliwice Canal. A more detailed view on correlations is shown in Fig. 26 a and b.

Fig. 26
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a Correlation matrix for the Oder River. b Correlation matrix for the Gliwice Canal

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The plot in Fig. 26 a and b displays correlation matrices for the Oder River and Gliwice Canal. There are a few interesting things that should be noticed. Firstly, it is very clear that there is a strong negative relationship between temperature and dissolved oxygen (the higher the temperature, the lower the oxygen level). Such a relationship is observed only in the Oder River (R =  − 0.8, p value = 6.69 × 10−42), whereas in the Gliwice Canal, such a relationship is very weak and positive (R = 0.2, p value = 1.07·10−4). There are also noticeable differences in conductivity and sulphate levels. The correlation between those variables in the Oder River is R = 0.67 (p value = 1.88·10−25), whereas for the Gliwice Canal, the Pearson's correlation coefficient adopts the value R = 0.95 (p value = 4.76·10−184). In the Oder River, pH does not correlate with sulphate, chloride, or sodium levels, whereas for the Gliwice Canal, there are slight correlations visible. In the case of correlations between sulphate and chloride levels, sulphate and sodium levels are lower in the Oder River than in the Gliwice Canal. A similar situation takes place for chlorides and sodium. Apart from the above-mentioned changes, the correlation matrix patterns are similar.

Below (Fig. 27 a, b and 28 a–d) correlation matrices for each of the cross-sections are provided. It can be noticed that despite some differences between cross-sections, the correlation matrix patterns are generally similar for each of the watercourses.

Fig. 27
figure 27

a Correlation matrix for data at the Oder Utrata measuring site. b Correlation matrix for data at the Oder Lipki Weir measuring site

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Fig. 28
figure 28

a Correlation matrix for data in the Gliwice Canal: Gliwice Marina measuring site. b Correlation matrix for data in the Gliwice Canal: Pyskowice measuring site. c Correlation matrix for data in the Gliwice Canal: Ujazd measuring site. d Correlation matrix for data in the Gliwice Canal: Rudziniec measuring site

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Principal component analysis (PCA)

The principal component analysis (PCA) transforms the dataset into a new space where the axes are aligned with the directions of the maximum variance for each data dimension. Such a proceeding allows us to analyse the relationships between the original variables in the new space.

A common way to display linear relationships between variables is a correlation circle. Such a plot presents the original variables in the form of vectors in the newly transformed space. If arrows point in similar directions, the variables are strongly positively correlated. If they point along the same line, but the heads of the arrows point in the directions opposite to each other, the correlation is strong but negative. If the arrows are perpendicular, the variables do not correlate. Below the PCA correlation circles are provided for each of the watercourses (Figs. 29 and 30). The following relationships can be noticed:

Fig. 29
figure 29

Principal component analysis: correlation circle for the Oder River

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Fig. 30
figure 30

Principal component analysis: correlation circle for the Gliwice Canal

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The Oder River

  • Oxygen negatively correlates with temperature

  • Oxygen moderately correlates with pH

  • There is a very strong correlation of variables: sodium, chlorides, and conductivity

  • Sulphates are moderately correlated with sodium, chlorides, and conductivity

The Gliwice Canal

  • Oxygen moderately correlates with pH

  • There is a very strong correlation of variables: sulphates, sodium, chlorides, and conductivity

  • pH moderately correlates with temperature

The plots of individual data points from the Oder River (Fig. 31) and Gliwice Canal (Fig. 32) are distinguished with respect to measurement site display showing no particular differences between measurement sites (point clouds overlap).

Fig. 31
figure 31

Individual data points plotted in the principal component space distinguished with respect to the Oder measurement site: Utrata and Lipki Weir

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Fig. 32
figure 32

Individual data points plotted in the principal component space distinguished with respect to measurement site GC (Gliwice Canal): Gliwice Marina, GC: Pyskowice, GC: Ujazd, and GC: Rudziniec

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Discussion and conclusions

Fluvial ecology disasters are a worldwide problem, with mass fish die-offs occurring in both natural river systems and artificial waterways or water facilities. Harmful algal blooms, high nutrient levels, and pollution are common triggers for these events, leading not only to contamination of the river itself but also of the reservoirs they flow into, i.e. seas and oceans, changing their chemical composition (Kosek et al. 2016; Kosek and Kukliński 2023). The impacts of these disasters extend beyond the affected river, affecting downstream ecosystems and even water facilities (man-made waterways). Recognising the global nature of these incidents is crucial for implementing effective management and mitigation strategies to protect fluvial ecosystems and the environments associated with them.

The quality of the tributary waters plays a key role in shaping all parameters determining the water quality of a first-order stream (Yao et al. 2023). In the summer of 2022 and later, this was the Gliwice Canal water that featured the highest levels exceeding standards for chlorides, sulphates, nitrates, and biogenic substances and first of all, the specific conductivity level (Kolada et al. 2022). However, it should be noted that the specificity of a water facility (man-made waterway) as a canal suppresses frequent water mixing. This is because the canal is not a river tributary, but just a facility that joins it. The amount of water introduced is fully adjustable; hence, the natural impact here should be ruled out (Kostecki et al. 2001). These tests were supposed to verify the level of the Gliwice Canal waters affect the water quality in the Oder River.

