Abstract
The aim of this paper has been to evaluate the ecological status of small watercourses in the capital city of Prague by analogy to the evaluation of water bodies pursuant to Directive 2000/60/EC. A total of eight sites were selected at seven watercourses (Šárecký, Dalejský, Radotínský, Libušský, Kunratický, Botič and Rokytka Streams). Both sections that are semi-natural and those that have been restored in various ways were selected. From May 2017 onwards, annual sampling of chemical indicators took place in selected sites and a standard multihabitat method was used to take samples of aquatic invertebrates (macrozoobenthos). Data related to water quality and macroinvertebrates populations were complemented with astacological research of the presence of native and alien crayfish species, which took place in Šárecký, Říčanský, Radotínský, Dalejský and Rokytka Streams.
On the basis of evaluated physicochemical parameters, all the above sites were classified as moderate and worse ecological status. An evaluation of total and phosphate phosphorus yielded the worst results and all the sites were classified as moderate and worse status. An evaluation of macroinvertebrates populations yielded the following results: the sites at Šárecký Stream, Rokytka below Hořejší Pond, Radotínský Stream and Libušský Stream were classified as moderate ecological status whereas Dalejský Stream, Kunratický Stream, Rokytka above Hořejší Pond and Botič Stream were classified as poor ecological status. None of the evaluated sites thus reached good status. The main reason for the evaluated poor water quality of small watercourses in Prague is pollution by nutrients, in particular phosphorus, which causes eutrophication of aquatic ecosystems. Poor water quality clearly also limits populations of aquatic invertebrates in restored and semi-natural watercourses. The presence of endangered narrow-clawed crayfish (Astacus leptodactylus) was established in Dalejský Stream at the upper boundary of the Řeporyje municipality. A negative finding is extinction of the population of critically endangered stone crayfish (Austropotamobius torrentium) in Radotínský Stream. This extinction was probably caused by strong municipal waste water pollution and by subsequently established crayfish plague. No alien crayfish population specimens were found during the research.
Introduction
According to the Strahler method, there are 357 km of small order 1 to 4 streams in Prague. A vast majority of them is administered by the capital city of Prague. Since 2000, Prague has been doing systematic measurement and evaluation of physicochemical and chemical parameters of water quality of 16 streams in 38 profiles and 55 water reservoirs [1]. Measurement takes place once a month close to the mouth of larger streams and once every two months in the remaining 33 profiles. Based on an overall evaluation, the above are classified into surface water quality classes pursuant to the ČSN Technical Standard 75 7221 [2].
Prague does not regularly monitor biological quality elements. Research of biological is either a subject of individual contracts or it is done by research institutions. Botič Stream in particular has been examined long-term by the Czech Technical University in Prague, which also evaluates aquatic invertebrates (macrozoobenthos) [3–6]. Other published data related to aquatic invertebrates in Prague are sporadic [7]. However, macroinvertebrates is one of the main biological components of the aquatic environment that has been systematically researched in the Czech Republic. Macroinvertebrates is a widespread and taxonomically diverse group of organisms that are very abundant, have different demands for the environment they live in and usually good bioindicator abilities, i.e., abilities to reflect the conditions of the environment they live in and also changes thereof. This capability may be used to evaluate the degree to which they are affected by various negative anthropogenic impacts or on the contrary remedial measures and that is why it is used to evaluate the ecological status of water, especially running water. Approaches to this evaluation are defined by the Water Framework Directive (WFD) [8]. The environmental significance of macroinvertebrates populations in watercourses is crucial. As part of the food chain, the individual taxons have various feeding strategies [9]. Among others, there are those that graze and scrape, feeding on a growth of algae, those that collect and cut, taking part in the decomposition of fallen leaves for example, then there are filter feeders who feed on fine particulate matter and also predators. At the same time, macroinvertebrates is an important component of what the fish stock feeds on.
