Biodiversity of Diatoms as Indicators of Water Quality and Landscape Sustainable Dynamics in the Zarafshan River, Uzbekistan

Abstract

For the first time, we have compiled a general list of diatoms for the Zarafshan River consisting of 428 species based on our own research and the literature data. Indicator species for nine water parameters were identified, making up more than 90% of the list. Bioindicators and statistical methods revealed that sections of the river around the city of Samarkand and further in the middle reaches reflect the complexity of the impact of the environment on diatom communities. The surveyed sections of the middle reaches of the river are divided into branches and the dynamics of water parameters and diatom communities are shown from the border with Tajikistan to the confluence of the Zarafshan with the Amu Darya. The indices of organic pollution, S, and toxic impact, WESI, were calculated. They show that there is an increase in salinity and turbidity and a decrease in organic pollution downriver. At the same time, the Navoi section is a source of water acidification. Nutrients and heavy metals, as well as phenol pollution, enter the river from various sources, mainly in the middle reaches of the river. The Zarafshan Nature Reserve in the catchment area of the upper section of the river within Uzbekistan is important for maintaining water quality. Bioindicators show an increase in self-purification, with an increase in the species richness and abundance of diatoms in the middle section of the Zarafshan River. The integrated index of river pollution, RPI, shows that most pollution comes from the northern canal of the river in the middle reaches. A general look at the Zarafshan River catchment basin and the dynamics of the identified water parameters and bioindicator species of diatoms shows that the river ecosystem successfully copes with incoming pollution, including transboundary impacts from Tajikistan. Such a conclusion could not be made based on chemical analysis of the water alone. This allowed us to recommend expanding state monitoring points to the lower section of Karakul while including biological indicators in the observations.

1. Introduction

Of great importance in the life of aquatic organisms, including algae, is the nature of the trophic state, the altitude above sea level, the types of soil or rocks, and other factors that determine the degree of water transparency, its temperature, the quantity and qualitative composition of the salts dissolved in it, and some other ecological features that are vital for the aquatic ecosystems [1].

Much attention is paid in the world to monitoring the state of large aquatic ecosystems, assessing the level of transformation of biota under the influence of strong anthropogenic and technogenic influence. The main waterways of Central Asia are the Syr Darya, Amu Darya, Zarafshan, Chirchik, Talas, Chu, and Murgob, the waters of which have been used for irrigation of cultivated fields since time immemorial. For this purpose, various irrigation and drainage canals, reservoirs, etc., were created [1]. Thus, water resources in the semi-desert and desert climate zone are vital, supporting the existence and economy of the countries of the region. It should be noted that almost all mountain water bodies (including the Zarafshan River), are basically fed by snow. Rain plays an insignificant role in this regard [2]. However, this resource replenishes the river mainly in its upper reaches, outside of Uzbekistan. In transboundary rivers, water flows from different geographic regions and the widespread use of water resources in agriculture and other economic sectors lead to changes in aquatic organism communities [3].

Water quality is a critical variable in conditions of limited water resources [4]. For Uzbekistan, such a main waterway is the Zarafshan River. This is the only river flowing through the entire territory of the country, supporting its economy, population, economy, and biodiversity. The upper part of the river’s catchment area is located outside the country, so the river is transboundary [5].

In most cases, population growth in the areas of large rivers, an increase in the number of livestock, and wastewater discharge lead to radical changes in river algal flora and the formation of new species in the basin flora [6]. Excessive input of nutrients such as nitrogen and phosphorus into rivers can have significant impacts on benthic algal communities [7]. In mountain streams, diatoms are the most abundant and diverse taxonomic group in benthic algal communities [8].

Diatoms are increasingly used to assess short-term and long-term environmental changes because they are informative, versatile, flexible, and powerful environmental indicators, as they respond rapidly to changes in many environmental factors [9,10]. Thus, through having data on the diversity of diatoms in a river, one can discuss changes in water quality and sources of its pollution. Moreover, the experience of ecological mapping of chemical pollution parameters and toxicity indices, as was performed for the Arys River basin in the nearby region of Kazakhstan [11], opens up opportunities for linking data to the landscape and allows for the identification of critical areas in the river catchment basin.