In general, in sections 0–IV, test parameters remained at a similar level. High variability in water quality related to the increase in sulphate, phosphate levels, pH, and dissolved oxygen was observed near sections V and VI, i.e. within the Gliwice Port. According to, the development and operation of the port itself are of great importance for the quality of water, regardless of whether the port is part of inland or marine waters. This anthropopressure is particularly manifested by the intensification of water eutrophication, which means an increase in biogenic substances such as sulphates and phosphates. In addition, in section IV, the Gliwice Canal shares the bed with the Kłodnica River which according to studies conducted by W. Nocoń et al. (2006) is a river heavily polluted by domestic and industrial wastewater. Furthermore, sections V and VI of the Gliwice Canal are supplied by water and wastewater originating from the heavily urbanised zone of the Upper Silesian metropolitan area (Hamerla, Pierzchała 2016). The above-mentioned factors can affect the values significantly different from the parameter values observed for the remaining sections under analysis. Regarding the data published in the report provided by A. Kolada et al. (2022) in those sections, mass fish die-offs occurred.

A completely different situation was observed in sections 0, 1, and 2, located in Kędzierzyn, while section 0 comprises a direct junction of the Gliwice Canal with the Oder River. The lowest pollution level in this section was found in tests performed in 2001 by M. Kostecki et al.

However, it should be clearly emphasised that the canal is not a natural hydrographic object and cannot be considered a typical tributary. The frequency of the canal water mixing fully depends on the frequency of lock operations. If the lock operations are limited, the water in individual canal sections can take on the features of immovable water (Szumińska 2008). In turn, immovable water is an environment where a particular intensification of eutrophication phenomenon can be observed (Bazrkar 2013). Water in the canal is especially endangered by eutrophication when water movement is restrained (by suspending lock operations) (Zhan et al. 2022), which took place in the summer months of 2022.

In section VI of the canal where fish die-offs were very intense, the highest phosphate concentrations contributing to algal development and bloom were found (Handa 1990). PO43− and NO3−, including especially phosphates, are indicated as especially good stimulants of algal development (Yang et al. 2008). The concentration of both phosphates and nitrates in sections V and VI, where the most massive fish die-offs took place, was very high.

The level of dissolved oxygen (DO) in the Oder River was lower in the summer. High water temperatures stimulate the reduction of oxygen solubility in water (Jabłońska 2008). The dissolved oxygen (DO) level can be the indicator of eutrophication level—its occurrence contributes to lowering the level of dissolved oxygen because it is taken in by the developing algae (Hanjaniamin et al. 2023).

A very high Cl− and Na+ levels in the water of the Gliwice Canal can result from the discharges of saline mine water (Kostecki et al. 2001). According to measurements taken by authors and data provided by the General Inspectorate of Environmental Protection (GIOŚ), the level of those substances in the Oder River downstream the mouth of the Gliwice Canal has increased. The amount of chlorides determines water salinity, and this stimulates its eutrophication process (Dissanayaka et al. 2022). The increase in salinity of the Oder River system has also a significant impact on ecosystem changes, including the expansion of alien and invasive species that tolerate or prefer salty waters. In the case of the ecological disaster in the Oder River, salinity was an important factor favouring the growth of the euryhaline species Prymnesium parvum. In addition, in 2022, we observed high temperatures, which could have supported algal blooms. In those conditions, thermophiles such as cyanobacteria and toxic haptophytes (e.g. Prymnesium parvum) grow fast (Sługocki and Czerniawski 2023).

The electrolytic conductivity of water indicates the amount of dissolved substances—the higher the content, the higher the conductivity, which is confirmed by statistically significant correlation values ​​of conductivity with the content of sulphates, chlorides, and sodium both in the Oder River and in the Gliwice Canal. This parameter depends also on water temperature—the higher the temperature, the higher the conductivity (Dewangan et al. 2023).

Based on the measurements taken by the GIOŚ in the Oder River at sites upstream and downstream of the Gliwice Canal mouth from August 2022 to April 2023, one can observe the relationship between fluctuations in temperature and the level of water conductivity. However, in the winter period despite low temperature, the value of conductivity remained at a relatively high level. Except for the increase in sulphate level and pH downstream the junction of the Gliwice Canal with the Oder River, both measurements taken by authors and provided by the GIOŚ show no increase or significant change as compared to the levels of those parameters detected upstream the junction with the canal.

The tests conducted have allowed us to confirm that the impact of the Gliwice Canal water does not significantly change the quality parameters of water in the Oder River. Because of the completely different character of the water facility (man-made waterway), the level of the same parameters has completely different values not only compared to the natural Oder riverbed but also in individual (closed lock-separated) sections of the canal, although seasonal variability is directly proportional. Because of this, the qualitative analyses of canal beds and other man-made water facilities should be conducted separately from the natural beds of watercourses (they should not be compared to each other). The achieved results clearly indicate that comparing parameters together for both Oder River and Gliwice Canal may cause false conclusion.

Although the research carried out focused on the impact of the Gliwice Canal and the Oder River, there are numerous water facilities connected to natural rivers in Europe or North America (similar latitude), whose water environment is similar to the facilities studied. Therefore, the observation resulting from this article indicates the need to adopt a new research approach to qualitative analyses of inland flowing waters (separating natural watercourses and water facilities).

Performed study clearly indicates that water in Gliwice Canal does not impact on the water quality in the Oder River in aspect of mass die-off of aquatic organisms in summer 2022. Although the Oder River with the Gliwice Canal is functionally linked to each other (they make up one of the few inland waterways in Poland) and special attention was paid to exceeding the standard values ​​of the canal's water quality during the period of ecological disaster in the Oder, this water facility (man-made waterway) has not been analysed extensively enough yet. Because of this and to confirm this thesis, further studies and analyses of seasonal variability of quality indicators for water in the Gliwice Canal are required.