Other researchers also studied the presence of crayfish in Prague [10], which is an attractive and very serious topic in urban environment with regard to the protection of native crayfish populations and limiting the spread of alien crayfish. Native crayfish (Noble crayfish, Narrow-clawed crayfish and stone crayfish) as the largest macroinvertebrates representatives may be regarded as umbrella species whose protection safeguards the protection of a whole range of other species who share the same biotope with crayfish. Crayfish do not feed only on specific foods but rather feed on diverse plants and animals. As opposed to native crayfish, invasive crayfish species (in particular signal crayfish and spiny-cheek crayfish) are far more aggressive to other aquatic animals. They have a far greater reproductive ability (they reproduce more often and have more offspring) and are thus able to completely destroy both the fauna and flora in a given site in a short period of time. Besides, they are carriers of crayfish plague, which is a disease that kills native crayfish species and is caused by a mold-like pathogen Aphanomyces astaci. Alien crayfish are often infected with this pathogen but are resistant to the disease. The presence of invasive crayfish can thus lead to the extinction of the whole population of native crayfish in a site and disrupt the stability of the aquatic ecosystem [11].
Two largest Prague Streams, Botič a Rokytka, are defined as water bodies within the meaning of the WFD [8]. State-owned enterprise Povodí Vltavy carries out regular monitoring of their representative profiles based on which it evaluates the ecological status of these water bodies [12]. However, only two out of numerous watercourses are evaluated in this way and besides that the representative profiles are located on the outskirts of Prague before the main impacts of the Prague agglomeration.
In 2017–2018, experimental work took place in several sites of Prague Streams within the “VODA PRO PRAHU” [“WATER FOR PRAGUE”] project. Its aim was to improve the ecological status of the streams by increasing their morphological variability and habitat offer for aquatic organisms [13]. Data were obtained within the project about physicochemical and chemical parameters and about populations of benthic invertebrates including the presence of crayfish in seven larger streams in Prague.
This paper aims at providing information that Prague has lacked to present day: namely at presenting an evaluation of the ecological status of small watercourses by analogy to the WFD procedures [14, 15]. At the same time, our aim has been to complement data about water quality from sites that are not monitored long-term. In this context, findings about the presence of crayfish have also been included since their protection is topical and closely linked to water quality.
Sites and methods
Sites
A total of eight sites at seven watercourses in the capital city of Prague were selected for the research. Their location and basic characteristics are presented by a schematic map in Fig. 1 and Tab. 1.
Selected sections of Botič, Radotínský, Libušský and Kunratický Streams may be characterised as unaltered and semi-natural watercourses. Forests and extensively used farming land with preserved river bank tree vegetation are in their vicinity.
Other four sites were selected in sections of watercourses where the stream bed has been restored in recent years. Rokytka above Hořejší Pond and Šárecký Stream in the Žežulka settlement are comprehensively restored watercourses. Dalejský Stream in the location of a former Klukovice outdoor swimming pool was irregularly broadened and now has a broad gravel bed. Rokytka below Hořejší Pond has an embedded high-capacity bed reinforced by boulders, which was, as far as the territory allowed, segmented by aggregate into sections with riffles. There are natural recreational areas in the vicinity of all the restored sections. A detailed description and rich picture documentation of the implemented restorations can be found on a website “Pražská příroda” [“Prague Nature”] [1]. Restoration modifications in these sites were completed two to four years before the sampling started.
Fig. 1. Schematic depiction of small watercourse sites in the capital city of Prague
Tab. 1. Location and basic characteristics of the sampling profiles
Water quality sampling
In May 2017, sampling of chemical indicators (CODCr, BOD5, SS105, nitrogen and phosphorus forms, calcium, magnesium, sulphate and chloride content) started in selected sites together with the measurement of physicochemical water quality indicators (water temperature, electrical conductivity, dissolved oxygen content and pH). Monitoring took place in monthly intervals for one year, i.e., until April 2018. Water samples were analysed in the TGM WRI Testing Laboratory of Technologies and Environment Components. Physicochemical parameters were measured directly in the sites by a measuring device HACH HQ40d multi. The Libušský Stream site was an exception because its location was almost identical to the location of the long-term water quality monitoring profile where measurement is done by the capital city of Prague and it was therefore not efficient to sample and evaluate the same profile. Samples from this sampling site were thus taken only three times over the monitoring period (5/2017, 10/2017 and 4/2018). Water samples from Rokytka were taken only in the Rokytka profile above Hořejší Pond, which is representative also for the lower site of Rokytka below Hořejší Pond. There are no important influences between both Rokytka profiles since Hořejší Pond is fed by a side channel with a negligible discharge. The Methodology for Evaluating General Physicochemical Quality Elements of the Ecological Status of Running Surface Water [15] was used to evaluate both chemical and physicochemical parameters. For the sake of clarity, it must be noted that Libušský Stream should not be evaluated by the above method due to the low number of only three taken samples. The evaluation methodology [15] recommends to use an annual dataset containing 12 data. If there are fewer monitored data, their number should not be lower than 6 and the data should represent the year-long monitoring cycle.