An analysis of the literature data on the diversity of diatoms in the Zarafshan River showed that a generalized assessment of its ecological state in terms of algal diversity has not yet been carried out. However, some fragmentary data are known. In previous years, the hydrochemical indicators and composition of periphyton algae in the Zarafshan River were given separately for the upper, middle, and lower parts of the river [12]. Several other publications are devoted to the study of the diversity of algae, including diatoms, in the rivers and reservoirs of Uzbekistan located in the catchment area of the Zarafshan River. While some references do not provide a link to a specific collection site [13,14] and were published about a hundred years ago in Russian, they may have only historical value. Others, published in Russian and in the form of abstracts [15,16,17], were included in the general reference [18] that was used by us as one of the sources of the biodiversity data within the following published paper [19]. A short analysis of the middle part of the river diatom flora was presented in [20]. The hydrochemical data on the section of the Zarafshan River that interests us are discussed separately in the work of Kulmatov et al. [21].

The aim of our study was a comprehensive assessment of the ecological state of the Zarafshan River, which is an environment-forming river for Uzbekistan, based on the species composition of diatoms and environmental indicators using statistical methods and bioindication in the conditions of a transboundary river in a semi-desert climate. Ecological mapping of the quality of river waters for organic and toxic pollution will link the chemical and biological data of the study to the landscape of the transboundary river.

2. Materials and Methods

2.1. Description of Study Site

The Zarafshan River originates at the junction of three mountain systems—the Turkestan, Zarafshan, and Gissar (Alai) ranges from the Zarafshan glacier at an altitude of about 5000 m outside of Uzbekistan. Zarafshan Valley is the largest intermountain depression in Central Asia. Its length is about 877 km [22].

The initial section of the current, about 300 km long, runs in a narrow and deep valley between the Turkestan (in the north) and Zarafshan (in the south) ranges (Figure 1). From the left southern side, it receives significant tributaries—the Fandarya, Kshtutdarya, and Magiandarya. The average annual water flow in this section fluctuates between 58–108 m³/s. The water level is high in July and August, and low in April [22].

Figure 1. Sampling stations over the Zarafshan River; numbers are described in Table 1.

The Zarafshan River, after merging with Magiandarya, passes through the territory of Tajikistan to the west to Penjikent and turns to the northwest into the territory of Uzbekistan (Figure 1). Below Penjikent, on the territory of Uzbekistan, a flat section of the course begins, where not one single significant tributary flows into the Zarafshan until the end of the river [22].

In Uzbekistan, the upper part of the riverbed has a high biodiversity, which is protected by the IUCN reserve. The Zarafshan State Reserve (Figure 1) was established in 1975 in the Samarkand region on the right bank of the Zarafshan River, 15 km from the city of Samarkand, at an altitude of 600 to 900 m above sea level. Its area is 2352 hectares, including a small area covered by forest of 868 hectares. The territory of the reserve extends for 35 km along the riverbank with a width of 150 m to 1400 m. Its main objective is to preserve, study, and restore the natural environment of forest plants and animals as well as the gene pool of rare and endangered plants and animals. The climatic conditions of the reserve are typical of the continental subtropics. The air temperature in summer reaches +4 °C, and in winter it drops to −27 °C. The average annual precipitation is 100–400 mm. The flora consists of trees, shrubs, and herbs such as poplar, willow, cypress, and chakanda. The reserve is home to 266 plant species, 172 bird species, and 8 fish species. The reserve protects the golden pheasant, and cage breeding has been studied. Since 2016, work has been underway in the reserve to acclimatize six Bukhara deer, and the work to breed this unique animal is being continued [23].

Further downstream, the riverbed is flat, has virtually no catchment area, and is constantly changing and anastomosing. Sampling points cover all three parts of the river on the territory of Uzbekistan (Table 1). As can be seen from the table, the altitude of sampling points in the upper part of the river is significantly higher and refers to its mountainous section. The other points are much lower, and the drop in altitude occurs smoothly, starting from the city of Samarkand to the confluence of the river with the Amu Darya River. In the middle part of the riverbed, the Zarafshan River divides into two approximately equal streams at stations 5 and 6, then reuniting into one river at station 7.

Table 1. Sampling sites, river part, code, coordinates, altitude, and distance from the Tajikistan border of the Zarafshan River.

