This study determined changes in the ecological status of a lowland river under the impact of an urban agglomeration after modernizing the wastewater management. For 174 years the Ner River was loaded with pollution from expanding industry. Construction of the Group Sewage Treatment Plant (GOŚ) in 1994 and its subsequent modernization should have improved the water quality. After 20 years of proper sewage treatment the ecological status of the river hasn't improved.

Study sites were established on the river above and below sewage disposal from GOŚ, and the ecological status was assessed on the basis of benthic diatoms. Diatom indices: IO, GDI, IPS were calculated in order to determine the ecological status and water quality. Hierarchical cluster analysis and multivariate analysis of diatom assemblages were used to reveal differences of the study sites. It was revealed that the river is divided into two sections: the first is above the sewage disposal and the second is below it. In the first section, domination of species sensitive to organic pollution Meridion circulare and Ulnaria ulna was noted, while in the second section species resistant and tolerant to organic pollution, Hippodonta capitata and Navicula gregaria, were noted. The ecological status at the first section was moderate to poor, while at the second it was poor to very poor.

The division of the urban river into two sections with different ecological status stems from the history of water management. The present location of the GOŚ outflow served in the past as a dump of untreated sewage from the industry of the Łódź Agglomeration. Development of the city led to the degradation of the ecosystem. Modernization of wastewater management did not reverse the degradation of the river. The persistence of accumulated industrial pollution in sediments caused by the long-term impact of urban agglomeration almost completely destroyed the ecosystem of the river.

Introduction

The development of industry in Poland in the 1970s caused the contamination of many rivers, especially those located near urban agglomerations. Sewage from expanding industries and municipal contamination were dumped directly into urban rivers until the 1970s, thus decreasing the water quality and the ecological status of rivers. The degradation of rivers was so high that according to the classification system they exceeded the class range for the most polluted rivers (Kwandrans et al., 1999; Prygiel, 2002; Rakowska, 2001; Bogaczewicz-Adamczak and Dziengo, 2003; Szczepocka and Szulc, 2009).

The Ner River, flowing through the Łódź and Wielkopolska Provinces, was classified as a heavily polluted, right-bank tributary of the Warta River. The Ner River has functioned as the sewage receiver since 1820 when textile and paint industries started. For 174 years, pollution was discharged directly into the river, which caused a high degradation of the river ecosystem (Kostrzewa, 1999). In 1994 the Group Sewage Treatment Plant in Łódź (GOŚ) was constructed and after a few years it was modernized, therefore the river has been relieved from untreated industrial and municipal sewage discharges. Decreasing the pressure caused by the Łódź Agglomeration on the river should have accelerated the river's auto-purification process; however it did not happen and as such the ecological status of the river is still very poor (Mosiej et al., 2007). This has been caused by the long-term accumulation of discharged pollution from the textile and paint industry in the sediments of the river from before the GOŚ was constructed (Penczak, 1975).

The Ner River was previously an object of biological research, including fish fauna (Penczak, 1969, 1975; Kostrzewa and Penczak, 2002; Penczak et al., 2010) and benthic macroinvertebrates (Tończyk et al., 2003). In addition, since 1996 the river has been under monitoring practices carried out by the Regional Inspectorate for Environmental Protection in Lodz. In 2011, monitoring was extended to assess the water quality with use of benthic diatoms.

The ecological status assessment of surface water bodies is currently based on biological, physico-chemical and hydromorphological elements (Regulation of the Minister of the Environment, 2011a,b). A combination of all three elements allows the ecological status and the water quality of aquatic ecosystems to be assessed. Biological assessment of aquatic ecosystems in Poland has been conducted since 2011 and it is based mainly on diatom phytobenthos. The main tool for the evaluation are the diatom indices, such as: IO – the diatom index evaluated in Poland for standing water ecosystems (Picińska-Fałtynowicz, 2006), or others widely used in Europe: IPS – Specific Pollution Sensitivity Index (CEMAGREF, 1982), IBD – Biological Diatom Index (Lenoir and Coste, 1996), GDI – Generic Diatom Index (Coste and Ayphassorho, 1991), or TDI - Trophic Diatom Index (Kelly and Whitton, 1995). Monitoring of the quality of surface waters with the use of diatom indices is carried out on a large scale around the world (Kelly and Whitton, 1995; Eloranta and Soininen, 2002; Prygiel, 2002; Potapova et al., 2004; Rey et al., 2004; Ector and Rimet, 2005; Harding et al., 2005; Potapova and Charles, 2005; Tison et al., 2005; Taylor et al., 2007; Kelly et al., 2008; Bathurst et al., 2010; Blanco et al., 2012; Venkatachalapathy et al., 2013).