Macroinvertebrates sampling
Macroinvertebrates was sampled by the “Perla” Czech national method [16, 17] between 2nd and 11th May 2017. A manual benthos net with 0.5 mm mesh size was used for the sampling that was done for three minutes proportionally from the occurring habitats. All taken samples were fixed by 4% formaldehyde and taken to a laboratory for processing. Individual taxons separated from the samples were subsequently determined up to the lowest possible determination level. When samples were taken, a basic hydromorphological description of the individual sites was made pursuant to a standard sampling protocol [18]. Macroinvertebrates populations were evaluated on the basis of a methodology for evaluating the ecological status of running surface water by means of the macroinvertebrates biological quality elements [14] using the Riverchange module, which is an extension of the IS ARROW evaluation system. Contrary to the methodology, which requires an evaluation of samples from both the spring and autumn seasons for an overall assessment of the ecological status, only the spring macroinvertebrates sample was evaluated. The resulting classification into ecological status classes must therefore be regarded only as approximate.
Astacological research
Since no comprehensive recent information about the presence of crayfish in Prague was available, astacological research was carried out as part of the research of aquatic invertebrates. It took place at the beginning of May 2017 in Šárecký, Říčanský, Rokytka, Dalejský and Radotínský Streams. Potential hiding places in sections of shallow streams were searched manually. Suitable crayfish hiding places include places underneath stones, dead wood, tree roots and dens in riverbanks. A total of 100 individual hiding places were searched in the respective sections if possible. When the watercourses had deeper parts that could not be searched manually, coops were placed there with bait (liver) and left in the sites overnight. A more detailed description of applied methods, characteristics and scope of the monitored sections is made when the results are presented for each site.
Results
Chemical and physicochemical water quality
Overall results of the individual indicators of chemical and physicochemical parameters regarding the monitored sites are presented by charts in Fig. 2 and Fig. 3.
Dalejský and Radotínský Streams, whose basin lies in the Barrandienne, had a higher calcium and magnesium ion content, which is also manifested by a higher pH and electrical conductivity. Šárecký Stream, which drains alkaline clay and siltstone layers in the upper part of its basin, has similar characteristics.
The maximum summer water temperatures of Rokytka and Botič exceeded 20°C, which was significantly more than of the other streams. The reason for that is presumably the outflow of warmed water from the epilimnion of large water reservoirs that are located above both monitored profiles (Kyjský Pond at Rokytka and Hostivař Dam at Botič).
Extremely high concentrations of total phosphorus in Radotínský Stream with maximum values around 2 mg/l are disconcerting. A vast majority of the concentrations were due to phosphate phosphorus. Concentrations of total, nitrate and ammonia nitrogen in Radotínský Stream also exceeded values established in the other streams severalfold.
Fig. 2. Chemical and physicochemical water quality parameters in sampling sites between 5/2017 and 4/2018. Site codes are described in Tab. 1. The box diagram presents the median, average (cross), 1st to 3rd quartile (box) and maximums and minimums (whiskers); distant values are presented by separate circles.
An evaluation into ecological status classes [15] provides a standardised perspective on the values of chemical and physicochemical water quality parameters. For the purposes of the Water Framework Directive [8] it is used to evaluate water bodies in the planning process. Limit values are set for individual parameters, which are specific according to the type of water body. The type of water body is determined according to the typological classification of water in the Czech Republic [19, 20]. Limits classify the values into three ecological status categories: high (blue), good (green), moderate and worse (yellow). The worst result from a group of evaluated parameters is used for the resulting classification. A high status corresponds to conditions in unaffected watercourses. Good status is regarded as a minor deviation from natural status and it is a target status in the process of improving polluted watercourses. An evaluation of general physicochemical quality elements plays only an auxiliary function with regard to biological quality elements. However, where monitoring of biological quality elements is absent, it is used for the evaluation of the ecological status of water bodies also independently.