Site	No.	River Part	Code	North	East	Altitude, m a.s.l.	Distance, km
Fandarya	1	Upper	Fan	39°23′2.91″	68°38′32.02″	1455	0
Upper Zeravschan	2	Upper	Up-Zer	39°25′1.48″	68°30′56.40″	1403	21
Pervomayskaya plotina	3	Middle	P-mays	39°32′16.87″	67°24′36.43″	880	188
Upper Choponata	4	Middle	Up-Chop	39°41′40.29″	67° 3′14.25″	702	38
Akdarya	5	Middle	Akda	39°53′20.18″	66°28′30.56″	522	62
Karadarya	6	Middle	Kara	39°58′26.02″	66°33′46.88″	523	55.5
Khatirchi	7	Lower	Khat	40°0′47.40″	65°56′34.06″	427	52.3
Pakhtakor	8	Lower	Pakh	40°1′59.89″	65°45′42.50″	396	32.11
Navoi	9	Lower	Navo	40°9′32.56″	65°19′47.08″	330	34.35
Gizhduvon	10	Lower	Gizh	40°3′48.40″	64°46′26.15″	277	54.74
Bukhara	11	Lower	Bukh	39°49′57.97″	64°19′38.60″	214	48.47
Karakul	12	Lower	Kara	39°29′53.98″	63°49′58.58″	197	63.12

2.2. Material

For a full-scale analysis of the dynamics of diversity and assessment of water quality in the Zarafshan River, hydrochemical data from 7 of the 12 stations from UzHYDROMET (Hydrometeorological Service Center (Uzgidromet) under the Cabinet of Ministers of the Republic of Uzbekistan) [24] were used, as well as data from previously conducted studies of algae diversity by the authors and other researchers.

For stations 1 and 2 of the upper reaches of the Zarafshan River, the data on diatoms published by A.M. Muzafarov [1] were used, and only 233 taxa identified in the riverbed were selected from the list, excluding lakes and other water bodies.

In the middle reaches of the Zarafshan River basin at stations 3–6, the diversity of diatoms was studied by J. Toshpulatov together with K. Alimjanova in 2005–2011 and amounted to 218 species from 81 samples [15,18]. Diatoms in the lower reaches of the Zarafshan River were studied at stations 7–12 [19] during 2009–2015 in relation to pollutant influx and water quality, and 198 species were identified from 195 samples. Both series of sampling in the four seasons and species definition were performed under the supervision of K. Alimjanova in 2005–2019 and published in detail in [15,18,19]. Diatom samples were collected throughout the river within the borders of Uzbekistan at 12 sampling stations. Periphyton was collected by scraping with a scalpel or knife from underwater substrates, aquatic vegetation, or dead plant substrate at a depth of 0 to 50 cm along the riverbanks from an area of 10 cm². Benthos samples were collected from the river bottom using improvised means. Samples were fixed in 4% neutral formaldehyde and transported to the lab in an icebox.

All these data on water chemistry and diatom diversity were combined by us into a database for this work. A list of species was compiled for each of the 12 stations. As a result, we have a set of hydrochemical data from state monitoring stations (2–7, 9) for analysis, as well as a set of biological data from stations 1–12, which we collected not only at state monitoring stations, but also in the upper reaches of the river near the border with Tajikistan, in the intervals between hydrochemical stations, and in the lower reaches of the river. Thus, biological data cover the river with a denser and more continuous network of monitoring stations.

The modern taxonomic names were updated with the help of algaebase.org [25]. The abundance of cells of each revealed species in each station community was assessed as scores from 1 to 6 according to a semi-quantitative method [26]. Index saprobity, S, was calculated according to each indicator species abundance and its species-specific value [27,28].

2.3. Methods

Bioindication properties of each identified species are taken from our collected database [28,29]. Bioindicator analysis was performed with species-specific ecological preferences of revealed taxa classified according to indicator systems [29,30]. Hydrochemical variables were classified from an ecological point of view [31]. Both hydrochemical and biological data were analyzed with the following statistical programs. Similarity trees were performed using the BioDiversity Pro 2.0 program [32]. The correlation network analyses were performed in JASP (Jeffreys’s Amazing Statistics Program) on the botnet package in R Statistica [33]. The heat map was constructed in the ExStartR program [34].

Calculation of the Water Ecosystem State Index (WESI) was performed for definition of the environmental impact to the diatom communities on each sampling station [31] as a function of the classification rank of Index S and rank N-NO₃.

