Algal community structure and water quality assessment in the Sattarkhan and Sahand Dams, East Azerbaijan Province, Northwestern Iran

Algae and cyanobacteria are the main primary producers in lotic and lentic freshwater ecosystems. Algal community structure, biomass standing crops, and species composition have been used to assess the ecological condition of rivers and reservoirs. Moreover, algae are abundant and cosmopolitan in their distribution, can be sampled rapidly, and have a wide range of structural (biomass, composition) and functional (metabolism) attributes (Stevenson & Bahls, 1999; Atazadeh et al., 2021). Therefore, the study of algal biodiversity for water quality investigations has long been important for ecologists and conservationists. Understanding the communities of aquatic biota is crucial for developing policies that provide potable water to surrounding communities (Godfray et al., 2002; Kahlert et al., 2016; Conix et al., 2023). Investigating algal flora is crucial for determining water quality for several reasons. For example, algae serve as excellent bioindicators, and algal flora helps in assessing the extent of nutrient pollution and its impact on water quality (Smith et al., 1999). Furthermore, different species thrive under specific environmental conditions, including nutrient levels, pH, temperature, and the presence of pollutants. By studying the presence, abundance, and diversity of algae, scientists can assess the overall health of aquatic ecosystems (Reynolds et al., 2006). Certain types of algae and cyanobacteria can form harmful algal blooms (HABs) under conditions of nutrient pollution, temperature, and eutrophication. These blooms can produce toxins harmful to aquatic life, animals, and humans. Monitoring algal flora helps identify the potential for HABs and allows for timely intervention to mitigate their impact (Atazadeh, 2023; Paerl & Otten, 2013). Excessive nutrients, particularly nitrogen and phosphorus from agricultural runoff, urban wastewater and industrial discharge, can lead to eutrophication, a process that accelerates algal growth, depletes oxygen levels in water bodies, and harms aquatic organisms.

On the other hand, algae play a vital role in aquatic food webs and ecosystem dynamics in terms of biogeochemical cycling. Changes in algal communities can disrupt the balance of ecosystems, affecting the abundance and distribution of other organisms, including fish and invertebrates. Monitoring algal flora helps assess the overall ecological health of water bodies and identify potential threats to biodiversity. In conclusion, investigating algal flora provides valuable insights into water quality assessment, helping policymakers, scientists, and water resource managers make informed decisions to protect and preserve aquatic ecosystems for future generations (Hillebrand et al., 1999). Several diatom species (Bacillariophyceae) respond quickly to environmental changes and have been used as effective climate indicators (Smol et al., 2005). Their specific range preferences, together with their distinctive morphological features, allow taxonomic differentiation to the species level. The durability of their siliceous cell walls is valuable for investigating current as well as past environments. However, to successfully investigate the ecology of diatoms, determine patterns in biogeography, and use species identity as indicators of environmental conditions (Dimitrovski et al., 2012; Kopalová & Nedbalová, 2013), their identification must be as unambiguous and verifiable as possible, that is, documented with images and traceable to archived materials and sources (Spaulding et al., 2021). Preserving biological diversity requires research on the richness of certain taxonomic groups across different natural territories.