An evaluation of physicochemical parameters of the monitored sites according to the above method is presented in Tab. 2. In the overall evaluation all the sites were classified as moderate and worse status. An evaluation of total and phosphate phosphorus yielded the worst results and all the sites were classified as moderate and worse status. As for nutrient parameters, the content of nitrate nitrogen was unsatisfactory in four streams, especially in the typical median value (RAD, DAL, SAR and LIB). The BOD5 limit of good status, which is a measure for available carbon content, was exceeded in four cases. The water temperature limit was not met by five of the monitored streams; in the case of Botič and Rokytka even both in median and maximum values. An evaluation of the concentrations of ammonia nitrogen, pH and oxygen saturation yielded relatively better results.
Fig. 3. Chemical and physicochemical water quality parameters in sampling sites between 5/2017 and 4/2018. Site codes are described in Tab. 1. The box diagram presents the median, average (cross), 1st to 3rd quartile (box) and maximums and minimums (whiskers); distant values are presented by separate circles.
Hydromorphological characteristics of the sites
The bottom of a majority of the monitored watercourses (SAR, DAL, RAD, KUN and LIB) was made of predominantly coarse and fine gravel with a small share of stones and boulders (Tab. 3). Boulders and stones dominated as a bottom substrate in Botič and likely came from a disintegrated bank reinforcement. A similarly big share of large aggregate was established in Rokytka below Hořejší Pond where this material had been used for stabilising and segmenting the stream bed during the implemented “urban” restoration that involved increasing the stream capacity. The bottom of the comprehensively restored Rokytka above Hořejší Pond was completely different and was created in a completely new meandering course in clay alluvial soil. A compact clay bottom was supplemented with only rarely present stones and gravel. Sections of running water were predominant in all sites whereas pools with calm water formed only a smaller area of the stream bed. Pools were even completely absent in the restored sections of Rokytka, Dalejský and Šárecký Streams. In the individual sites, their maximum depth in the streamline was 20 to 100 cm and mean depths ranged between 10 and 40 cm. Mean width of the watercourses at water level was estimated at 2 to 6 metres.
Tab. 2. Evaluation of chemical and physicochemical water quality parameters pursuant to a standard methodology used for evaluating watercourses [15]. Typical values established in the individual profiles are colour-coded according to ecological status classes: blue – high, green – good, yellow – moderate and worse status
Tab. 3. Basic hydromorphological characteristics of the monitored sites
Macroinvertebrates
The evaluation of samples taken from the individual sites is summarised in Tab. 4. The highest quantity was established in a sample from Šárecký Stream,
which contained over 13,000 specimens. On the contrary, Rokytka above Hořejší Pond and Botič only had approximately one third of that quantity. Quantities in the other sites ranged between 7 and 9,000 specimens.
Widespread taxons of aquatic invertebrates were established in the samples. A sample from Radotínský Stream was the richest in species, containing 60 taxons. On the contrary, the poorest sample was from Libušský Stream where only 31 taxons were present. Between 46 and 54 taxons were determined in the remaining sites. The number of established families correlated with the number of taxons: the highest number of families (33) was established in Radotínský Stream and the lowest (17) in Libušský Stream. The established abundance and richness in species are reflected by the Margalef diversity index. This index is a ratio of the number of established taxons and a logarithm of the overall number of specimens in a sample. The higher the index, the higher the diversity of a population [14]. This index reached its highest values in Radotínský Stream and Rokytka above Hořejší Pond, whilst the Libušský Stream population had the lowest value.
The colour part of Tab. 4 presents EQR (Ecological Quality Ratio) values of the individual metrics of the macroinvertebrates population. The values range between 0 and 1 and have different weights (depending on the type) in a calculation of the final multi-metric macroinvertebrates index. Its value is segmented into five ecological status classes (high, good, moderate, poor and bad). For the sake of clarity, the colours used for coding the ecological status classes are also used to present the EQR values of the individual metrics.
All the monitored sites reach only poor or moderate status in the multi-metric index. Based on the macroinvertebrates population, Šárecký Stream, Rokytka below Hořejší Pond, Radotínský and Libušský Streams have moderate ecological status. Dalejský Stream, Kunratický Stream, Rokytka above Hořejší Pond and Botič are classified as poor ecological status. Overall, it may be said that the EQR value in all sites oscillates around the threshold of poor and moderate status (0.4) and none of the sites are close to the threshold of the desirable good status (EQR > 0.6). According to the EQR values of the saprobic index, it is clear that all monitored watercourses suffer from a high organic substances load, which is also evidenced by the low abundance of EPT macroinvertebrates groups (Ephemerida, Plecoptera and Trichoptera).
