Introduction

The Caspian Sea is regarded as the largest close lake with brakishwater on Earth (Grigorovich et al. 2003). Ecological and morphological traits of the open seas are not seen in this ecosystem; this is regarded as a sea because of its vast area. Based on physical, geographical, and hydrological characteristics, the Caspian Sea is classified into three parts: Northern, Central and Southern Caspian. Rivers like Sefidrud, Gorganrud, Tajan, Haraz, Shiroud, Sardabrud, Talar, Babolrud, etc. are discharged into the Caspian Sea. One of the unique features of the Caspian Sea habitat is the diversity of its origins. The most abundant species are endemic (about 75% of the total number) and a few from Mediterranean (6%) and the Arctic (3%), and also the immigrants from freshwater. About 15 phytoplankton species have been introduced to the Caspian Sea, as the result of the artificial connecting of the Caspian Sea with the Azov-Black Sea basin by the Volga-Don Canal since 1952 (Kosarev and Yablonskaya 1994).

Bandar-e Anzali is the largest port in northern Iran and is one of the most active ports of the Caspian Sea. Anzali’s main importance is its harbor location, which plays an important role in the transportation of water of the Caspian Sea, fisheries and aquatic environments on the southern shores of the Caspian Sea (Dumont 1998). Phytoplankton is considered as an important and main element in aquatic environments, since it composes the first ring of the food chain in the ecosystem (Boni 1925). Phytoplanktons are useful bioindicators of changing environmental conditions and the advantage of using biological indicators over chemical or physical ones (Griffith et al. 2002). On the other hand, a number of introduced species have ecological negative effects on ecosystems. Therefore, the identification and qualitative studies of phytoplankton populations not only have an important role in a collection of historical data, but are also necessary to understand the polluted and under stress environment of the Caspian Sea (Nasrollahzadeh Saravi et al. 2014).

Phytoplankton is beneficial to evaluate the lakes because of suitable characters such as short life cycle, high cell abundance and species diversity, sensitivity to different kinds of organic and inorganic pollution, various zone of tolerance against contaminates and environmental stressors (Castro and Huber 1991).

Primary production in the Caspian Sea is dependent on nutrients content and type of compounds in sea water. Most nutrient compounds are entered through rivers and drainages, as annual discharge of silica, phosphorous and nitrogenous into the Caspian Sea by the rivers is 95, 90, and 80%, respectively. Vertical distribution of the nutrients in deep waters of the Central and Southern Caspian Sea is influenced by the intense of water mixing resulted from convection flow and increased with increased depth (Ghasemov and Bagherov 1983). In the Caspian Sea, 449 phytoplankton species consist of 163 diatoms, 139 Chlorophytes, 102 Cyanophytes, 39 Dinofagellates, 5 Euglenophytes and 1 Chrysophyta during 1962–1974. In addition, different kinds of algae have been identified as 414 species in the north, 225 species in the center and 71 species in the south.

Therefore, the number of phytoplankton is decreased from north to south due to a decline in the number of freshwater phytoplankton species (Proshkina-Lavrenko and Makarova 1968; Kosarev and Yablonskaya 1994). Of the Caspian phytoplankton compounds, diatoms are dominant in species number (163 species), but their population becomes less than green algae at the northern Caspian in summer for some years (Ghasemov and Bagherov 1983; Salmanov 1987).

Phytoplankton distribution in this ecosystem is not uniform during different seasons of the year (Salmanov 1987). At present, there are several problems in this sensitive and vulnerable ecosystem which are considered as major environmental concerns.

Population growth of cities in the Caspian basin, increased discharge of different wastewaters (industrial, agricultural and civil effluents), oil extraction development and the connection of this sea with the open seas through Volga-den canal have caused many problems (Bagheri et al. 2010) like as increased water temperature, comb jellyfish invasion, and eutrophication at the end of twentieth century (Izaguirre et al. 2001). Phytoplankton is the main group that plays a dynamic and active role in the marine food web, in exploiting primary products at higher levels of nutrients and elements in the global cycle of crops. By itself, it contains 1% of organic carbon and 45% of annual primary production on Earth. Phytoplankton include proteins, carbohydrates, lipids, vitamins and mineral salts, that they are directly and indirectly considered as food sources for all marine organisms. In addition, the oxygen produced by the phytoplankton is a result of photosynthesis, a vital component of the system on the Earth (Falkowski and Raven 1997).

