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Article

Plankton Community Structure and Biomass in the Eastern Middle Caspian Sea

by
Tatyana Fedorovna Kurochkina
1,
Botagoz Murasovna Nasibulina
1,
Shima Bakhshalizadeh
2,
Nikolai Popov
3,
Gulnur Kuanysheva
4,
Francesco Fazio
5,* and
Attaala Muhaysin Ali
6
1
Department of Ecology, Nature Management, Land Management and Life Safety, Faculty of Geology & Geography, Innovative Natural Institute, Astrakhan State University, P.O. Box 414000 Astrakhan, Russia
2
Department of Marine Science, Caspian Sea Basin Research Center, University of Guilan, Rasht 4183753689, Iran
3
Kazakh Research Institute of Fisheries, Atyrau 060001, Kazakhstan
4
Department of Ecology, Safi Utebayev Atyrau Oil and Gas University, Musa Baymukhanov Str. 45A, Atyrau 060027, Kazakhstan
5
Department of Veterinary Sciences, University of Messina, Viale Palatucci, 98168 Messina, Italy
6
Department of Marine Biology, Hadhramout University, Mukalla 50512, Yemen
*
Author to whom correspondence should be addressed.
Water 2023, 15(1), 138; https://doi.org/10.3390/w15010138
Submission received: 6 November 2022 / Revised: 16 December 2022 / Accepted: 27 December 2022 / Published: 30 December 2022
(This article belongs to the Section Biodiversity and Functionality of Aquatic Ecosystems)
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Abstract

:
Despite the role of planktonic organisms in the fishery value of the Caspian Sea and in addition to the increase in the plankton population in the Caspian Sea, there are few detailed studies regarding the determination of the planktonic organisms in the Caspian Sea. In the present investigation, we aimed to study the cell abundance and diversity of the phyto- and zooplankton in the eastern part of the Middle Caspian Sea in the spring of 2016. The composition of phytoplankton consisted of 50 species belonging to 4 systematic groups which were registered in the eastern part of the Middle Caspian: Cyanophyta (7), Bacillariophyta (24), Dinoflagellata (13), and Chlorophyta (6). Zooplankton were represented by 9 taxa: Cladocera—6, Copepod—1, and other groups—2. The most important phytoplankton species were Rhizosolenia calcar-avis, Anabaena bergii, Exuviaella cordata, and Binuclearia lauterbornii. The 0–50 m layer in the eastern Middle Caspian was the most productive, where the biomass varied from 98 mg/m3 to 109 mg/m3, consisting largely of diatoms. The cladoceran Evadne anonyx typica, the copepod Acartia tonsa, and the larvae of barnacle crustaceans (Cirripedia) were widely distributed in this layer. Zooplankton abundance decreased markedly with the depth. The maximum zooplankton concentrations were confined to depths down to 200 m.

1. Introduction

The Caspian Sea is a special type of ecosystem that is extremely vulnerable to external factors and is regarded as the largest closed lake with brackish water on Earth [1]. The biodiversity of the Caspian Sea flora and fauna is derived from its long geological history and isolation, allowing sufficient time for speciation. There are more than 400 endemic aquatic taxa in the Caspian Sea, including 124 species of molluscs, and 140 taxa of crustaceans; of the molluscs, about 114 are endemic [1,2]. There are 159 species of fish, many of which are rare and migrate from the Caspian Sea to the rivers to spawn, and 99 of them are endemic [3]. The Caspian Sea is noted for a great number of endemic species of fauna. It is considered an independent zoogeographical region due to its diversity, specificity and the endemism of its fauna [2]. Most of the native Caspian fish belong to the herring (Clupeidae) and bullhead (Acipenseridae) families, of which seven species and subspecies of sturgeon are of commercial importance. The Caspian is the only area with a significant sturgeon fishery, which represents up to 90% of the world catch. Among Caspian molluscs, 119 endemic and subendemic species belong to two families of bivalves (Dreissenidae and Lymnocarduidae) and 7 families of Gastropods [2]. High light intensity and solar energy potential typical for the arid Caspian Sea climate provide optimal physico-chemical conditions to stimulate the productivity of phytoplankton, zooplankton, and zoobenthos. However, currently, the Caspian, and especially the eastern region of the Middle Caspian, is experiencing a high anthropogenic impact, leading to the loss of certain species and a significant expansion of many others [1,3,4,5,6]. Plankton, as the base of the food chain, are considered the most important main element in aquatic environments and also useful bio indicators of changing environmental conditions [6]. On the other side, invasive introduced species by ballast water as the result of the artificial connecting of the Caspian Sea with the Azov–Black Sea basin by the Volga–Don Canal could have ecologically negative effects on these closed ecosystems [1,3]. Moreover, phytoplankton are an important water quality indicator because of their sensitivity to environmental changes and short life span. Phytoplankton are also a useful indicator of high nutrient concentrations in water because of their propensity to proliferate rapidly. Under the adequate conditions that are species-specific, phytoplankton can undergo rapid population growth to form blooms [4]. Between 1962 and 1974, the total number of phytoplankton species in the Caspian Sea was reported to be 449 [5]. The species distribution decreased from the north (414 species) to the middle (225) and to the southern region (71) mainly due to the loss of fresh-water taxa towards the south [6]. Recently, Kideys et al. (2005) reported a significant increase in phytoplankton abundance, particularly in harmful species [7]. Moreover, Nasrollahzadeh et al. (2008) and Roohi (2009) observed two- to four-fold increases in phytoplankton abundance in 2003–2004 and 2005 compared to previous years [8,9]. The fauna and flora in the Caspian Sea are largely native to this expansive water body, and they are therefore susceptible to external influences [6].
Therefore, qualitative and identification studies of plankton are very crucial for monitoring changes. The results of the present survey will be the base of the next research study to monitor and predict changes under environmental and biological conditions which could affect the plankton diversity in the biggest closed lake in the world. This study was carried out to estimate the diversity and abundance of plankton in the insufficiently explored eastern part of the Middle Caspian Sea, with the aim of further determining its productivity and environmental and economic importance.