The aim of this study was to determine the changes in ecological status which have taken place in a Polish lowland urban river ecosystem after modernizing the wastewater management. A proper wastewater management has been implemented for 20 years by the Group Wastewater Treatment Plant in Łódź, therefore the ecological status of the river should be improved. The crucial question is: how persistent are the negative effects for the lowland urban river ecosystem of long-lasting industrial and municipal sewage inputs? In this article we try to answer this question with the use of biological assessment methods, amongst which the best “tools” reflecting not only the ecological status of rivers, but also the state and condition of sediments, are the phytobenthic diatoms.

Materials and methods

Study area

The Ner is the longest river passing through the city of Łódź (Bieżanowski, 2001). The length of the river is 134 km, and the catchment area is 1866 km2. The natural daily flowing regime reaches 200,000 m3. The river spring area is located at the Łódź Hills Scarp, 250 m a.s.l. The river flows as a right bank tributary of the Warta River. Research was conducted in the upper part of the Ner River, which is under direct impact of the Łódź Agglomeration (Central Poland). Four study sites were selected for the research: (1) Łódź (city), Zastawna Street, (2) Łódź, Sanitariuszek Street, (3) Konstantynów Łódzki (town), Łaska Street, (4) Mirosławice (town), Konstantynowska Street (Appendix 1, available in the online supplementary information). Water samples for chemical analysis were collected from sampling sites in April and June. Parameters such as: biological oxygen demand (BOD5+2), dissolved oxygen, total organic carbon (TOC), nitrates, phosphates, and silica content were analyzed in an accredited laboratory of the Regional Inspectorate for Environmental Protection in Lodz.

Analysis of diatom assemblages

Phytobenthic samples were collected from the river study sites once a month from June to October 2010. Diatoms were extracted from the benthic samples and were embedded in Naphrax® resin and analyzed according to the Żelazna-Wieczorek (2011) procedure.

Qualitative analysis was made on the basis of the diatoms' cell wall morphology, following Krammer (1997, 2000, 2002, 2003), Krammer and Lange-Bertalot (1986, 1988, 1991a,b), Lange-Bertalot (2001), Levkov et al. (2007), Hofmann et al. (2011) and Werum and Lange-Bertalot (2004). A quantitative proportion of the diatoms was estimated in each permanent slide, by counting 400 diatom valves (Cholnoky, 1968). Species which had equal to or more than 5% abundance were indicated as dominant.

Hierarchical cluster analysis and multivariate analysis were made on the basis of qualitative and quantitative analysis of the diatom assemblages with use of MVSP v 3.2 software (Piernik, 2008). Cluster analysis was estimated on the basis of percent similarity of samples by use of the average linkage method. For multivariate analysis, principal component analysis (PCA) and canonical correspondence analysis (CCA) were estimated.

Ecological status assessment of the Ner River

Assessment of the ecological status and water quality was made with the use of three diatom indices: IO – Diatom Index (Picińska-Fałtynowicz et al., 2006), GDI – Generic Diatom Index (Coste and Ayphassorho, 1991) and IPS – the Specific Pollution Sensitivity Index (CEMAGREF, 1982). To evaluate GDI and IPS, OMINDIA 4.1 software was used. Ranges of diatom indices with corresponding ecological status and water quality class were carried out according to the following: for IO according to OJ No 258, pos. 1549 from 2011, and for IPS and GDI according to Dumnicka et al. (2006) (Appendix 2, available in the online supplementary information).