A worse ecological status of the Rokytka site above Hořejší Pond compared to below it and compared to Šárecký Stream is an interesting finding. This is in spite of the fact that the restoration implemented there was the most semi-natural compared to the two latter sites.
Astacological research
Šárecký Stream was explored from Nebušický Stream up to the confluence with Vltava River. Its sections were first searched manually in the potential hiding places. Even though enough suitable hiding places were found, no crayfish were discovered. In the deeper parts of the stream bed that could not be searched manually coops with bait were placed and left in the sites overnight. However, not even the coop method led to the discovery of crayfish.
Říčanský Stream was explored below the Dubeč municipality from the Panská zahrada Park downstream in the direction of Lítožnice Ponds. A manual search method discovered no crayfish in the hiding places in this section. River bank areas of Rohožník Pond in Dubeč were searched using the same method and no crayfish were found there either. The search was done only in the daytime when crayfish do not leave their hiding places because of the rich fish stock.
Astacological research in Rokytka Stream was done in the vicinity of a footbridge across the stream, in the lower part of Hořejší Pond. Manual search of hiding places did not lead to the discovery of any crayfish. However, hiding places there are not very suitable for crayfish. Although the stream bottom in the section is stony, the stones are often solidly embedded in the river bottom or freely placed in the bank parts with fine mud sediment.
Radotínský Stream is known as a site where stone crayfish (Austropotamobius torrentium) is present, which belongs among Natura 2000 priority species and among critically endangered species pursuant to Annex III to Decree No. 395/1992 Coll. [21]. The search took place on 9/6/2017 and hiding places in the permanent monitoring area (Natura 2000) were searched below a weir near the Zmrzlík tributary. Since no crayfish were found in that section, the search continued above the weir. However, no crayfish specimen was found there either. During the search, water in the stream had an unnatural blackish colour and was covered by foam in some places. The search was then carried out in another permanent monitoring area above the Choteč municipality and no specimen was found either. The bottom of Radotínský Stream was covered with fine black sediment there, which covered all hiding places including empty dens that often used to serve as hiding places for crayfish in this section.
Dalejský Stream was searched from Holyně up to Řeporyje. The stream has the character of a natural watercourse in this section with a large number of hiding places under stones, dead wood and tree stumps and roots in bank areas. According to experience, the hydromorphological status and flow speed correspond to habitats of native crayfish species. However, no crayfish specimens were discovered in this section. A water reservoir at Dalejský Stream at the beginning of the Řeporyje municipality was also searched. A specimen of Narrow-clawed crayfish (Astacus leptodactylus) that is an endangered species was found there.
Discussion
Chemical and physicochemical indicators
As part of state monitoring grids, water quality in Prague is regularly monitored in five profiles of small watercourses. Three profiles are situated in the mouth of Rokytka (Libeň), Botič (Nusle) and Radotínský Stream (Radotín). The two remaining profiles are located on the outskirts of Prague, in Rokytka in Běchovice and Botič in Křeslice. Pursuant to the WFD, attention in the water planning process is paid only to watercourses that are defined as water bodies. Out of the small watercourses in Prague, only Rokytka (DVL_0750) and Botič (DVL_0740) are water bodies. Even though they are in both cases defined from the spring up to when they empty into Vltava River, their representative profile where evaluation takes place is located in the above profiles on the outskirts of Prague. Evaluation of both water bodies is therefore affected neither by the actual impact of the Prague agglomeration, large water reservoirs such as Počernický and Kyjský Ponds at Rokytka and Hostivař Reservoir at Botič nor by any of their tributaries. According to an evaluation of water bodies published by the state-owned enterprise Povodí Vltavy [12], the resulting evaluation of both water bodies is classified as “moderate and worse status”, i.e., the same categories that resulted for Rokytka and Botič in the profiles that we had monitored. However, the list of indicators that do not meet a limit for good status is slightly different.
The median of ammonia nitrogen concentration drops significantly in Botič between Křeslice and the profile we monitored (up to the high status level) and to a smaller degree the median of total phosphorus drops too (however, not beyond the moderate and worse status limit). Undoubtedly, in both cases this is an effect of nitrates retention and denitrification and phosphorus retention in Hostivař Reservoir, which deteriorates temperature conditions of the watercourse by warming up in summer months.