So far, the algal diversity of Iran has been poorly studied. The first studies on the algal flora of Iran were by Löffler (Löffler, 1959, 1961). The works of Löffler can be considered the first authentic studies on the algal flora of Iran. Later, Hirano (Hirano et al., 1973) and Wasylik (Wasylik, 1975) reported 406 infraspecific taxa. Studies on the algal community of water reservoirs in Iran include studies on the biodiversity of the diatom population in the Masuleh River in Guilan (northern Iran) (Ramazanpour et al., 2013), a floristic study of phytoplankton in the Yamchi Dam in Ardabil (northwestern Iran), and the biodiversity of diatoms and their relationship with environmental factors in the Ahar Chai River in northwest Iran (Yadollahi & Atazadeh, 2024).  Overall, some studies on algae of Iran were conducted including algae from the deserts of Iran (Compère, 1981), Caspian Sea (Fallahi, 1993), Zayandeh Rood (Afsharzadeh & al., 2003), Lake Neure (Nejadsattari, 2005), Gharasou River (Atazadeh & al. 2007; Atazadeh & Sharifi, 2012), Streams in Ramsar (Soltanpour-Gargari et al., 2011), Karaj River (Kheiri et al., 2018), Balikhli River (Panahy-Mirzahasnlou et al., 2018), Taleghan River (Naseri et al., 2022), Western Rivers of Lake Urmia (Mehrjuyan & Atazadeh, 2022), Aras River (Parikhani et al., 2023), Ahar Chai River (Yadollahi & Atazadeh, 2024), Sufi Chai river (Charandabi & Atazadeh, 2025). The taxonomic composition of benthic diatom communities has been widely used for monitoring water quality (Atazadeh et al., 2021). Therefore, floristics surveys are important for long-term ecological investigations and monitoring.

Similar studies in neighboring countries include the study of the phytoplankton community in the Vaghchi River in Armenia (Hambaryan et al., 2016) and the evaluation of the composition and seasonal changes of epilithic diatoms in the Yedikir Dam Lake in Amasya, Türkiye (Maraslioglu & Soylu, 2017). This study aims to document the species composition of diatoms, green algae, and cyanobacteria in the freshwater bodies of northwestern Iran, to create a voucher flora rich in images to be used as a diagnostic tool in future studies.

Materials and methods

Sampling Site Descriptions

The Sattarkhan Dam is located 15 km west of Ahar city in East Azerbaijan Province, on the Ahar Chai River. This dam is located at an elevation of 1,426 m a.s.l., along the Ahar–Varzeghan road, with geographic coordinates of latitude 38.476682 and longitude 46.877076. The Sahand Dam is located southwest of Hashtroud city, at an average elevation of 1,641 m a.s.l., with geographic coordinates of latitude 37.41114 and longitude 46.90402 (Figure 1). Sampling was carried out in the Sattarkhan and Sahand dam reservoirs in four seasons using a plankton net and from a depth of half a meter in the dam reservoir and scraping the algal periphyton at the margin of the dams. At each sampling station, multiple samples were collected for the analysis of algal biomass standing crop and species composition. Algal communities can exhibit great diversity, yet their structure and composition may vary depending on the nature of the substrate. Algae colonize in the water column and substrates such as cobbles, stones, mud, rock, woody debris, and emergent or submerged plants. Sampling of phytoplankton was carried out over two years (2023–2024) in the freshwater habitats of the dams. Samples were collected using standard APHA methods and EPA periphyton protocols (American Public Health Association, 1926; Utermöhl, 1958; Stevenson & Bahls, 1999).

Laboratory and statistical techniques

Live algal units, including green algae and cyanobacteria, were analyzed within two days after sampling using Olympus and Zeiss optical microscopes. However, a different method was used for diatoms. Samples were digested with 35% hydrogen peroxide in a beaker at 90°C on a hotplate for 2 hours, after which two drops of 35% hydrochloric acid were added. The beakers were filled with distilled water and left to settle overnight, after which the supernatant was discarded. This process was repeated four times. Subsamples of 800 µl were air-dried on coverslips and mounted using Naphrax. Optical microscopes (Olympus and Zeiss) were used for examination of prepared slides and taking light micrographs. Species identification was carried out using internationally recognized floras, books, and articles, including: Hillman et al.,1974; Krammer et al., 1986, 1988, 1991a, 1991b; Lange-Bertalot, 1997; Vuuren et al., 2006; Levkov & Ector, 2010; Moresco et al., 2011; Bellinger & Sigee, 2015; Lange-Bertalot & Hofmann, 2017; McGregor, 2018; Solak et al., 2019; Joseph & Subramani, 2021. In addition, running water from each dam was measured for EC, TDS, and pH at the time of sampling (American Public Health Association, 1926). Measurements of EC, pH, and TDS were performed on-site using a Hanna HI9811 device. Data was analyzed to examine the relationship between ecological factors and species distribution using Canoco software and CCA analysis. The significance of differences in physicochemical data, based on four replicates, was assessed using GraphPad InStat software and Tukey's test. Relevant graphs were generated using Microsoft Excel.