Index WESI = rank of Index S/rank N-NO₃

where Rank S is the rank of water quality according to the range of Sládeček’s saprobity indices [27] calculated for the sampling site; Rank N-NO₃ is the rank of water quality according to the range of nitric nitrogen concentration [28,31].

The index values vary from 0 to 9. If WESI is more than one, the aquatic ecosystem stayed in good condition, whereas for affected communities exposed to toxic pollution inhibiting photosynthesis, WESI is below one.

The integral River Pollution Index (RPI), according to Sumita [35], was calculated for chemical and biological data in the sampling sites and the distance between them (Table 1) according to equation:

$$RPId={\displaystyle \frac{\sum _1^n\left(Di+Dj\right)\times l}{2L}}$$

where Di, Dj are the variable values of entire format units for the adjacent sites i, j; l is the distance between two adjacent sites (km); and L is the total river length.

3. Results

3.1. Physicochemical Properties of the Zarafshan River Water

Averaged chemical variables over stations 2–7 and 9, defined by [24] for the period of 2009–2015 in the Zarafshan River, are represented in Appendix A Table A1. Appendix A Table A1 shows that the amplitude of fluctuations in the values of the main environmental variables remains relatively constant throughout the year, as indicated by the value of the standard deviation. The value of pesticides was minimal. However, the pollution variables Total Suspended Solids (TSS), Total Dissolved Solids (TDS), and Chemical Oxygen Demand (COD) varied widely in years of monitoring.

The most important thing in this study is to consider changes in the indicators of the environment at the river stations. Even though not all the stations that we examined were represented in the monitoring system, there is still data for each of the three sections of the river. Figure 2 shows the change in the main indicators at the river stations. It is evident that all the stations in the upper and middle reaches of the river have similar and low indicators, whereas starting from station 6 their values increase significantly, excluding oxygen, which in contrast decreases.

Figure 2. Comparative heat map of chemical variable distribution over sampling stations in the Zarafshan River, Abbreviations in the y-axis and station numbers in the x-axis are the same as in Table 1. The color of the cells changes from blue to red depending on the magnitude of the value of each variable in the amplitude of the values of all variables.

We examined separately the dynamics of the macro indicators in Figure 3. COD, TSS, and TDS are critical variables because their values are dramatically changed down the river. The values of mineral pollution increased significantly, several times, starting from station 6, which belongs to the northern branch of the middle reaches of the river.

Figure 3. Distribution of critical chemical variables over sampling stations in the Zarafshan River.

3.2. Biological Characteristics of the Zarafshan River

The diversity of diatoms in the Zarafshan River, which crosses the entire territory of Uzbekistan, amounted to 428 species (Appendix A Table A2). The diatom species distribution over the sampling stations shows the greatest species richness from 96 to 109 at stations 1, 11, and 12. Here, as well as at stations 5–7 and 9, the abundance of algae in the communities was also the greatest. Despite significant fluctuations in the species composition of diatom communities at the river stations, the species richness in the river sections remains approximately the same: upper—185 species, middle—200 species, and lower—194 species (Appendix A Table A2). The most abundant species in the upper part were Diploneis ovalis and Odontidium hyemale that were presented across all stations of the part. The middle part communities were dominated by Diatoma vulgaris across most of the sampled stations. In the lower part stations, Navicula rostellata prevailed in most communities.

Figure 4 shows comparative fluctuations in the variables of the biotic part of the Zarafshan River ecosystem. The smallest number of species was at station 4, and it was also low at stations 8 and 10. The trend line for this species distribution sags in the middle part of the river, showing a vulnerable area. Despite significant variability in the abundance of diatoms in communities, the trend line shows a significant increase in abundance towards the mouth of the river. At the same time, the saprobity index, S, varied within the 2 and 3 water quality classes, and its trend had a slight increase in the middle of the river.

Figure 4. Distribution of biological variables: species number, sum of scores, and Index saprobity, S, over sampling stations in the Zarafshan River. Poly. Is the polynomial trend line.