Hence, the goal of this investigation was to identify species diversity and abundance of the main phytoplankton groups in south west of the Caspian. Finally, this present study shows data of phytoplankton in recent years.

Materials and methods

The present survey was conducted in Bandar-e Anzali in the west southern Caspian Sea (37º0.27′59″N 49º0.27′59″E) and has a coastline of 40 km. Sampling was performed at 12 stations located at four transects (Hasanrod, Taleb abad, Bandar coast, Sangachin) (a half line perpendicular to the coast) with 3 stations at surface water, depths of 5 and 10 m (Fig. 1, Table 1). The sampling stations were fixed near shore. Sampling was performed seasonally, during spring 2012 to spring 2013.

Fig. 1
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Bandar-e Anzali Map from the southwestern Caspian Sea, showing the sampling sites

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Table 1 Sampling transects and the location of stations in Bandar-e Anzali from the southwestern Caspian Sea
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In order to study phytoplankton, water samples were collected in Van Dorn water bottle (Ruttner) (Vollenweider 1974) and poured into plastic containers. Then, 7 ml of Lugol iodine solution was added to the containers (0.7 ml per each 100 ml of the sample) and after 1 h, 40 ml of 40% concentrated formaldehydes was added to the vessels. Therefore, the final solution contained 4% of formalin (APHA 2005). Three replicates were obtained at each station from surface water, with 5 and 10 m depth.

Phytoplankton samples were transported to the laboratory and stayed for 48 h. Then, the supernatant was siphoned in order to make the sample denser. The remaining was centrifuged for several times for 5 min at 3000 rpm to gain 30 ml volume of each sample (Centrifuge Model Number D-7200 tutilinge7).

To identify phytoplankton species, each sample was shaken well to obtain a homogenized sample. Thereafter, three replicates of each sample (1 ml) were pipetted from the 30 ml sample and poured into a Sedgewick-Rafter counting chamber under an invert microscope (Vollenweider 1974; Newell 1977). Species identification was performed using valid identification keys (Prescott 1962; Kasymov 2000; Tiffany and Britton 1971). Statistical analysis was done using the SPSS software (version 11.5). The data were tested to check for the normality. One-way analysis of variance (ANOVA) was used for the existence of significant differences (P < 0.05) between transects seasons and depths. Multivariate regression was used in this study. Phytoplankton phyla were considered as dependent variables and transect and depth were considered as independent variables.

Discriminate function analysis was applied to indicate how different abundances of phytoplankton phyla vary across transect. Shannon–Wiener Index was used to study species diversity. The values of this index are divided into five qualitative classes (Marques et al. 2009) (Table 4). Shanon Index is calculated by the following formula:

$$H^{\prime} = - \varSigma \, \left( {ni \, /N} \right) \, \ln \, \left( {ni \, /N} \right),$$

N is the total population density, ni is the species density i.

Results

In this trail, 72 species from 59 genera, 45 families, 31 orders, and 5 main phytoplankton branches were identified (Table 2). Of these identified phytoplankton, 7 orders, 11 families, 16 genera, and 17 species were allocated to Green algae (Chlorophyta); 3 orders, 5 families, 10 genera and 6 species to blue-green algae (Cyanophyta); 5 orders, 6 families, 7 genera and 16 species to Dinoflagellata (Pyrrophyta); 1 order, 1 family, 3 genera, and 4 species to Euglenoids (Euglenophyta); 15 orders, 17 families, 23 genera and 29 species to diatoms (Bacillariophyta).

Table 2 Identified phytoplankton species from southern Caspian Sea in all seasons
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Among the phytoplankton species, 29 species (40%) were from diatoms (Bacillariophyta), 17 species (24%) from green algae (Chlorophyta), 16 species (22%) from Dinoflagellata (Pyrrophyta), 6 species (8%) from blue-green algae (Cyanophyta) and 4 species (6%) from Euglenophyta. The number of recorded species in spring, summer, autumn, and winter was 46, 50, 41 and 39, respectively.

Diatoms showed the most species diversity in winter (22 species) and autumn (21 species), while the lowest one was observed in spring (13 species). The highest species number of Pyrrophyta was detected in spring (15 species), Cyanobacteria in summer (6 species), Chlorophyta in spring (14 species) and Euglenophyta in summer (4 species) (Table 3).