No decrease in nutrients is apparent in Rokytka between the profile in Běchovice and the profile we monitored above Hořejší Pond. A reason for this may be a persistent nutrient load of Počernický and Kyjský Ponds as well as the incomparability of both profiles. Rokytka measurements in Běchovice take place before the polluted Říčanský and Běchovický tributaries.
The other monitored Prague streams did not meet the good status limit for general physicochemical quality elements most often in the phosphate phosphorus and total phosphorus indicators as well as in nitrate nitrogen. All these streams drain densely populated areas of urban and suburban settlements and treated waste water from local waste water treatments stations is led there. The highest median values of total and phosphate phosphorus were established in Radotínský and Dalejský Streams. However, at the same time these two streams had very low BOD5 values, corresponding to a high status. This seeming paradox can easily be explained by fast degradation of easily available organic carbon in running and fairly well-oxygenated water in both streams whereas phosphate in the shady stream bed gets assimilated into the green algae biomass rather slowly.
When making a comparison with water bodies of the sub-basin of the Lower Vltava [12], a conclusion may be drawn that Prague streams do not significantly differ from other watercourses as regards the characteristics of nutrient enrichment. Out of 79 running water bodies, not a single one met the requirements of general physicochemical quality elements of the ecological status. The most common reasons for that included unsatisfactory status of phosphorus and nitrogen nutrient load.
The capital city of Prague carries out a detailed monitoring of physicochemical and chemical water quality parameters of small watercourses in its territory [1]. Numerous profiles enable a comparison with the results of our paper but their overall evaluation is done into water quality classes pursuant to the ČSN Technical Standard 75 7221 [2].
Approximately since 2015, increased concentrations of BOD5, CODCr, ammonia nitrogen and total phosphorus have been measured in profile No. 18A in Radotínský Stream near Rutenický Mill. These values are roughly in line with our results and show that the extraordinary nutrient load of Radotínský Stream began already approximately two years before our research. However, we cannot agree with a comment attached to the evaluation that the increasing concentrations of total phosphorus are attributed to municipal waste water from the porous piping system from the old buildings in the municipality. Our results and one-time observations clearly indicate that the waste water treatment stations that are in operation are in serious disrepair. There is a highly likely impact of strong municipal pollution in the source part of Radotínský Stream where overloaded waste water treatment stations cannot keep step with the new development.
The impact posed by reducing torrential rain load may have gone unnoticed in the laboratory analysis results. This impact cannot be captured by standard monitoring. However, the volume of pollution that gets to watercourses in that case may be a significant contribution to eutrophication [22].
Macroinvertebrates
An evaluation of macroinvertebrates by a multi-metric index in accordance with the WFD methods brings a number of questions. Radotínský Stream was evaluated as moderate ecological status, i.e., the “better” half of monitored streams. Values of individual metrics in Radotínský Stream strongly oscillate between the EQR of the saprobic index at the level of bad status up to B index values and the Margalef diversity index corresponding to good status. The extremely high concentrations of total phosphorus and high concentrations of nitrogen forms that indicate a significant nutrient load of the watercourse should also be noted. These nutrient parameters have a direct link to autotrophic biological quality elements (phytobenthos and macrophytes) and have an indirect impact on macroinvertebrates via the nutrient cycle in the watercourse [23]. The monitored section of Radotínský Stream is semi-natural from both the hydromorphological perspective and composition of the stream bed substrate. There the population of benthic invertebrates reaches the highest diversity out of the monitored watercourses, which is in line with the fact that macroinvertebrates is significantly affected by the habitat diversity of a watercourse. Organic water pollution in the watercourse, reflected in the fully unsatisfactory EQR of the saprobic index, presumably prevents the population from reaching a better than moderate ecological status and is likely to be the main barrier in reaching good ecological status.
Radotínský Stream is the most striking example of a difference in the evaluation of the EQR of individual metrics, which can be observed in other sites too (Šárecký Stream, Rokytka below Hořejší Pond, Kunratický Stream and Botič). Saprobic index is a metric based on the sensitivity of selected taxons to water load by easily degradable organic matter. The metric primarily indicates organic pollution but it also positively correlates with eutrophication intensity, farming use of the alluvial plain and basin as well as several elements of hydromorphological degradation [14]. The B index is calculated by a prediction model. It models the theoretical composition of macroinvertebrates populations in specific sites based on seven values of environment variables (distance from the source, altitude, latitude, longitude, inclination of the watercourse, catchment area and stream order). The index is the result of a subsequent comparison of predicted and actually established population in the evaluated site [14]. Differences in the EQR of saprobic and B indices therefore reflect the fact that from the hydromorphological perspective and composition of the stream bed substrate, the respective watercourses are semi-natural or restored into a semi-natural hydromorphological status but organic water pollution of the watercourse is unsatisfactory.