3.3. Bioindicators

Based on the database of identified species and their ecological preferences (Appendix A Table A2 and Table A3), indicator taxa of diatoms were determined at 12 stations of the Zarafshan River. Diatom indicator species were grouped by three sections of the river according to nine groups of bioindicators (Figure 5). The proportion of benthic species increased downstream of the river (Figure 5a). Water temperature indicators showed water warming in the lower third of the river (Figure 5b). The proportion of slightly acidic and neutral water indicators increased noticeably, and the proportion of alkaliphiles increased (Figure 5c). A slight increase in the proportion of stagnant water species towards the river mouth corresponded to a decrease in water oxygen saturation (Figure 5d). The trend towards water salinization downstream was reflected by chloride indicators, with an increase in mesohalobes in the lower third of the river (Figure 5e). At the same time, the indicators of organic pollution of waters according to Watanabe [36] (diatoms only) showed a noticeable increase in the proportion of saproxenes, inhabitants of water unpolluted by organics, in the lower third of the river (Figure 5f). The indicators of the type of nutrition were 80–85% represented by autotrophs, but in the second and third parts of the river mixotrophs (hne, hce) appeared in the communities, the development of which is not hindered by the presence of toxicants (Figure 5g). A noticeable increase in the proportion of oligotrophy indicators downstream of the river shows a decrease in the trophicity of its ecosystems (Figure 5h).

Figure 5. Distribution of bioindicators over the upper, middle, and lower parts of the Zarafshan River. Abbreviation for ecological groups: Substrate preferences (a): B, benthic; P-B, planktonic-benthic; P, planktonic. Water temperature (b): cool, cool-loving species; temp, temperate temperature water inhabitants; eterm, eurythermic species; warm, warm water inhabitants. Water pH (c): acb, acidobiontic species; acf, acidophilic species; ind, indifferent species; alf, alkaliphilic species; alb, alkalibiontic species. Oxygen (d): aer, aerophiles; str, streaming waters inhabitant; st-str, low streaming waters inhabitant; st, standing water inhabitant. Water salinity (e): hb, halophobe; i, oligohalobious-indifferent; hl, oligohalobious-halophilous; mh, mesohalobious; eh, euhalobous. Organic pollution, Watanabe (f): sx, saproxenes; es, eurysaprobes; sp, saprophiles. Autotrophy-Heterotrophy, Nutrition type as Nitrogen uptake metabolism (g): ats, nitrogen-autotrophic taxa, tolerating very small concentrations of organically bound nitrogen; ate, nitrogen-autotrophic taxa, tolerating elevated concentrations of organically bound nitrogen; hne, facultatively nitrogen-heterotrophic taxa, needing periodically elevated concentrations of organically bound nitrogen; hce, nitrogen-heterotrophic taxa, needing elevated concentrations of organically bound nitrogen. Trophic state (h): ot, oligotrafentic; o-m, oligo-mesotraphentic; m, mesotraphentic; me, meso-eutraphentic; e, eutraphentic; o-e, oligo- to eutraphentic; he, hypereutraphentic. Water quality class of organic pollution according to species-specific index saprobity S of Sládeček (i).

3.4. Species Environment Relationships

Figure 6, Figure 7 and Figure 8 were constructed to compare the importance of chemical, biological, and combined data results at the same set of stations. It was the limited set of stations where chemical data were available that determined the order of data selection for all three plots. Therefore, statistical comparison of the data was performed using three combinations based on Appendix A Table A1 and Appendix A Table A3. To construct the similarity tree in Figure 6, only hydrochemical environmental parameters (Appendix A Table A1) were selected, which were available for seven stations only. It is evident that significant similarity was observed only for the upper stations from 1 to the southern channel of station 5.

Figure 6. Bray–Curtis tree of similarity for chemical variables in sampling stations 2–9 of the Zarafshan River.

Figure 7. Bray–Curtis tree of similarity for chemical variables and bioindicators in sampling stations 2–9 of the Zarafshan River.

Figure 8. Bray–Curtis similarity tree of bioindicators in sampling stations 2–9 of the Zarafshan River.

The tree in Figure 7 included, in addition to Appendix A Table A1 data, also bioindicator parameters (Appendix A Table A3), but for the same seven stations. The similarity of the constructed Figure 6 and Figure 7 is obvious, indicating the predominance of chemical data in the assessments. Figure 8 is constructed using bioindicator data only, but for the same seven stations. A significant similarity of bioindicators is observed for stations 2, 5, 6, 8 and 9, while the data from stations 3 and 4 fall into separate branches. Stations 3 and 4 belong to the Samarkand region. This shows the importance of more detailed assessments of the bioindicators separately, but for the entire data set over all studied stations (Appendix A Table A3). Therefore, we continue the data comparison with other statistics. For a detailed analysis of the links between biodiversity and chemical data, graphs were constructed in the JASP program.