Results

In this study, the biodiversity and seasonal distribution of diatoms and soft algae species, along with their relationship to physicochemical factors, were investigated in the reservoirs of the Sattarkhan and the Sahand dams. A total of 115 taxa belonging to 55 genera were identified across four seasons, with 40 species shared between both dams. According to the data presented in Table 1, five species were recorded exclusively in the Sahand Dam, and ten only in the Sattarkhan Dam. In terms of taxonomic composition, 5 genera and 6 species of cyanobacteria, 16 genera and 31 species of green algae, 31 genera and 75 species of diatoms, and one genus and species each from the Euglenaceae (Euglena), Peridiniaceae (Peridinium), and Phacaceae (Phacus) families were documented. Among the genera, Nitzschia (17 species) and Ulnaria (6 species) were the most diverse among diatoms, Scenedesmus (6 species) and Pediastrum (5 species) among green algae, and Oscillatoria (2 species) among cyanobacteria. Among diatoms, only four genera, Cyclotella, Melosira, Lindavia, Cyclostephanos and Thalassiosira, belonged to centric (radial) types, while the remaining taxa were pennate. Centric diatoms were among the dominant groups in both dams. Cyclotella meneghiniana was the most abundant species in Sattarkhan Dam during all four seasons, with relative abundances of 3.11% in summer, 2.44% in autumn, 4.00% in winter, and 3.11% in spring, followed by Melosira varians. A similar pattern of dominance was also observed in the Sahand Dam. The genus Surirella (pennate diatom) was consistently present in all seasons and both reservoirs. Nitzschia, the most diverse genus, was recorded year-round in both locations. Among green algae, desmid genera such as Cosmarium and Staurastrum were absent in summer but were observed with moderate abundance in autumn, winter, and spring. Scenedesmus species were recorded in both dams throughout all seasons. In the case of cyanobacteria, Anabaena and Nostoc were abundant and detected in every season in both reservoirs. Aphanizomenon flos-aquae was exclusive to the Sattarkhan Dam during summer, while Oscillatoria species were observed in all seasons except winter, and only in Sahand Dam.

Table 1. lists the identified species, their families, and the corresponding references.

According to Figures 2 and 3, which show the number of identified species of diatoms, green algae, and blue-green algae across different seasons in both dams, diatoms were the least abundant in summer, with 37 species in the Sattarkhan Dam and 39 species in the Sahand Dam. However, their numbers increased in autumn, winter, and spring in both locations. Green algae showed relatively stable abundance across seasons in both dams. Blue-green algae were most abundant in summer, with 7 species recorded in the Sattarkhan Dam and 6 in the Sahand Dam, while the lowest abundance was observed in winter, with only 2 species recorded in both dams. The canonical correspondence analysis (CCA) identified that electrical conductivity (EC) had the most significant effect on the algal assemblage and structure compared to other environmental variables (Figure 4).

Figure 2. The number of species observed in three algal groups: diatoms, green algae, and blue-green algae in different seasons in the Sattarkhan Dam.

Figure 3. The number of species observed in three algal groups: diatoms, green algae, and blue-green algae, in different seasons in Sahand Dam.