As regards Prague streams, regular evaluation of the population of aquatic invertebrates into ecological status classes is done only for Rokytka and Botič, which are water bodies within the meaning of the Water Framework Directive [8]. As described above, sampling is carried out by the state-owned enterprise Povodí Vltavy in representative profiles on the outskirts of Prague. The current Draft Catchment Area Plan for the 3rd planning cycle [12] evaluates macroinvertebrates in Rokytka as Class 4 ecological status (poor status). Botič is classified as Class 3 (moderate ecological status) in the document. Both water bodies were evaluated in the same way also in the previous planning cycle. The differences compared to our results are presumably due to different impacts on the watercourses and microhabitat characteristics of the sampling profiles. In the summer months, water flowing away from Hostivař Reservoir and Kyjský Pond is enriched with the phytoplankton of algae and cyanobacteria, which negatively affects representation of the feeding groups of macroinvertebrates [23]. The sensitivity of the evaluation method regarding microhabitat conditions of watercourses may be documented on the different result in the profiles of Rokytka below and above Hořejší Pond (Tab. 4). The flow section below Hořejší Pond with stream bed substrate made of aggregate was evaluated by a class better than the sampling section above Hořejší Pond. It is a paradox that the latter site was comprehensively restored into a broadly meandering stream bed. However, sampling took place there only two years after the completion of restoration modifications. What we consider to be the main reason for classifying the site as having poor status is the uniform conditions of the of the restored stream bed, which had been dug in compact clay. Even after two years, neither gravel alluvia have formed there nor have the pools deepened. It should be noted in this context that the staff of the Prague City Hall quickly responded to our preliminary findings and had the stream bed enriched with gravel and river wood.
Other data about the ecological status evaluation of small watercourses in Prague according to macroinvertebrates are rather exceptional. Older papers document the failure to reach good status in Botič profiles on the basis of saprobic and ASPT indices [3, 4]. In 2017, macroinvertebrates sampling in Botič was done by Lískovcová [6]. Her evaluation based on individual metrics in profiles below Hostivař Reservoir oscillated between moderate and bad status.
No evaluation of smaller Prague streams that are not water bodies has been done to present day using methods analogical to the evaluation of water bodies. This is the merit of our paper, which besides Rokytka and Botič evaluates also another five streams of stream order 3 according to Strahler. The resulting evaluation as moderate and poor status classes is not surprising. It was presumably not improved even by a deviation from the standard methodology when we evaluated only spring samples that tend to be richer. There are numerous unfavourable impacts on watercourses in urban environment. In this paper we looked in particular at water quality and partly at hydromorphological characteristics. Besides that, urban watercourses experience negative impacts of hydraulic stress [5], dramatic water quality deterioration due to reducing the torrential rain load of sewerage systems [22] or an impact of old environmental burdens [24]. The result of an evaluation of water bodies in the Lower Vltava sub-basin based on macroinvertebrates [12] gives a rough comparison of the status of small watercourses in Prague. Only a half of water bodies achieved good status in the ecological quality elements of macroinvertebrates. The second half of water bodies was evaluated as having moderate or poor status.
Astacological research
When evaluating the presence of crayfish, we need to strictly distinguish between native and alien species. A population of alien species of spiny-cheek crayfish (Faxonius limosus), which has spread to the Czech Republic up the Labe River, has been present in Prague already for several decades. This invasive species usually does not spread to small watercourses but may be actively brought there by people. This risk holds true especially for ponds and water reservoirs. An example of this was Rohožník Pond on Říčanský Stream in Dubeč. The strong local population of spiny-cheek crayfish was suppressed there in 2014 by draining the pond and removing mud from there. Our research was motivated by the aim to find out whether this intervention had been successful. There was an assumption that when the pond was being drained, a large number of specimens could have escaped to Říčanský Stream. Our searching confirmed their presence neither in the watercourse nor in the newly filled pond. The section of the stream below Rohožník Pond is a fast flowing one, which is not a suitable habitat for crayfish. Říčanský Stream is highly polluted by municipal waste water [1] and therefore it is presumably unsuitable even for the invasive spiny-cheek crayfish, which is less sensitive to pollution compared to native crayfish [25]. Even though no crayfish were established when the section was searched, later on the presence of spiny-cheek crayfish was confirmed in Kyjský Pond by a mobile app “Raci v ČR” [“Crayfish in the CR”].