2. Study Area Description

The hydrochemical regime of the Caspian Sea is formed under the influence of eutrophication processes, regulation of river runoff, and the transformation of biogenic runoff in river mouths. As a result, the increased intake of mineral forms of nitrogen and phosphorus from the Volga and Ural drains (2.1 and 3.4 times more, respectively, than in natural conditions), the processes of production, and the destruction of organic matter are actively developing, and zones of deep hypoxia are forming in the Northern and Middle Caspian, changes in the trophic structure of biocenoses and an increase in bacterial production [7]. The eastern (Kazakh) part of the Middle Caspian Sea, the study area, is located within the rainless region—the Mangyshlak Peninsula—and has a long shelf. Its productive properties are determined in summer due to the rise of deep waters (upwelling) and in winter due to the compensatory inflow of warmer waters from the Southern Caspian [8,9]. The features of the sea bottom relief in this area make it different from other parts of the sea with respect to water circulation, salinity, transparency, and other hydrological characteristics. This eastern coastal region of the Caspian Sea lacks sources of fresh river water because there are no rivers flowing into it, and the nearest sources of fresh water are the Ural and Volga rivers, which flow into the northern part of the sea, making it brackish [3–8‰] this salinization extends to increase the concentration in the central part of the sea to make it more brackish [12–14‰]. In the north, the sea is almost fresh, and in the direction of the southeast, the sea brine becomes more concentrated. Seasonal changes in salinity in the middle part of the sea, from season to season varies by no more than 0.2‰. In summer, the salinity is less than in winter. In May 2016, the air temperature in the area varied from 19.1 to 25.2 °C. The average temperature was 21.4 °C. In September, the weather was stable; the morning temperature varied from 18.3 to 21.7 °C, and the average morning temperature was 20.1 °C. The water temperature of the surface water layer (0–5 m) fluctuated in May between 15.3 and 19.7 and in September between 18.4 and 22.3. At the deepest station (400 m), the water temperature fluctuated between 4.6 near the bottom and 24–26.8 at the surface in both months. The pH values for the study stations were mostly homogeneous and varied within insignificant limits. On average, in the study area, the pH values in the spring period were 8.2 and in the autumn 7.9. The pH values in all observation periods were stable and were practically at the same level.

3. Materials and Methods

Phytoplankton sampling was conducted mainly in 2013–2016 and was carried out in the area of the eastern Middle Caspian Sea by the Safi Utebayev cruises of Atyrau Oil and Gas University at depths of down to 400 m (Figure 1). The samples were collected using a 1.6 L Ruttner sampler at six sampling stations (10–20, 20–50, 50–100, 100–200, 200–300, 300–400 m) [10] and then the samples were kept in a 5-L plastic bottle, one bottle for each depth with three replicates within 2–3 h. The subsamples were fixed with stock 37% formalin to a final concentration of 4%. The temperature, salinity, pH, and oxygen of the water column at the stations were measured using a after preliminary CTD probing using the Seabird 19 plus profiler (Sea-Bird Scientific company). After delivery to the land laboratory, each sample was divided into two plastic jars of 250 mL each. The contents of one jar went for further processing of quantitative and qualitative phytoplankton, and the other went for other research. The first jar was kept at 4 °C and shaded, and the samples were allowed to sediment for 10 days [11,12]. Samples were processed by standard methods. Phytoplankton samples were identified and counted by sedimentation in a separable (sedimentation cylinder) plankton chamber with the inverted microscope [13]. Zooplankton was identified in the laboratory with the 6 mL Bogarov chamber according to the standard method [13,14]. The samples were concentrated by draining the middle layer of water from the container (95% of water was removed within 5 days). The volume of the sample was adjusted to 50 mL. Then the water above the sediment was sucked off to a volume of 5 cm3. Then, two secondary samples were taken from them, each with a quantity of 1 mL, each of these was scattered on the microscopic plankton counting cell. Cell counting was carried out in a Nageotte chamber of 0.001 mL. It was examined and counted under magnification of 200–400 times, considering all cellular squares with extreme accuracy. In some cases, a Petroff–Hausser counting chamber was also used. To examine and diagnose taxonomic groups, a Zeiss microscope (Axioskop, 2plus, Carl Zeiss Microscopy, Jena, Germany ) was used. Phytoplankton abundance is expressed as the number of cells per million in 1 m3 of water. The cell biomass was measured by the volumetric technique. The biovolume was calculated based on the cell shape of a particular species being equated with the geometrical figure closest to its shape [15,16].
The identification of microalgae was conducted by an inverted microscope (Nikon Eclipse TS 100) according to the published regional literature [17,18,19,20]. To determine the structure of the community, the Shannon–Weaver index was used [21,22]. Zooplankton was sampled with a Juday net with sieve No. 70 (50 cm in diameter and 70 µm mesh size) by filtration of the water column from the lower targeted level to its top. Samples were fixed with stock formalin to a final concentration of 10%. For counting, zooplankton subsamples were analyzed in a 6 mL Bogorov counting chamber. In general, the sampling strategy of Newell and Newell (1977) for the examination of planktonic population was followed [18]. The individual biomass of zooplankters was determined by the equations of linear-weight dependence on the basis of individual measurements with a large volume of samples, and only 10 specimens of each size group were measured and weighed; with a small volume, all specimens were measured. The samples were identified under Stereomicroscope Altami CM0745 with eyepieces SWF30X/9, magnification 7X-135X, Saint-Petersburg, Russia. The number and weight of zooplankters was calculated as individuals/m3 (ind./m3) and mg/m3, respectively. The degree of complexity of the communities was established using the Shannon–Weaver index [23,24]. Zooplankton taxa were identified using the published literature [25,26,27,28].