Figure 4. Canonical Correspondence Analysis (CCA) of species data and environmental variables

Physicochemical data

According to Figures 5 to 10, which illustrate the seasonal variation of physicochemical parameters, the pH of water in Sattarkhan Dam (Ahar) was relatively alkaline in all four seasons, ranging between 7 and 8. In the summer, however, pH was significantly lower than in the other seasons (P < 0.05). The electrical conductivity (EC) values varied seasonally. In Sattarkhan Dam, the lowest EC was recorded in winter, while the highest value was observed in summer, which was statistically significant (P < 0.05). Similarly, the total dissolved solids (TDS) level was at its lowest in winter and highest in summer, again showing a statistically significant seasonal difference (P < 0.05). For Sahand Dam, the trend was similar to that of Sattarkhan Dam. The pH ranged between 7 and 8 in all seasons, but the highest pH was recorded in winter, showing a statistically significant increase (P < 0.05). The EC in Sahand Dam was also lowest in winter and highest in summer, both changes being statistically significant (P < 0.05). Likewise, the TDS level followed the same pattern, with the lowest value in winter and the highest in summer, again with a statistically significant difference (P < 0.05).

Figure 5. Seasonal variation of TDS in Sattarkhan Dam. The columns represent the mean of four replicates ± SD. Columns sharing the same letter are not significantly different (P < 0.05).

Figure 6. Seasonal variation of pH in Sattarkhan Dam. The columns represent the mean of four replicates ± SD. Columns sharing the same letter are not significantly different (P < 0.05).

Figure 7. Seasonal variation of electrical conductivity (EC) in Sattarkhan Dam. The columns represent the mean of four replicates ± SD. Columns sharing the same letter are not significantly different (P < 0.05).

Figure 8. Seasonal variation of total dissolved solids (TDS) in Sahand Dam. The columns represent the mean of four replicates ± SD. Columns sharing the same letter are not significantly different (P < 0.05).

Figure 9. Seasonal variation of pH in Sahand Dam. The columns represent the mean of four replicates ± SD. Columns sharing the same letter are not significantly different (P < 0.05).

Figure 10. Seasonal variation of electrical conductivity (EC) in Sahand Dam. The columns represent the mean of four replicates ± SD. Columns sharing the same letter are not significantly different (P < 0.05).

Data from microscopic studies

The plates A to K show selected microscopic images of species observed in the present study.

Plate A). (1) Nitzschia linearis; (2) Nitzschia sigmoidea; (3) Nitzschia sp; (4) Nitzschia cf sigma; (5) Nitzschia brunoi; (6–7) Nitzschia sp; (8) Tryblionella hungarica; (9–10) Nitzschia capitellata; (11) Cyclostephanos dubius; (12) Melosira varians; (13) Cyclotella meneghiniana; (14) Thalassiosira weissflogii; (15) Lindavia affinis. Scale bar=10μm.

Plate B).  (1) Nitzschia vermicularis; (2) Nitzschia gracilis; (3) Nitzschia cf reversa; (4) Tryblionella hungarica; (5) Nitzschia dissipata; (6) Nitzschia calida; (7) Nitzschia thermaloides; (8) Nitzschia calida; (9) Nitzschia salinarum; (10) Nitzschia debilis; (11) Platessa oblongella (dorsal and venral views) ; (12) Fragilaria capucina; (13) Tabellaria fenestrata; (14) Fragilaria vaucheriae; (15) Staurosira binodis; (16) Lemnicola hungarica. Scale bar=10μm.

Plate C).  (1) Fragilaria nanana; (2) Staurosirella sp; (3) Fragilaria mesolepta; (4) Fragilaria cf henryi; (5, 6) Ulnaria ulna; (7) Fragilaria saxoplanctonica; (8) Ulnaria capitata; (9) Synedra subrhombica; (10) Ulnaria biceps; (11) Ulnaria danica; (12) Nitzschia recta; (13) Fragilaria corotenensis; (14) Fragilaria sp; (15) Luticola mutica; (16) Gomphonema exilissimum; (17) Aulacoseira granulata; (18) Planothidium rostratoholarcticum; (19) Nitzschia vitrea; (20) Nitzschia brevissima; (21) Diatoma tenuis; (22) Caloneis bacillum; (23) Hantzschia amphioxys. Scale bar=10μm.