Tab. 4. Overall evaluation of macroinvertebrates populations. The colours mark the division of the EQR into ecological status classes (high – blue, good – green, moderate – yellow, poor – orange, bad – red). A more detailed description of the metrics and calculation of the multi-metric index is presented in a methodology by Opatřilová et al. [14]
Presence of the alien spiny-cheek crayfish was suspected also in Rokytka. Several spiny-cheek crayfish specimens were found near Podvinný Mill in Libeň in Prague in recent past [J. Patoka, pers. comm.]. Fortunately, when we searched Rokytka on the boundary between Hloubětín and Vysočany did not confirm the presence of spiny-cheek crayfish.
When Šárecký Stream was being searched, the possibility that native Narrow-clawed crayfish (Astacus leptodactylus) may be present there was considered. One specimen of this species was found in the Zlatnice area in January 2015 [J. Svobodová, unpublished]. It was an uncommon discovery of crayfish outside its hiding place in winter, which shows that the crayfish was not in a good condition. Most likely, it may have been a specimen from one of the ponds, perhaps from the nearest one near Dubový Mill.
The finding of Narrow-clawed crayfish in Dalejský Stream at the upper boundary of the Řeporyje municipality is gratifying but not surprising. The site is located close to the former Řepora open air museum in whose ponds Narrow-clawed crayfish has been found [26]. However, its greater spreading further downstream is presumably hindered by the municipal pollution of Dalejský Stream below Řeporyje.
Perhaps the most significant negative finding of the astacological research is that the population of stone crayfish (Austropotamobius torrentium) in Radotínský Stream has become extinct. In the past, stone crayfish used to be present long-term in an approximately one-kilometre-long section above the Choteč municipality up to Radotín, i.e., roughly in five kilometres of Radotínský Stream [26]. The best situation occurred in 2010 when crayfish abundance amounted to over 60 crayfish in the section. Over time, their abundance started to decline, e.g., in 2014 there were only 13 crayfish per 100 suitable hiding places [26] but the population was still stable.
As described in the chemical analyses results, the Radotínský Stream basin is affected by a strong municipal waste water load. As shown by accidental findings as well as inspections of the state of the watercourse above the monitored site, the degree of its treatment ranges from very low to being in serious disrepair. An accidentally captured water sample from Radotínský Stream in Tachlovice (approximately 7 km above the monitored profile) on 14/9/2017 provides evidence of episodic statuses incompatible with the lives of a majority of aquatic organisms (dissolved oxygen content 2 mg/l, ammonia nitrogen 8.15 mg/l, BOD5 47 mg/l) [P. Kožený, unpublished].
The poor water quality in Radotínský Stream was also looked into by the staff of the state-owned enterprise Povodí Vltavy. In 2017, blackish turbid water was repeatedly established in the stream below Tachlovice over the course of approximately 14 days [Mr. J. Válek, pers. comm.]. Its source was presumably a malfunctioning waste water treatment station in the source area but it was not provably traced. In autumn 2017, crayfish plague was also confirmed in the site.
No stone crayfish specimens have been found in Radotínský Stream to present day. Unfortunately, it seems that the presence of this critically endangered species in Prague is a thing of the past.
Conclusion
The outcome of the evaluation of the ecological status of small watercourses in Prague is not positive. None of the evaluated profiles met general physicochemical quality elements limits for a good ecological status. An evaluation of the ecological status according to macroinvertebrates classified the monitored profiles as moderate status at the best and as poor status at the worst.
The main reason for the evaluated poor water quality of small watercourses is pollution by nutrients, in particular phosphorus, which causes eutrophication of aquatic ecosystems. Poor water quality clearly limits also the populations of aquatic invertebrates in restored and semi-natural watercourses. Without a consistent focus on the quality of waste water treatment including phosphorus removal, the long-term efforts of the Prague City Hall aimed at improving water quality in reservoirs and in watercourses may prove futile.