4. Results

In May 2016, 50 microalgal species, varieties and forms belonging to 4 taxonomic groups were found in the eastern part of the Middle Caspian: Cyanobacteria (7), Bacillariophyta (24), Dinoflagellata (13), and Chlorophyta (6) (Table 1). In the Middle Caspian region, the number of species per station varied from 7 to 24. The most abundant species were Rhizosolenia calcar-avis, Anabaena bergii, Exuviaella cordata, and Binuclearia lauterbornii from green algae.
In relation to salinity, five ecological groups were noted (Figure 1). Marine taxa were represented by 19 species, of which 11 were diatoms and 8 dinoflagellates. In brackish water, only 3 diatom and 3 dinoflagellate species were observed.
The brackish–freshwater community included 10 species: 3 species of cyanobacteria, 4 diatoms, 2 dinoflagellates, and 1 chlorophyte species. In the freshwater community, 13 species were distinguished (3 cyanobacteria, 5 diatoms, 5 chlorophytes). The ecological group “others” included two species with unknown affinity to salinity. Phytoplankton species of all five ecological groups developed in the coastal zone, with the members of the brackish and marine communities being most active in deeper water with higher salinity. The mean phytoplankton abundance and biomass were 4.5 × 106 cells/m3 and 80.7 mg/m3, respectively (Table 2 and Table 3). Diatoms dominated in terms of both abundance (up to 47%) and biomass (up to 91%).

5. Discussion

The results of our study showed that in the eastern Middle Caspian, the biomass varied from 98 mg/m3 to 109 mg/m3 in the 0–50 m layer, largely due to diatoms (Table 2), which is in accordance with Tatarintseva and Terletskaya [28]; however, there was less biomass in deeper layers (Table 3) [29]. In the water column, the values of the Shannon–Weaver diversity index varied from 1.60 to 1.78 and from 0.77 to 1.21 in biomass, so the level of diversity in phytoplankton community was low (Table 4).
Twenty phytoplankton species with a known saprobic valency were found. The main ones were Anabaena spiroides var. contracta, Gomphosphaeria aponina var. multiplex, Melosira granulata, Rhizosolenia calcar-avis, R. fragillissima, Thalassionema nitzschioides, Thalassiosira caspica, Nitzschia distans, and Exuviaella cordata. The values of the saprobity index varied from 1.47 to 1.69 (Table 4). Its maximum value did not exceed the limits established for the β-mesosaprobic, moderately polluted zone.