A statistical comparison of chemical and bioindicator variable values with JASP at seven stations of the Zarafshan River showed that the river can be divided into two sections (Figure 9). Cluster 1 combines the values at the stations of the upper and middle sections, while cluster 2 includes only the stations of the lower section of the river.

Figure 9. JASP Network plot of correlation on the level greater than 50% for chemical and bioindicator variable values for seven stations (St.) of the Zarafshan River. The line thickness between stations reflects the correlation value (only significant results are represented); blue is positive, red is negative. Clusters 1 and 2 are outlined by color dashed lines.

However, if all river stations for which data are available (St. 1–St. 12) are included in the analysis, then only biological indicators form the basis for the analysis. The JASP Network plot (Figure 10) groups biological data into two clusters. Cluster 1 includes the upstream stations and two middle-section stations 5 and 6, located on two parallel branches of the river. The remaining stations are united by cluster 2, which includes all the stations of the downstream section of the river plus two stations 3 and 4, related to the middle course of the river. Thus, the communities at stations 3 and 4, as well as 5 and 6, reflect the complexity of the impact of the environment around the city of Samarkand.

Figure 10. JASP Network plot of correlation on the level greater than 50% for bioindicator variable values only for twelve stations (St.) of the Zarafshan River. The line thickness between stations reflects the correlation value (only significant results are represented); blue is positive, red is negative. Clusters 1 and 2 are outlined by color dashed lines.

3.5. Integral Indices of Water Toxicity (WESI) and River Pollution (RPI)

Based on the available chemical and biological data, the WESI was calculated, showing the inhibition of photosynthesis in the studied communities at the river stations. The index was calculated considering that the river is divided into two branches and flows in one branch through station 5 and in the second branch through station 6, then merges. This is why in Figure 11, the index data are separate for stations 5 and 6. Figure 11 shows that the communities at the lower reaches of the river stations 7 and 9 were inhibited, since the index was less than one. The remaining communities vegetated successfully, but at station 5 of the northern branch of the river they experienced some toxic effect, which was higher than at station 6.

Figure 11. Index toxicity (WESI) distribution over sampling stations 2–9 in the Zarafshan River. The dynamics of the WESI values can be traced along the entire river through the southern branch cross station 5 with the orange color and through the northern branch cross station 6 with the blue color.

Although the toxic heavy metal pollution is particularly high at the river headwaters at station 2 and then at the end of the riverbed at stations 7 and 9, the toxic organic pollution fluctuates significantly depending on the source (Appendix A Table A1). The fluctuation of pesticides Alpha-HCH (Hexachlorocyclohexane) and DDT (dichloro-diphenyl-trichloroethane) is particularly noticeable, with its highest concentration at the river headwaters at station 2 and then repeated at stations 4, 5, and 7, where the drainage channels flow in. At the same time, the pollution caused by dissolved organic matter is not as high. The calculated saprobity indices, S, characterize the studied sections of the river according to water quality class from 2 to 3. Based on these data, we compiled a map of organic pollution class and accordingly colored not only the riverbed but also the section of the catchment basin related to each station. Figure 12a shows that organically enriched waters of Class 3 come from the out-border area of the Zarafshan River. Stations of the upper (3, 4, and 6) as well as the middle parts of the river (8, 9, and 10) were also marked as Class 3 in terms of water quality. All other parts of the river basin contain low organic matter in its water. We did not reveal any stations with waters of Classes 4 and 5. At the same time, the map of WESI distribution over the river (Figure 12b) takes attention to the middle part of the basin with stations 7, 8, and 9. This is part of the river basin with a narrow catchment area but is very enriched by settlements and saturated with enterprises discharging wastewater from processes with heavy metals. Both these maps show that the toxic effect in the middle part of the Zarafshan River landscape is due to the influence of metal discharges, but not organic pollution. Overall, the river has a high potential for self-purification, as can be seen on the maps in Figure 12, where the downstream stations already contain higher-quality non-toxic water.

Figure 12. Distribution of Water Quality Class in organic pollution (a) and Index WESI (b) over sampling stations and catchment basin areas in the Zarafshan River.