Plate D). (1) Diploneis calcilacustris; (2) Diploneis parma; (3), Cocconeis lineata; (4) Cocconeis pediculus (ventral view); (5) Cocconeis placentula; (6) Cocconeis pediculus (dorsal view); (7) Cyclostephanos sp; (8) Pantocsekiella cf shaumanii; (9) Surirella angusta; (10-12) Surirella ovalis; (13) Achnanthes inflata; (14) Placoneis symmetrica; (15) Navicula lesmonensis; (16) Placoneis undulata; (17) Frustulia vulgaris; (18) Placoneis clementis; (19) Placoneis gastrum. Scale bar=10μm.

Plate E). (1) Pinnularia joculata; (2) Encyonopis cf microcephala; (3, 4) Amphora ovalis; (5) Amphora lange-bertalotii; (6) Craticula ambigua; (7) Caloneis amphisbaena; (8) Cymbella affinis; (9) Cymbella caespitasum; (10–12) Cymbella neocistula; (13, 14) Cymatopleura solea; (15, 16) Cymatopleura elliptica.

Plate F). (1) Diatoma mesodon; (2) Rhopalodia operculata; (3) Epithemia sorex; (4) Gyrosigma attenuatum; (5, 6) Gyrosigma acuminatum; (7) Navicula rostellata; (8) Navicula cryptofallax; (9) Pinnularia cf brebissonii; (10) Pinnularia borealis; (11) Gomphonema olivaceum.

Plate G). (1) Tabularia fasciculata; (2) Fragilaria cf radians; (3, 4) Rhopalodia gibba; (5) Halamphora normanii; (6) Hantzschia abundans; (7) Pseudostaurosira subconstricta; (8) Melosira varians; (9, 10) Rhoicosphenia abbreviata; (11) Stauroneis anceps; (12) Eunotia arcubus; (13) Neidium dubium; (14) Encyonema leibleinii; (15) Epithemioa turgida; (16) Fallacia lenzii; (17) Navicula antonii; (18) Gomphonema acuminatum.

Plate H). (1) Desmodesmus abundans; (2) Pectinodesmus pectinalus; (3) Desmodesmus sp.; (4) Scenedesmus bijugatus; (5, 6) Scenedesmus quadricauda; (7) Scenedesmus perforatus; (8) Pediastrum duplex; (9, 10) Monoraphidium grifithii; (11) Oocystis cf lacustris; (12) Chroococcus minutus; 13 Phormidium sp.; (14) Desmodesmus sp.

Plate I). (1) Stauridium tetras; (2) Pseudopediastrum boryanum; (3) Pseudopediastrum cf pearsonii; (4) Pediastrum duplex; (5) Coelastrum pseudomicroporum; (6) Coelastrum pulchrum; (7) Ankistrodesmus cf falcatus; (8) Oedogonium sp; (9) Oocystis parva; (10) Staurastrum anatinum; (11) Staurastrum bicorne; (12) Staurastrum boreale; (13) Staurastrum gracile; (14) Peridinium cf cinctum; (15) Closterium acerosum; (16) Pectinodesmus pectinalus; (17) Scenedesmus cf bijugatus (18, 19) Oscillatoria limosa

Plate J). (1) Actinastrum sp; (2) Phacus cf pleuronectes; (3) Batrachospermum turfosum; (4) Sphaerocystis schroeteri; (5) Stigeoclonium tenue; (6) Microcystis aeruginosa; (7)  Euglena sp; (8) Nostoc pruniforme.

Plate K). (1) Oscillatoria sp; (2) Tolypothrix sp; (3) Oscillatoria sp; (4) Nostoc sphaericum; (5) Cosmarium phaseolus; (6) Cosmarium subprotumidum; (7) Franceia ovalis; (8) Golenkinia radiata; (9) Spirogyra sp; (10) Arthrospira sp; (11) Aphanizomenon flos-aquae; (12) Anabaena sp.