5.1. Phytoplankton Distribution with Depth

In the studied area of the Middle Caspian, the number of phytoplankton species per sample varied from 7 to 9. The highest species richness was found at depths of 50–100 m (Figure 2). Diatom species diversity was provided mainly by the genera Coscinodiscus, Navicula, and Nitzschia.
Phytoplankton abundance varied from 2.6 × 106 to 5.7 × 106 cells/m3 and, and the biomass from 62.4 to 104.2 mg/m3. The highest abundance of phytoplankton was observed at depths of 20–50 m, with a maximum biomass at depths of 10–20 m and belonging to four taxonomic groups (Figure 3). Of these, the most abundant (in 2016) were Anabaena bergii, Rhizosolenia calcar-avis, Exuviaella cordata, and Binuclearia lauterbornii. In the uppermost 50 m layer, R. calcar-avis was the most dominant; however, its abundance decreased with increasing depth. At the uppermost 50 m layer depths, the brackish–freshwater chlorophyte Binuclearia lauterbornii was the most numerous. Anabaena bergii did not contribute significantly to the algal population, while Exuviaella cordata was of moderate importance (Figure 3). The most dominant species was Rhizosolenia calcar-avis, which was found at the highest concentrations across the depth profile, and other remaining species of phytoplankton had a minor portion of the total biomass. The Shannon–Weaver index varied from 1.02 to 1.71 along the water column. The maximum value was noted in the layer of 50–100 m, with the values of the index gradually decreasing with depth. The Shannon–Weaver index varied little with the number of species. The maximum values for the saprobity index (1.69) were calculated for the stations with site depths of 50–100 m; the minimum value (1.47) was found at stations with site depths of 20–50 m. Thus, in the spring (May) of 2016, phytoplankton on the eastern section of the Middle Caspian consisted of 50 species. The most abundant diatoms were Rhizosolenia calc-aravis, the dinoflagellate Exuviaella cordata, and the chlorophyte Binuclearia lauterbornii. Phytoplankton biomass varied from 4.5–80.7 mg/m3. The maximum number of species recorded in May 2013 was 50 taxa. In 2013–2016, the composition of phytoplankton varied by year and season from 15–74 species (Table 5). The most diverse community was represented in spring (50–74 species), mainly due to differences in diatom populations. However, this spring bloom was not apparent in the Middle Caspian Sea in 2016. The highest phytoplankton biomass was reported in the spring of 2014. It consisted mainly of large-celled diatoms. In May 2016, phytoplankton biomass was similar to that in 2013 but did not reach the highest values calculated for 2014. In earlier studies, the biomass of phytoplankton in this part of the sea in the spring of 2003 varied over a wide range from 40.5 to 535.0 mg/m3 [29].
The Shannon–Weaver diversity index values varied from 1.09 to 1.98 in abundance and from 0.57 to 1.98 in biomass. The highest values of the indexes for abundance and biomass were reported in the autumn of 2015; the minimum values were reported in the spring of 2014, which coincided with the lowest number of species noted during this period of research. In 1962–1974, in the entire Caspian Sea, 449 phytoplankton species were found [9]. They included 163 diatoms, 139 chlorophytes, 102 cyanobacteria, 39 dinoflagellates, 5 euglenophytes, and a chrysophycean. The total number of phytoplankton species decreased from the north (414) to the middle (225) and the southern region (71), mainly due to the loss of freshwater species towards the southern end [10]. Recently, a significant increase was reported in the Caspian Sea for phytoplankton abundance as well as an invasion of the jellyfish Mnemiopsis leidy [30,31]. Moreover, there was a reported increase in phytoplankton abundance in 2001–2002 and 2005 as compared to previous years [32,33].

5.2. Zooplankton

In May 2016, the zooplankton of the eastern section of the Middle Caspian consisted of 9 taxa, of which Cladocera—6, copepods—1, and optional plankters—2 were most prevalent (Table 6). The number of species included in each community varied from 2 to 6. The crustaceans Evadne anonyx typica, copepods Acartia tonsa, the larvae of barnacle crustaceans, and Cirripedia were widely distributed, with Cirripedia prevalent in all areas. Zooplankton were distributed homogeneously. The cladoceran crustacean Podon polyphemoides, which are a common component of zooplankton, were rarely found in single samples, which may be due to differences in lower water temperature. Obviously, the low frequency of occurrence of other common Caspian organisms is also associated with the temperature factor. Quantitative indicators of zooplankton organisms showed that there was an average of 18,103 ind./m3 and 392 mg/m3 (Table 7 and Table 8). They were dominated by copepods, which accounted for 56% to 97%, while Cladocera comprised up to 40%. Cladocerans formed 97.7% of the biomass with the most common being Acartia tonsa (up to 97%) within Copepods and Evadne anonyx (up to 20%) within Cladocera. Of lower prevalence were the barnacles—13% of the total biomass. The facultative plankter, Cirripedia, consisting mostly of the larvae of barnacle crustaceans, formed 13% of the biomass. With a limited number of species entering the community and a marked dominance of only a few of them, the diversity of zooplankton was low (Table 9). Average Shannon–Weaver index values were 0.47 bit/ind. and 0.42 bit/mg. Among the diversity of zooplankton (9 taxa) identified, saprobic valences are not known for any species, which makes it impossible to assess the level of organic pollution of the marine water column using the Pantle–Bucca method.