The integrated index of river pollution, RPI, was calculated based on the available chemical and biological data, also considering the division of the river into two branches with stations 5 and 6. Table 2 shows in bold the index values for the river parameters passing through station 6, which exceed those for the southern branch of the river passing through station 5. The greatest difference in the RPI indices was observed for TSS and TDS. Apparently, the excesses of the index for zinc, copper, phenols, fluorine, and detergents are associated with the influx of turbid and salted waters. The excesses of nitrites and COD indicate that the northern branch of the river is actively decomposing the influx of both organic and mineral pollutants. However, the relatively close values of the index for the abundance and number of species in both streams passing through the northern and southern branches of the river indicated that, in general, the river ecosystem successfully copes with the pollution entering the northern branch and the impact is leveled out.

Table 2. River pollution index (RPI) for environmental and biological variables crosses two different channels of the Zarafshan River.

Variable	RPI Cross 5 Average	RPI Cross 6 Average
km	479.7	469.1
O2 mg L−1	7.40	7.77
BOD, mgO L−1	1.10	1.11
COD, mgO L−1	7.54	8.01
N-NH4, mg L−1	0.05	0.06
N-NO2, mg L−1	0.55	0.90
N-NO3, mg L−1	0.95	0.96
Fe, mg L−1	0.02	0.02
Cu, mcg L−1	2.84	3.30
Zn, mcg L−1	2.96	3.29
Phenols, mg L−1	0.002	0.003
Oil, mg L−1	0.01	0.02
Detergents, mg L−1	0.00	0.01
TSS, mg L−1	380.3	507.0
DDT, mg L−1	0.0	0.0
Alpha-HCH, mcg L−1	0.0004	0.0004
Gamma-HCH, mcg L−1	0.0	0.0
Cr VI, mcg L−1	0.48	0.46
F, mg L−1	0.33	0.36
As, mcg L−1	0.92	1.09
TDS, mg L−1	505.7	545.8
No of Species	58.48	58.65
Sum of scores	154.03	155.07
Index S	1.53	1.56

Note: Maximal critical values of RPI are given in bold.

4. Discussion

For the first time, a study was conducted on the dynamics of environmental parameters and diatom communities along the entire length of the Zarafshan River, which crosses the territory of Uzbekistan from the border with Tajikistan to its confluence with the Amu Darya. The transboundary river has not been previously fully surveyed. However, according to the hydrochemical data of the upper transboundary section, a study was conducted [21] and showed a high water quality, with an excess of some metals as echoes of discharges from ore processing industries in Tajikistan. A complete list of algae inhabiting the riverbed in Uzbekistan and responsible for the self-purification processes of waters has not yet been compiled. Separate works were devoted to algal communities of some sections of the river, its tributaries, and reservoirs of its subordinate system [18,20]. Our study identified 428 species of diatoms, 98% of which can be used as indicators for nine environmental parameters for water quality monitoring that have not been used by UzHydromet up to now.

Our research in rivers has shown that the RPI can show the dynamics of chemical or biological indicators, but also indicate those places in the river channel where negative changes occur [37]. The RPI showed that the northern channel introduces noticeable pollution into the riverbed, of which the TSS values had the greatest gradient compared to the southern channel. Along with turbidity, the influx of metals and phenols was detected. However, in general, the dynamics of hydrochemical indicators underwent sharp changes at the stations of the lower reaches of the river, where TSS and TDS increased sharply. However, the saprobity index, S, was generally within the limits of 2–3 water quality classes, which indicates a large influence of inorganic pollution, rather than organic matter

Statistical analysis also confirms this conclusion, since separately calculated similarity coefficients for chemical data, for biological data, and for both together show the prevalence of the influence of chemical components of the ecosystem on its species composition. An assessment of the ecological state for the lower third of the river, carried out by us earlier [19], showed the sources of pollution and how the river ecosystem responded to their entry into the water, especially highlighting the stations near Navoi and Khatirchi. Arsenic and zinc entries were detected here. However, when analyzing the entire Uzbek section of the river, our attention was drawn to the stations near the city of Samarkand, where the middle reaches of the river begin. Bioindicator analysis of river communities, divided into three sections, showed that as the temperature and salinity of the waters as a whole increase towards the mouth of the river, oxygen and pH decrease. Organic pollution also decreases down the river. Communities tend to occupy substrates downriver rather than inhabit the water column and were mostly autotrophs. Along the river, the water became more oligotrophic, remaining at quality Class 2–3.