Table 3 Number of identified phytoplankton species from southern Caspian Sea in different seasons
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Seasonally phytoplankton diversity in the southern Caspian Sea in Bandar-e Anzali was investigated based on Shannon-Diversity Index in the present study and exhibited the most diversity index in Autumn (2.532) and the least index in Spring (2.201) with an annual mean of this index (2.8) (Fig. 2). Eventually, Shannon index in Bandar-e Anzali is classified according to Table 4 in the moderate class.

Fig. 2
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Seasonality phytoplankton diversity in the southern Caspian Sea

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Table 4 Water quality classification, based on Shannon Diversity Index
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During the first three seasons of the year, the highest abundance was discovered at 10 m depth while in winter it was observed at depths of 5 and 10 m. In different depths the most and the lowest abundance were recorded at depth of 10 and surface water, respectively. Transect and depth had significant effect on density of phytoplankton phyla (P < 0.05) (Table 5), but the interaction effect of depth by transect showed no significant effect (P > 0.05) (Fig. 3). The number of Cyanobacteria and Euglenophyta species is completely similar in different transects located at the southern Caspian Sea, whereas the species number of diatoms was variable at different transects. 12 species of diatoms, 9 species of Pyrrophyta and 13 species of other branches were revealed in Sangachin (Fig. 4). In fact, the main phytoplankton groups with most species number in Bandar-e Anzali were diatoms, green algae and Dinoflagellata. Moreover, Sangachin and Bandar coast transects had the highest and the least species diversity and cell abundance, respectively.

Table 5 Multivariate tests (MANOVA) analyzing the effect of transect and depth on abundances of phytoplankton phyla in the southern Caspian Sea
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Fig. 3
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Comparison between densities of phytoplankton phyla (a Chlorophyta, b Cyanophyta, c Pyrrophyta, d Euglenophyta, e Bacillariophyta) in different depths and transects

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Fig. 4
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Comparison between diversity of different phyla of phytoplankton in different transects

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Discriminant Function Analysis was applied to better find any differences among transects based on phytoplankton cell abundance. As depicted in Fig. 5, DFA could indicate the discrimination between transect 1 (Hasan rod) and 3 (Bandar coast) as well as transect 2(Taleb abad) and 4(Sangachin). Transect 3 was well discriminated from other transects.

Fig. 5
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Canonical Discriminant Function graph based on phytoplankton cell abundance at different transects

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According to the results of DFA, among phytoplankton phyla isolated from the Caspian Sea, three phyla including Cyanophyta, Pyrrophyta and Euglenophyta had major role in discriminating the transects as their abundance among various transects showed significant differences (P < 0.05).

The following equations obtained from DFA indicate the relationship between phytoplankton phyla and transects.

$$\begin{aligned} {\text{DA1}} &= \, - 0. 2 5 2\;{\text{chlorophyta }} + \, 0. 4 9 5\;{\text{cyanophyta }} \hfill \\ &\quad + \, 0. 8 4 6\;{\text{pyrrophyta }} + \, 0. 2 9 5\;{\text{euglenophyta }} + \, 0. 3 6 3\;{\text{diatoms}} \hfill \\ {\text{DA2}} &= \, 0. 7 5 5\;{\text{chlorophyta }} + \, 0. 1 6 8\;{\text{cyanophyta }} \hfill \\ &\quad + \, 0. 1 7 6\;{\text{pyrrophyta }} - { 1}. 2 7 9\;{\text{euglenophyta }} + \, 0. 2 80\;{\text{diatoms}} \hfill \\ {\text{DA3}} &= \, 0.0 8 2\;{\text{chlorophyta }} + \, 0. 9 9 8\;{\text{cyanophyta }} \hfill \\ &\quad - \, 0. 4 6 1\;{\text{pyrrophyta }} - \, 0. 3 9 2\;{\text{euglenophyta }} + \, 0. 3 2 2\;{\text{diatoms}} \hfill \\ \end{aligned}$$

The following equations obtained from DFA indicate the relationship between phytoplankton phyla and depths.

$$\begin{aligned} {\text{DA1}} &= { 1}.0 9 2\;{\text{chlorophyta }} + \, 0. 20 3\;{\text{cyanophyta }} \hfill \\ &\quad + \, 0. 1 5 7\;{\text{pyrrophyta }} - \, 0. 5 70\;{\text{euglenophyta }} + \, 0. 4 5 9\;{\text{diatoms}} \hfill \\ {\text{DA2}} &= \, - 0. 8 7 1\;{\text{chlorophyta }} + \, 0. 8 1 7\;{\text{cyanophyta }} \hfill \\ &\quad + \, 0. 50 2\;{\text{pyrrophyta }} + \, 0. 60 4\;{\text{euglenophyta }} + \, 0. 2 3 8\;{\text{diatoms}} .\hfill \\ \end{aligned}$$