5.3. Distribution of Zooplankton with Depth

The eastern section of the Middle Caspian is characterized by steep increases in depth, from 13.8 to 377.0 m. The number of species in the zooplankton community varied insignificantly, from 2 to 6 species. The greater diversity detected at depths of 100–300 m was due to the prevalence of Evadne anonyx varieties and random components—shell crustaceans (Ostracoda) and the larvae of barnacle crustaceans. The less prevalent species were equally distributed throughout the water area, regardless of depth (Table 6).
The relatively low abundance of zooplankton varied widely—from 52 to 22,738 ind./m3, with fluctuations from 0.5 to 621 mg/m3. The quantitative indices of planktonic invertebrates tended to decrease as the depth increased over 200 m (Figure 4, Figure 5 and Figure 6). Maximum concentrations of zooplankton were confined to areas with depths up to 200 m. The total number of plankton invertebrates reached 20,866–29,625 ind./m3 in this zone, with biomass of 488–704 mg/m3. In the deep-sea zone, the values of the indices decreased to 3165 ind./m3 and 69 mg/m3 (Table 10).
The spatial dynamics of the abundance and biomass of the zooplankton community was determined by a change in the abundance of Cladocera and copepod crustaceans Evadne anonyx, Acartia tonsa, and the larvae of barnacle crustaceans Cirripedia. The quantitative indices of the populations of these species, as well as of the entire community, decreased several times with the increase in depth (Figure 7).
The population of Evadne anonyx was of median density at depths of 10 to 300 m. The lowest population was found in the shallowest and deepest zones (less than 50 m and more than 300 m) with an average of 43 ind./m3. With depth changes, the most noticeable decrease was the population of the dominant species, Acartia tonsa. At 10–20 m, its abundance averaged 13,737 ind./m3, which increased to 22,014 ind./m3 at 50–100 m. This number decreased further to 2301 ind./m3 at 300 m. As for other species included in the composition of zooplankton, the minimum number of A. tonsa was found in the deepest zone (300–400 m) with an average of 2301 ind./m3. The average number of larvae of the barnacle crustaceans Cirripedia in areas 10–100 m deep did not change significantly, from 544 to 2273 ind./m3, and then changed sharply to decrease to 242 ind./m3 at depths of 200–300 m and up to 52 ind./m3 at depths of 300–400 m.
The diversity of zooplankton, estimated by the Shannon–Weaver index, varied from 0.28 to 0.77 bps in the water area and from 0.26 to 0.76 bps (as density in mg). To a depth of 50 m, the values of the diversity index were close to average values, and then they increased sharply between 100–300 m and continued to increase slightly to reach maximum values. With an almost unchanged composition and number of species, the spatial dynamics of the values of the Shannon–Weaver index were due to a change in the proportion of background species in the community. In May 2016, nine taxa were identified as part of the zooplankton of the eastern section of the Middle Caspian. A comparison with the data of previous years showed that such a low diversity of zooplankton is characteristic of the surveyed locations. In some years and seasons, 8 to 13 zooplankton taxa were present in the zooplankton community (Table 6 and Table 10). The most diverse community of zooplankton organisms is represented in the summer period with 13–19 taxa represented, mainly due to the thermophilic species of branchy crustaceans and rotifers.
Differences in composition of zooplankton across the Caspian were significant, so, while the number of species found in the eastern Middle Caspian is only 9 (present study), the number of species encountered in the Dagestan waters of the Caspian Sea was 112 in 2006, and in the southern Caspian Sea (Iran waters), the number was 45–51 species [34,35,36].
Among various studies of plankton in different areas of the Caspian Sea, the study of Abdurakhmanov et al. (2010) showed facts that cannot be ignored [33]. It became clear that the number of species registered in the current study (9) is a real phenomenon as they are identical to the results of Guseinova and Abdurakhmanov (2009) where, for example, in August 2001, only 7 species and forms of zooplankton were found [37,38,39]. Of these, 55% were larvae or juveniles of plankton/benthic organisms. Many rotifers were completely absent from the plankton. Of the copepods, only Limnocalanus grimaldii and the Azov–Black Sea invader of the 1980s were noted. The native species, Acartia clause, apparently disappeared from the plankton. The reasons for this, in the opinion of many experts, are the presence of the out-climatizer pelagic predator mackerel Mnemiopsis leidyi and the mass consumption of zooplankton in addition to the intrinsic characteristic of the Caspian Seas level changes and the impact of natural abiotic factors on the sea ecosystem [37].
It is useful to add that the analysis of the species diversity of the Middle and Southern Caspian has shown over the period 2006–2010 (Tarasova and Nikulina, 2011) the number of dominant groups of organisms is decreasing [39].
In the Middle Caspian, after the introduction of the ctenophore (Mnemiopsis leidyi) in 2000, the species diversity of zooplankton narrowed from 27 to 8 species, and in the Southern Caspian from 25 to 9 species. The most common species have been removed by Mnemiopsis, which saw their numbers reduced from 11 to 1 species in the Middle and Southern Caspian. Among the copepods of the zoocenoses that have become extinct, the main food of the sprat, Eurytemora grimmi and E. minor, are no longer found in these parts of the sea [39]. In recent years, the species diversity of zooplankton has expanded somewhat. However, only one type of Acartia tonsa dominates its total biomass. At the same time, the biomass of zooplankton remains low. The latter is not only associated with the presence of M. leidyi, which is well known to feed actively on zooplankton, but it also influences phytoplankton development. Compared with the period of the 1990s, after 2001 the total biomass of phytoplankton decreased in the Middle Caspian by a factor of 10–20. As for the feed source for zooplankton, the biomass of E. cordata was reduced from 43 mg/m3 in 1991 to 1 mg/m3 in 2006, while it was not found in 2007. The degradation of phytoplankton is associated with a sharp decrease in the concentration of mineral nitrogen in the waters of these parts of the sea.