Results

Chemical conditions of the river

Average concentration of dissolved oxygen in the water was 10.5 mg l−1; however, in summer it decreased locally to 3.1 mg l−1. Biological oxygen demand ranged between 3.7 and 8.7 mg l−1 and it decreased along the river, with distance from the city. The average amount of total organic carbon was 12.8 mg l−1, with the highest value in June – 16.8 mg l−1. In spring, the concentration of silica was lower than in summer: from 9.4 mg l−1 in April to 14.3 mg l−1 in June. Water samples collected from the 1st and 2nd study sites were characterized by a lower concentration of nitrates (average 3.8 mg l−1, relative standard deviation – RSDs 2.2) and a higher concentration of phosphates (average 0.9 mg l−1, RSDs 1.1) than samples collected from the 3rd and 4th study sites. Along the river – at the 3rd and 4th study sites, concentration of nitrates increased to 25.5 mg l−1 (RSDs 6.7), and phosphates decreased to 0.41 mg l−1 (RSDs 0.16).

Canonical correspondence analysis was used to find the relationships between the abiotic components differentiating the studied sites. The eigenvalues were greater than unity for the first two axes and they account for 46.3% of the total variance. Environmental conditions at the 1st study site were influenced and were modified by the concentration of phosphates and total organic carbon. In addition, at this site the highest value of biological oxygen demand was noted. However, along the river these parameters showed a decreasing trend in the influence on environmental conditions of the river. Conditions at the 3rd and 4th study site were under the highest influence of nitrates (Figure 1).

Analysis of diatom assemblages

In the benthic water samples collected from four study sites, 87 diatom species were identified. The number of species in each site was quite similar: 68, 62, 60 and 65. The species indicated as dominants for the studied part of the Ner River were: Aulacoseira granulata (Ehrenb.) Simonsen, Cocconeis placentula Ehrenb., Cyclostephanos dubius (Fricke) Round, Cyclotella meneghiniana Kütz., Gomphonema parvulum f. saprophilum Lange-Bert. & Reich., Hippodonta capitata (Ehrenb.) Lange-Bert., Metz. & Witk., Melosira varians Ag., Navicula gregaria Donk., Navicula slesvicensis Grun., Nitzschia palea (Kütz.) W. Sm., Nitzschia paleacea (Grun.) Grun. in V. H., Planothidium frequentissimum (Lange-Bert.) Lange-Bert., Stephanodiscus hantzschii Grun. in Cl.& Grun., Ulnaria ulna (Nitzsch) Comp.

Hierarchical cluster analysis based on percent similarity of the samples separated the samples into two clusters, A and B, whose similarity reached a value of 61% (Figure 2). Cluster A grouped the samples from the 1st (Łódź, Zastawna Street) and 2nd (Łódź, Sanitariuszek Street) study sites. These sites are located on the river before sewage disposal from the GOŚ. Cluster B grouped the samples from the 3rd (Konstantynów Łódzki) and 4th (Mirosławice) study sites, which are located below the sewage disposal from the GOŚ.

Multivariate analysis confirmed the differentiation of study sites located above and below the GOŚ outflow. The second axis of the PCA divided the study sites into two groups: I and II. Group I was characterized by a high incidence of diatom species such as: Melosira varians (MVAR), Ulnaria ulna (UULN), Meridion circulare (MCIR) and Cyclotella meneghiniana (CMEN), while group II was characterized by a high incidence of species such as: Navicula gregaria (NGRE), Hippodonta capitata (HCAP), Nitzschia palea (NPAL), Navicula slesvicensis (NSLE), Hantzschia amphioxys (HAMP) and Nitzschia capitellata (NCPL) (Figure 3).

Ecological status assessment of the Ner River

Depending on the diatom index and the study site, water quality in the river ranged from class III to class V on the water class scale. Diatom indices values evaluated for samples in the Ner River and corresponding water quality class are supplied in Appendix 3 (available in the online supplementary information). Diatom Index (IO) values ranged between 0.33 and 0.47. This index revealed the class III water quality at every study site. The Generic Diatom Index (GDI) values ranged between 10.6 and 13.9. GDI classified the river as mainly water quality class III, with the exception of the 2nd study site in August, the 3rd study site in June and July, and the 4th study site in October, where it increased the value and corresponded to water quality class IV. The Specific Pollution Sensitivity Index (IPS) varied between 8.8 and 12.1, and classified the river mainly as water quality class IV, with the exception of the 2nd study site in October (water quality class III), and the 3rd study site in June and the 4th study site in July, when both sites were classified as water quality class V.