Discussion

Comparison of the results of this study with those of Yamchi Dam in Ardabil Province (Mirzahasanlou et al., 2018), which shares similar habitat conditions, showed a high degree of consistency. In both studies, Bacillariophyceae, Chlorophyceae, and Cyanophyceae were the most diverse algal groups. Nitzschia was identified as the largest phytoplankton genus in both locations. In contrast, a comparison with the study conducted on the Ahar Chai River (Yadollahi & Atazadeh, 2024), which represents a flowing-water habitat, showed differences mainly in the composition of centric and radial diatoms. In the river, benthic species exhibited high diversity and abundance, likely due to the dynamic flow, whereas in the present study, which involved a more stagnant habitat, planktonic species were more dominant. In Ahar Chai, the most abundant genera were Cymbella, Nitzschia, Navicula, and Gomphonema, while in the current study, Cyclotella, Melosira, and Nitzschia were dominant. The genus Nitzschia was present in both seasons and dams, likely due to its high tolerance to variations in salinity and alkalinity. The diversity and persistence of this genus may also reflect its ability to utilize various mineral and organic nutrients, consistent with observations by Karthick et al., 2013. Bacillariophyta are known to be adapted to low temperatures (Lira et al., 2011; Basavaraja & Narayana, 2013). As shown in Figures 2 and 3, the abundance of diatoms increased in the cooler seasons following summer. In contrast, cyanobacteria and green algae (Chlorophyta) thrive in warmer conditions (Varol, 2019), explaining the high abundance of cyanobacteria in the summer, when elevated temperatures support their dominance. In colder seasons, especially in winter, the abundance and diversity of cyanobacteria decreased significantly. Among the blue-green algae identified in Sattarkhan Dam, Aphanizomenon flos-aquae, observed only in the summer, is a toxic species known to form algal blooms worldwide (Baykal & Açikgöz, 2004). Anabaena and Nostoc, which were recorded in all four seasons with relatively high abundance, are also bloom-forming filamentous cyanobacteria. Anabaena is particularly known for its nitrogen-fixing capability and ability to form symbiotic associations with certain plants, such as ferns. It is also one of four cyanobacterial genera that can produce neurotoxins. Oscillatoria, which was present in all seasons except winter in Sahand Dam, is known to release 2-methylisoborneol, geosmin, and cyanotoxins during growth (Wu & Jüttner, 1988). These semi-volatile compounds are responsible for earthy and musty odors in water, even at very low concentrations (Newcombe & Dixon, 2006). The abundance of desmid genera such as Cosmarium and Staurastrum—especially in autumn, winter, and spring- suggests oligotrophic (nutrient-poor and clean) water conditions (Habib et al., 1997; Reynolds, 1998). Their low abundance in summer, combined with the increased presence of cyanobacteria, indicates eutrophic conditions during that season. This shift may be due to reduced water flow, higher temperatures, and increased nutrient loads, all of which favor cyanobacterial dominance in summer. By analyzing species composition in Sattarkhan and Sahand Dams alongside physicochemical parameters (TDS, EC, and pH), it was observed that the results from both sites were largely similar. The pH was significantly more alkaline in winter and more acidic in summer (P < 0.05) in both dams. According to Figures 2 and 3, diatoms showed the lowest abundance in summer, whereas cyanobacteria peaked during this season. In contrast, green algae did not exhibit significant seasonal variation in abundance. Canonical Correspondence Analysis (CCA) indicated that EC had the most significant impact on algal community structure. Given the high EC values in summer, cyanobacteria were dominant in this season. In contrast, lower EC values in autumn, winter, and spring corresponded with higher diatom abundance, suggesting that nutrient-rich conditions and other factors, such as stagnant water, increased nitrogen and phosphate levels, and strong light availability, in summer contributed to this shift. Reduced inflow and localized nutrient accumulation may also explain the seasonal eutrophication observed in both reservoirs. Additional work is needed to define ecological gradients controlling algal assemblage and distribution in both reservoirs.