5.4. Conclusions

The study revealed that the Phytoplankton in the eastern section of the Middle Caspian was represented by 50 species in the spring of 2016. The main representatives of phytoplankton were Rhizosolenia calcar-avis from diatoms, Exuviaella cordata from the pyrophytic, and the green algae Binuclearia lauterbornii. Quantitative indicators of phytoplankton amounted to 4.5 million cells/m3 and 80.7 mg/m3. The predominant form of phytoplankton was diatom algae. In the study area, cladocerans zooplankton were widespread and dominated by Evadne anonyx typica, copepods by Acartia tonsa, and the larvae of the barnacle crustaceans Cirripedia. Quantitative indicators of plankton organisms were at an average level, with a mean of 18,103 ind./m3 and 392 mg/m3. The basis of biomass was formed by copepods represented by a single species, Ostracoda. In relation to qualitative differences in recent years, the number of phytoplankton species in the eastern part of the Middle Caspian (50) has been similar to that in the western Middle Caspian (Dagestan coast) and in the south Iranian waters, with 58 and 39 species found, respectively [37,38]. In all these studies, the basis for taxonomic diversity is related to diatoms within phytoplankton and the Copepods within zooplankton.
The average biomass of the phytoplankton in the Kizlyar Gulf was 10.4 g/m3, and the abundance was 1653.1 cells/m3 [37].
It is noticeable here that by comparing our results with previous reports, we have noted that the phytoplankton are characterized by a zonation of distribution, whereas it was found that the qualitative composition in the west and south of the sea included the six known categories of Diatoms, Chlorophytes, Cyanophytes, Dinoflagellates, Euglenophytes, and Chrysophyte [5,37,38]. While in the eastern part of the middle region, it was limited to the existence of the first four categories, where no species of Euglenophytes and Chrysophyte (golden algae) were found (present study). We think this is due to the scarcity of fresh water and the specificity of the physicochemical indicators [39].

Author Contributions

Author contributions: B.M.N., N.P. and G.K. contributed to the design and conception of the study. Data sampling was performed by T.F.K. and B.M.N. Laboratory and analyses were carried out by A.M.A., F.F. and S.B. The first draft of the document was written by A.M.A. and B.M.N. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

The data that support findings of this study are available from corresponding author.

Acknowledgments

This study was supported by Astrakhan State University and University of Guilan, Rasht, Iran, Kazakh Research Institute of Fisheries, Atyrau, Kazakhstan; and Safi Utebayev Atyrau Oil and Gas University, Atyrau, Kazakhstan.

Conflicts of Interest

The authors declare no conflict of interest.