In accordance with the diatom indices values, the ecological status of the river ranged from moderate to poor (Appendices 2–4, available in the SI). IO had the lowest variation in values and all the studied sections of the Ner River were characterized by a moderate ecological status. Likewise, the GDI also showed a moderate status, however, at times the 2nd, 3rd and 4th study sites declined to poor. IPS revealed that the river has a poor ecological status, with a temporary improvement at the 2nd station to moderate, and a temporary decrease at the 3rd and 4th study site to very poor status.

Discussion and conclusions

Water quality monitoring of the Ner River, which was performed before the GOŚ in Łódź was constructed, revealed that the river ecosystem was highly degraded. According to the State Inspectorate for Environmental Protection report in 1994, basic parameters such as oxygen indicators, nitrates and phosphates, several times exceeded the standards for the lowest water quality class. The river was defined as an “open sewer” Penczak (1975). In sediments of the river a high load of carcinogenic heavy metals and aromatic hydrocarbons were detected (Penczak et al., 2010).

The pressure of the Łódź Agglomeration on the Ner River, which has lasted over 150 years, has permanently changed this river ecosystem. The GOŚ construction in 1994, and subsequent modernization of the sewage treatment system, did not improve the river water quality. Part of the river located beyond the Łódź city borders (the 3rd and 4th study sites) and below the GOŚ still exceeds the standards for the lowest quality class. In contrast, part of the river within the Łódź city borders (the 1st and 2nd study sites) which is located above the GOŚ is characterized by better water quality. As Penczak (1975) noticed, degradation of the fish fauna in the river started from the point where sewage was disposed directly into the river, and only the spring area of the river avoided the degradation. The choice of sampling sites was to show the differences between the spring area – the 1st and 2nd study sites, and the river flow below the GOŚ – the 3rd and 4th study sites. It allowed us to determine if the ecological state has improved after the modernizing of the wastewater management.

Hierarchical cluster analysis and principal component analysis, which take into consideration not only the species composition but also the quantitative proportion of diatoms in assemblages, divided the studied river into two sections. The first section, where the 1st and 2nd study sites are located, was characterized with different diatoms' microflora than the second section, where 3rd and 4th study sites are located. The change in diatom composition together with relatively equal species diversity is often observed among diatoms. Rebuilding diatom assemblages results from diatoms' quick response to changes in the state and in the environmental conditions (Moravcová et al., 2013).

Cluster A grouped the 1st and 2nd study sites, with dominant diatom species such as Cyclotella meneghiniana, Melosira varians, Meridion circulare, and Ulnaria ulna. PCA showed that the structure of diatom assemblages at these sites was strongly associated with the listed species. These species are characterized by a high sensitivity to organic pollution (Hofmann et al., 2011). At both study sites diatom indices varied between water quality class III and IV (IO – III, IPS – IV, GDI – III), and ecological status from moderate to poor. Cluster B grouped 3rd and 4th study site, and was differentiated due to high incidence of group of species resistant or tolerant to organic pollution, such as Gomphonema parvulum f. saprophilum, Hippodonta capitata, Navicula gregaria, N. slesvicensis, and Nitzschia palea. The dominance of these species suggests that the water quality has decreased, which was related to higher saprobity. They tolerate organic pollution, therefore they widely occur even in polisaprobic zone (Hofmann et al., 2011). Their large presence was noted in other lowland rivers characterized with equal ecological status, such as the Bzura, Jasień and Pilica (Szczepocka, 2005; Szczepocka and Szulc, 2009; Rakowska and Szczepocka, 2011). The lowered value of evaluated diatom indices at this section of the river also indicates that the water quality below the GOŚ has decreased.

The better quality and better ecological status of the water at the first two study sites is due to the fact that they are located above the discharge of sewage from the Łódź Agglomeration. Currently the section between the 2nd and 3rd study sites is the place of sewage disposal from the modernized Grouped Sewage Treatment Plant in Łódź. However, before modernization of the sewage treatment station, the Sewage Treatment Plant, which was based only on an insufficient mechanical treatment method, was located here. Before the sewage treatment process was implemented in this section of the river all the sewage from the textile and paint industries were directly disposed of (Bieżanowski, 2005). Over 100 years of anthropogenic impact arising from the Łódź agglomeration caused a significant deterioration of water quality. This deterioration was noticeable at the 3rd and 4th study sites, located below the sewage disposal from the Łódź agglomeration.