The overall total average of cell abundance was approximately 43.55 ± 5.10 × 104 cells/m3. Actually, annual average dominance of cell abundance was discovered in diatoms with 14.99 ± 16.97 × 104 cells/m3 (Fig. 6). Because of high abundance of diatoms, the highest average abundance was detected in winter related to Thalassionema nitzschiodes at a depth of 10 m. The lowest and the highest cell abundance of diatoms were reported in summer and winter (5970 cells/m3 and 359,370 cells/m3, respectively) (Fig. 7) and their cell abundance showed significant differences at various seasons (P < 0.05).

Fig. 6
figure 6

The annual distribution changes in the average cell abundance (cells/m3) of phytoplankton phyla

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Fig. 7
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Comparison between densities of different phyla of phytoplankton in different seasons

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Of Dinoflagellata, the most and least cell abundance was in spring (144,720 cells/m3) and autumn (44,790 cells/m3) with significant differences among seasons (P < 0.05) and the highest cell abundance was of Exuviaella cordata at a depth of 10 m (Fig. 7). By contrast, Cyanobacteria had their highest cell abundance in spring (25,792.5 cells/m3) and their lowest was displayed in summer (187,890 cells/m3). and highest cell abundance related to Oscillatoria limosa at a depth of 10 m. There were significant differences in cell abundance of this group among various seasons (P < 0.05) (Fig. 7). In spring and summer, the lowest (14,490 cells/m3) and the most cell abundance (107,610 cells/m3) of green algae were observed, respectively. And this index exhibited a significant difference between summer with other seasons (P < 0.05) whereas no significant differences were displayed among other seasons (P > 0.05) (Fig. 7). Euglenoids had less abundance than other phytoplankton groups and showed their lowest and the most abundance in autumn and summer (9570 cells/m3 and 86,460 cells/m3, respectively). Euglenoids abundance in summer revealed a significant difference with other seasons (P < 0.05), while there were no significant differences among other seasons (P > 0.05) (Fig. 7).

At the end, the highest cell abundance was observed in the spring, summer, autumn and winter seasons that belonged to species Exuviaella cordata, Oscillatoria limosa, Thalassionema nitzschiodes and Thalassionema nitzschiodes at a depth of 10 m, respectively.

The maximum and minimum average abundance of phytoplankton was observed in winter and spring respectively (Fig. 8).

Fig. 8
figure 8

The total average of cell abundance (cells/m3) of phytoplankton in different seasons

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Discussion

The main phytoplankton taxa in Caspian Sea are Bacillariophyta, Pyrrophyta and cyanophyta (Ghasemov and Bagherov 1983; Salmanov 1987; Ganjian et al. 2009; Ganjian 2007). In this study, Diatoms were the main group of phytoplankton, with the highest diversity of 29 species and 40% of the total taxa. The number of identified species was 46 in spring, 50 in summer, 41 in autumn, and 39 in winter. The highest and lowest diversity was observed among phytoplankton groups in summer and autumn, respectively, which were similar to the finding in 2005–2006 recorded by Ganjian et al. 2010. They identified 101 species in the south of Caspian sea that the main phytoplankton group was Bacillariophyta (diatoms) with more than 71 species and 43% of the total taxa. The highest number of phytoplankton species was recorded in summer with 101 species in the southern part of the Caspian Sea. Temperature seems to be an important seasonal factor that controls the biological processes of phytoplankton communities (Vereshchaka and Anokhina 2014). Bat et al. (2011) reported Dinophyceae and Bacillariophyceae are the most dominant families for the number of species (respectively, 173 and 89 species) in the Black Sea. Also, Ozgur et al. (2010) published that a total of 129 taxa were recorded from neritic waters in Samsun Bay, including 76 Bacillariophyta, 2 Cyanobacteria, 1 Chlorophyta, 1 Euglenophyta and 48 other Phyla. Diatoms were dominant among phytoplankton groups except in November 2002 and June 2003.