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Water 15 00138 g001 550
Figure 1. Map of the Caspian Sea showing the study area in the eastern part of the Middle Caspian Sea (Water 15 00138 i001 Study stations).
Figure 1. Map of the Caspian Sea showing the study area in the eastern part of the Middle Caspian Sea (Water 15 00138 i001 Study stations).
Water 15 00138 g001
Water 15 00138 g002 550
Figure 2. Distribution of phytoplankton in the eastern part of the Middle Caspian by ecological groups. The number of phytoplankton species per sample varied from 7 to 9. The highest species richness was found at depths from 50 to 100 m in Marin water with salinity 11.8–12.4‰ at depths 50–100 at a distance 90 km from the shore.
Figure 2. Distribution of phytoplankton in the eastern part of the Middle Caspian by ecological groups. The number of phytoplankton species per sample varied from 7 to 9. The highest species richness was found at depths from 50 to 100 m in Marin water with salinity 11.8–12.4‰ at depths 50–100 at a distance 90 km from the shore.
Water 15 00138 g002
Water 15 00138 g003 550
Figure 3. Change in the number (A) and biomass (B) of background phytoplankton species depending on the depth of the eastern section of the Middle Caspian, 2014.
Figure 3. Change in the number (A) and biomass (B) of background phytoplankton species depending on the depth of the eastern section of the Middle Caspian, 2014.
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Water 15 00138 g004 550
Figure 4. Dynamics of the number of species of zooplankton depending on the depth of the eastern section of the Middle Caspian, May 2016.
Figure 4. Dynamics of the number of species of zooplankton depending on the depth of the eastern section of the Middle Caspian, May 2016.
Water 15 00138 g004
Water 15 00138 g005 550
Figure 5. Changes in the number of zooplankton taxonomic groups with depth of the eastern section of the Middle Caspian, May 2016.
Figure 5. Changes in the number of zooplankton taxonomic groups with depth of the eastern section of the Middle Caspian, May 2016.
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Water 15 00138 g006 550
Figure 6. Change in the biomass of taxonomic groups of zooplankton with depth of the eastern section of the Middle Caspian, May 2016.
Figure 6. Change in the biomass of taxonomic groups of zooplankton with depth of the eastern section of the Middle Caspian, May 2016.
Water 15 00138 g006
Water 15 00138 g007 550
Figure 7. Change in the number (A) and biomass (B) of background zooplankton species with depth of the eastern section of the Middle Caspian, May 2016.
Figure 7. Change in the number (A) and biomass (B) of background zooplankton species with depth of the eastern section of the Middle Caspian, May 2016.
Water 15 00138 g007
Table
Table 1. Species composition and frequency of occurrence of phytoplankton in the eastern part of the Middle Caspian.
Table 1. Species composition and frequency of occurrence of phytoplankton in the eastern part of the Middle Caspian.
SpeciesFrequency of Occurrence, %
10–20 m20–50 m50–100 m100–200 m200–300 m300–400 m
CYANOPHYTA- Blue-green algae
Oscillatoria sp.2510221
O.geminata Menegh.1102622
O. subtilissima (Kütz.)1-4-1-
Anabaena bergiiOstf.2307271
A. contracta (Szafer)Geitl.-11---
A. spiroides v.contracta Kleb.--2---
Gomphosphaeria aponina v. multiplex Nyg.--4---
Total 76463010124
BACILLARIOPHYTA- Diatoms
Melosira granulata (Her.) Ralfs424124132
Thalassiosira caspica Makar.1155271
Coscinodiscus granii Ehr.152122
C. gigas Ehr.125111
C. perforatus Ehr.172212
C. perforatus v.pavillardii (Forti H.)1-2-1-
C. perforatus v.cellulosus Grun.--12--
Actinocyclus echrenbergii Ralfs-61-2-
Rhizosolenia calcar-avis M.Schltze5382111136
Rh. Fragillissima Bergon.-92212
Thalassionema nitzschioides Grun17-111
Fragilaria virescens Ralfs-21-2-
Navicula cincta (Ehr.) Ktz.-45-4-
N. peregrina (Ehr.) Ktz.-14-1-
N. placentula (Ehr.) Grun.-1122-
N. cryptocephala Kutz.274512
N. lanceolata (Ag.) Kutz--211-
Hyalodiscus sphaericum Makar.272122
Nitzschia distans Greg.1106251
N. acicularis W.Sm.-111--
N. seriata Cl.-55-4-
N. closterium(Ehr.)W.Sm.16654-
N. sigma (Kutz.) W.Sm.-11---
Gomphonema constrictum Her.-1----
Total 242115969436822
PYRROPHYTA- dinoflagellates
Exuviaella cordata Ostf.5371510135
Ex. marina Clenk.1101054-
Prorocentrum scutellum Schrod.---111
Gymnodinium variabile Herdm.-2----
Peridinium latum Pauls.11-1-2
P. latum v. halophila (Lind.) I.Kissel.-22-1-
P. achromaticum Levand.--2-1-
P. subsalsum Ostf.--1---
Peridinium trochoideum (Stein.) Gemm.-2----
Goniaulax digitale (Pouch.)Kof.-21-1-
G. polyedra Stein-11---
G. spinifera (Clapar.et Lachm.) Dies.1-2---
Glenodinium lenticula (Berg.) Schiller--1---
Total 138573517218
CHLOROPHYTA
Pediastrum duplex Meyen-11-1-
Dictyosphaerium pulchellum Wood-11---
Ankistrodesmus convolutus Corda-1----
Binuclearia lauterbornii Schmidle429154104
Mougeotia sp.--1---
Spirogyra sp.-1----