Alarming levels of some of the legacy persistent organic pollutants (POPs) may pose adverse effects on the aquatic species dwelling in the riverine communities [38]. However, in the case of the Zarafshan River in the Uzbekistan part, it can be seen that the Zarafshan Nature Reserve plays a very important role in decreasing the risk of pesticide impact.

In the nearby region of the southern Caspian Sea, rivers flowing in from the south [39] have, as in our case, fluctuations in water quality, and the most polluted stations there were, on the contrary, in the upper, mountainous sections, which the authors associate with anthropogenic activity. The presence of heavy metals is also characteristic of mountainous sections of rivers in southern Europe, since it is associated with the presence of industry in the region [40]. In large European rivers, the combined effects of both heavy metals and pesticides lead to the suppression of biodiversity because of the combined toxic effects [38,41]. Among the large rivers in the immediate region, studies on hydrochemical indicators are known in the Tigris River, Iraq [42]. Here, the opposite trend of increasing organic pollution downstream and deteriorating water quality is noted. In the case of the Zarafshan River, we see a trend towards improving water quality in both hydrochemical and biological indicators. Even though pollution is significantly greater in the northern branch of the middle course that is reflected in a decrease in biodiversity and the abundance of diatoms, and comes from subsequent industrially saturated stations, the general state of the river ecosystem, as a whole, is improving. It can be assumed that the protected area of the Zarafshan Nature Reserve, which reduces transboundary pollution, plays a significant role in this.

Our data on indicator assessments were obtained for the first time for a transboundary landscape-forming river flowing through the entire territory of Uzbekistan. Currently, the catchment area of the Zarafshan River has a very limited volume even in the flat sections of the river. The influx of pollutants is, therefore, limited by the properties of the landscape, i.e., it comes from the mountainous transboundary area of the upper reaches and the water flows of the irrigation system of the middle and lower reaches. At the same time, the analysis of water for irrigation reduces the water content of the river and leads to a decrease in the self-purification capacity of its ecosystem. This is also facilitated by the high turbidity of the water, which suppresses the development of algae. Our previous studies in the Central Asian region have demonstrated the effectiveness of ecological mapping of water quality indicators and indices for the Arys River basin in Kazakhstan [11]. Our first basin-based water quality map for the transboundary Zarafshan River showed problem areas where elevated pollutant levels were also detected and, therefore, the effectiveness of mapping for future monitoring. Water quality monitoring in Uzbekistan is currently carried out only by hydrochemical indicators. Our work has shown that biological indicators should also be included in the monitoring system, since transboundary impacts and internal sources lead to a decrease in diversity. For this purpose, we have already created a database of diatom indicator algae for the largest river, the Zarafshan.

5. Conclusions

For the first time, we compiled a general list of 428 diatom species in the Zarafshan River and conducted a comprehensive ecological assessment of water quality and the state of the aquatic ecosystem based on their communities using bioindication and statistics. Of the entire list, 91 percent of the species were indicators for 9 environmental parameters. Community at stations 3 and 4, as well as 5 and 6, reflect the complexity of the impact of the environment around the city of Samarkand and further in the middle reaches of the river. The indicator species reflect the dynamics of water quality in the river downstream, where organic pollution and general toxic impact are not yet tracked by the monitoring system. Analysis of the distribution of bioindicators, organic pollution indices and WESI, as well as statistics, show an increase in salinity and turbidity and a decrease in organic pollution downriver. At the same time, the Navoi section is also a source of water acidification. Our analysis shows that nutrients and heavy metals, as well as phenol and pesticides pollution, enter the river from various sources mainly in the middle reaches of the river. The small catchment area in a dry climate can be regarded as a positive factor, reducing the irrigation area and the pollution coming from it. The integrated index of river pollution RPI shows most pollution comes from the northern canal of the river in the middle reaches. For the first time in Uzbekistan, ecological mapping of the river was used to visualize problem areas of the river. Based on the results of bioindication, an increase in self-purification with an increase in the species richness and number of diatoms in the middle section of the Zarafshan River was found. This indicates that the river ecosystem is successfully coping with incoming pollutants. Such a conclusion could not be made based on chemical analysis of the water alone. This allowed us to recommend the inclusion of biological indicators and the expansion of state monitoring points to the lower section of the Karakul.