The accumulation of heavy loads of pollutants in sediments of the river still has consequences to the present day (Kobojek, 2005). Despite the sewage treatment system construction, water quality still ranges beyond the lowest class range, and the impact of the treated sewage by the modernized sewage treatment plant is not sufficient enough to dilute the pollution and improve the water quality of the Ner River. After 20 years of treating the sewage by the GOŚ, no improvement of the water quality has been reported, resulting from the long-term, strong, negative anthropogenic impact on the lowland river.

Benthic diatoms made it possible to observe the influence of urban agglomeration on the ecological status of the urban lowland river. The agglomeration heavily contaminated the river and caused the accumulation of pollutants in the urban river sediments. After implementation of modernized wastewater management, thanks to which for 20 years treated sewage has been disposed of into the river, an improvement of the river status is unnoticed, with only the section above the treatment plant maintaining better water quality. In their studies Lukavský et al. (2006) and Moravcová et al. (2013) didn't note significant changes in the structure and composition of diatom species after the discharge of sewage. However, it may depend of the river type, because their research was conducted on mountain rivers in the northern part of the Czech Republic. The morphology of mountain rivers is very different from lowland rivers, characterized by distinct hydrological conditions and a distinct structure of communities, and therefore they have a different tolerance to urban agglomeration impact.

A significant decrease in the water quality at the section of the Ner River below the sewage disposal from the Łódź agglomeration was also observed in fish fauna investigations (Penczak, 1969, 1975). The modernization of the GOŚ and the collapse of the textile and paint industries resulted in a relief to the river from heavy pollution inputs (Kostrzewa, 1999; Bonisławski, 2008). However due to the long-term accumulation of pollutants in the river sediments, which are still gradually released into the water, the river is still considered as one of the most polluted rivers in the Łódź Province (Anonymous, 2012). The study on benthic macroinvertebrates and fish fauna conducted after 2000 allowed us to observe a very slow rebuilding of macroinvertebrates and fish communities (Kostrzewa et al., 2002; Tończyk et al., 2003; Penczak et al., 2010), which support the assumption that the auto-purification processes of the river are at function, but the persistence of pollution in sediments slows down the natural ability of the river to auto-purify.

A lowland river located in an urban area is highly influenced by urban agglomerations. These agglomerations impact on the river ecosystems, which is revealed by an elevation of nutrients and pollutions, influence the structure and composition of aquatic organisms. Due to this it is possible to observe an increase in the number of species tolerant to pollution (Meyer et al., 2005; Walsh et al., 2005). Along the river, the more it flows away from cities, the less the anthropogenic impact, and the river increases the intensity of the auto-purification process and improves its ecological status (Keraita et al., 2003; Feld et al., 2011). The spring section of the Ner River is located within the city borders, and then flows away beyond the borders of the city. Despite the fact that the spring area is located directly in an urban area it is characterized by a better ecological status than the part along the river. This study did not reveal the improvement of water quality along the river flow. Only the value of biological oxygen demand showed an improvement from within the city to downstream sites. An improvement of the oxygen conditions and distancing the river flow away from the city, however, did not otherwise result in an improvement of the ecological status of the river.

Changes in the composition of species along the river are widely and often observed (Jüttner et al., 2003; Morse et al., 2003; Collier et al., 2009; Violin et al., 2011; Kopp et al., 2012a,b). Such changes result from varying conditions of the habitats, i.e. elevation of pollutant concentration. It is considered that the most important parameters which accelerate the taxonomic diversity of diatoms are inorganic nitrogen compounds (Marcus, 1980; Lukavský et al., 2006; Veraart et al., 2008; Moravcová et al., 2013). In the Ner River a high concentration of nitrates was observed at the section of the river below the sewage disposal. At the studied sites the taxonomic diversity and abundance of diatoms were changed below the sewage disposal.

Funding

This research was supported by the National Science Centre, under the grant number N N304 354239.

Supplemental material

Supplemental data for this article can be accessed on the publisher's website.

The text of this article is only available as a PDF.

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