Formerly diatoms were the most abundant in the Caspian Sea (Kosarev and Yablonskaya 1994). After the diatoms, chlorophytes and cyanophytes were the most abundant groups in the north of the Caspian Sea, while dinoflagellates dominated in the middle and southern parts of the Caspian Sea (Kosarev and Yablonskaya 1994; Ganjian 2007).

Pyrrophyta has the highest cell abundance in spring, which may be the environmental conditions during the spring lead to the growth. In the summer, cyanophyta had the highest density, which could be attributed to the increase in water temperature as seen in the Blanes Bay (Hense and Beckmann 2006), and the highest cell abundant of diatoms in the winter and autumn. Different researchers have reported that temperature was an important factor in fluctuations in the composition of phytoplankton and changes phytoplankton population with changing season and temperature differences, and diatoms were dominant species of the cold season in the Caspian Sea (Kideys et al. 2005; Resende et al. 2007; Bagheri et al. 2012). Compared to the previous findings, an increasing trend was observed in phytoplankton cell density. For example, the annual average density of phytoplankton was reported by Kosarev and Yablonskaya (1994) (1.4 × 104 cell L−1), Kideys et al. (2005) (4.0 × 104 cell L−1), and Bagheri et al. (2010) (2.10 × 105–3.90 × 105 cell L−1) with the highest abundance of Pyrrophyta (51–56%) and Cyanophyta (22–29%). Dominant phytoplankton species were Prorocentrum cordatum of Pyrrophyta and Oscillatoria sp. of Cyanobacteria, and the present study (43.55 × 104 cell/m3) showed that there is a large variation in the density of phytoplankton in the Caspian Sea.

Also in the same sea, Ozgur et al. 2010 showed that the annual average density of phytoplankton in the south Black Sea Samsun Bay was less than 0.01 × 106 in February 2003 and reached maximum value of 1.20 × 106 cells L−1 in July.

This survey showed that diatoms such as Thalassionema nitzschiodes and pyrrophyte Exuviaella cordata and cyanophyte Oscillatoria limosa were prevalent in Phytoplankton populations at the site.

Bagheri et al. (2014) showed that diatom communities were represented by a few dominant species such as D. fragilissimus and T. nitzschioides in the system.

In a previous research by Nasrollahzadeh Saravi et al. (2014) identified phytoplankton species, Exuviaella cordata, Oscillatoria sp., Pseudonitzschia seriata, Thalassionema nitzschioides, Cerataulina pelagica and Chrysochromulina sp. indicated the most abundance. Cyanophyta are living organisms from the natura ecosystem. Pointed out that warm (more than 20 ºC), calm and stratified water in summer are desirable for the growth of Oscillatoria sp. (Chorus and Bartram 1999), Oscillatoria sp. is one of dwelling species of the Caspian Sea. However, this high abundance of the species in 2009 was introduced in the category of “harmful and potential bloom species “in the region.

In this study, dominant species had the highest density at a depth of 10 m. Also, the highest cell abundance of phytoplankton was observed in Bandar coasts transect. In this transect, due to the presence of breakwater, some species have come from the Anzali Wetland toward the sea, which increased the density of the phytoplankton compared with other transects.

In the Caspian Sea, various factors such as hydrological regimes, climate anomalies, increasing detergents, increasing the flow of fresh water through rivers, and increasing the level of nutrients due to the discharge of industrial and agricultural waste can affect the diversity and density of phytoplankton in coastal water of Caspian Sea (Kideys et al. 2008; Bilio and Niermann 2004).

Bagheri et al. (2014) theorized that hydro-biological changes in the Caspian Sea, Black Sea, and Baltic Sea during the 1990s and 2000s could be definitely correlated with climatical and hydrological characters.

Conclusion

There was no evidence of bloom in Bandar-e Anzali in southwestern of Caspian Sea. Although, species were observed that have a potential for blooming on this site, which should be controlled before reaching the bloom. As mentioned, increasing the detergents, fertilizers and nutrients resulting from industrial and agricultural sewages to the wetlands and coastal water of Caspian Sea and increase in deforestation are important elements for nutrient sources, and increase in these materials is an important factor in increasing the production of phytoplankton. The two top points were observed in the summer and winter. The top points were formed with O. limosa and T. nitzschiodes species from the phyla of Cyanophyta (29%) and diatom (34%), respectively. Opportunistic reproduction, toxin production and harmful characteristics of dominant species in the two seasons are the evidences of the under stress features of the Caspian Sea, which should be considered for environmental solutions.