Total 6433184114
Sum 50 spp.392951527411238
Mean284342
Table
Table 2. Phytoplankton abundance in the eastern part of the Middle Caspian (2016).
Table 2. Phytoplankton abundance in the eastern part of the Middle Caspian (2016).
Depth, mCyanophytaBacillariophytaDinoflagellataChlorophytaTotal
Abundance, Million Cells/m3
10–200.40 ± 0.062.26 ± 0.520.98 ± 0.530.92 ± 0.414.56 ± 1.52
20–500.43 ± 0.092.99 ± 0.410.85 ± 0.121.42 ± 0.315.69 ± 0.93
50–1000.58 ± 0.211.84 ± 0.191.35 ± 0.201.29 ± 0.315.06 ± 0.91
100–2000.22 ± 0.021.47 ± 0.250.80 ± 0.170.08 ± 0.042.57 ± 0.48
200–3000.27 ± 0.082.58 ± 0.300.78 ± 0.220.56 ± 0.264.19 ± 0.86
300–4000.44 ± 0.201.58 ± 0.221.24 ± 0.391.58 ± 0.684.84 ± 1.49
Mean0.39 ± 0.112.12 ± 0.321.00 ± 0.270.98 ± 0.344.49 ± 1.03
Table
Table 3. Biomass of phytoplankton in the eastern part of the Middle Caspian (2014).
Table 3. Biomass of phytoplankton in the eastern part of the Middle Caspian (2014).
Depth, mCyanophytaBacillariophytaDinoflagellataChlorophytaTotal
Biomass, mg/m3
10–202.44 ± 0.4398.04 ± 12.273.54 ± 1.310.22 ± 0.13104.24 ± 14.14
20–501.96 ± 0.2791.47 ± 6.063.91 ± 0.530.71 ± 0.2098.05 ± 7.06
50–1002.68 ± 0.4965.11 ± 8.546.96 ± 1.280.21 ± 0.0574.96 ± 10.36
100–2001.56 ± 0.1656.93 ± 10.093.91 ± 0.830.03 ± 0.0262.43 ± 11.10
200–3001.55 ± 0.4970.22 ± 11.343.88 ± 0.830.43 ± 0.2476.08 ± 12.9
300–4001.42 ± 0.5261.58 ± 6.575.16 ± 0.830.54 ± 0.2368.70 ± 8.15
Mean1.94 ± 0.3973.89 ± 9.154.56 ± 0.940.36 ± 0.1680.74 ± 10.62
Table
Table 4. Variety of phytoplankton according to the Shannon–Weaver index and water saprobes.
Table 4. Variety of phytoplankton according to the Shannon–Weaver index and water saprobes.
Depth, mStructural Indices of PhytoplanktonSaprobes, S
Number of SpeciesH, SpecimenH, mg
10–2071.770.771.62 ± 0.09
20–5081.600.891.47 ± 0.04
50–10091.751.211.69 ± 0.06
100–20071.781.081.60 ± 0.08
200–30081.700.991.56 ± 0.04
300–40071.641.201.54 ± 0.07
Mean8 ± 0.351.71 ± 0.071.02 ± 0.131.58 ± 0.06
Table
Table 5. Comparative characteristics of the phytoplankton structure in the eastern part of the Middle Caspian.
Table 5. Comparative characteristics of the phytoplankton structure in the eastern part of the Middle Caspian.
Indicators2013201420152016
MayMaySeptemberAprilMay
Total number of taxa7415495950
Number, million cells/m310.155.9 ± 12.989.0 ± 0.636.4 ± 0.364.5 ± 1.03
Dominant group by abundanceBacillariophytaBacillariophytaBacillariophytaBacillariophytaBacillariophyta
Biomass, mg/m380.1301.2 ± 39.776.2 ± 8.0693.0 ± 4.9580.7 ± 10.62
Dominant group by biomassBacillariophytaBacillariophytaBacillariophytaBacillariophytaBacillariophyta
Shannon–Weaver index, bit/ind.-1.09 ± 0.201.98 ± 0.131.65 ± 0.091.71 ± 0.07
Shannon–Weaver index, bit/mg-0.57 ± 0.121.98 ± 0.11.01 ± 0.161.02 ± 0.13
Saprobity index1.72-1.80 ± 0.11.46 ± 0.031.58 ± 0.06
Table
Table 6. Species composition and frequency of occurrence of zooplankton in the eastern part of the Middle Caspian, May 2016.
Table 6. Species composition and frequency of occurrence of zooplankton in the eastern part of the Middle Caspian, May 2016.
SpeciesFrequency of Occurrence, %
10–20 m20–50 m50–100 m100–200 m200–300 m300–400 m
Copepoda
Acartia tonsa Dana100100100100100100
Cladocera
Evadne anonyx typica G.O. Sars100100100100100100
Evadne anonyx producta G.O. Sars07911113
Evadne anonyx prolangata G.O. Sars000130
Evadne anonyx deflexa G.O. Sars000130
Podon polyphemoides Leukert012200
Podonevadne camptonyx podonoides G.O. Sars000010
Others
Ostracoda055452
Cirrpedia
Balanus improvisus Dorwin		100100100100100100
Average366885
Table
Table 7. Abundance of zooplankton in the eastern part of the Middle Caspian, May 2016.
Table 7. Abundance of zooplankton in the eastern part of the Middle Caspian, May 2016.
Depth, mCcopepodsCladoceraOthersAll
Abundance, ind./m3
10–2013,737 ± 31341760 ± 419684 ± 21416,181 ± 3101
20–5022,738 ± 6534994 ± 2931427 ± 18629,160 ± 785
50–10022,014 ± 5154838 ± 1972773 ± 11529,625 ± 665
100–20015,517 ± 3093896 ± 1351452 ± 11420,866 ± 448
200–3007362 ± 2532014 ± 59242 ± 139619 ± 300
300–4002301 ± 171810 ± 3952 ± 113165 ± 186
Mean13,945 ± 80483052 ± 17581105 ± 100418,103 ± 10,597
Table
Table 8. Biomass of zooplankton in the eastern part of the Middle Caspian, May 2016.
Table 8. Biomass of zooplankton in the eastern part of the Middle Caspian, May 2016.
Depth, mCopepodaCladoceraOthersAll
Biomass, mg/m3
10–20206 ± 4835 ± 81 ± 0.4247 ± 51
20–50473 ± 15104 ± 625 ± 5639 ± 17
50–100387 ± 1393 ± 410 ± 0.5488 ± 14
100–200621 ± 5777 ± 36 ± 0.5704 ± 57
200–300159 ± 740 ± 12 ± 0.2202 ± 7
300–40050 ± 518 ± 1.20.5 ± 0.169 ± 6
Mean316 ± 21461 ± 358 ± 9392 ± 257
Table
Table 9. The diversity of zooplankton on the Shannon–Weaver index in the eastern section of the Middle Caspian, May 2016.
Table 9. The diversity of zooplankton on the Shannon–Weaver index in the eastern section of the Middle Caspian, May 2016.
Depth, mStructural Indicators of Zooplankton
Number of SpeciesHh, bit/copyHb, bit/mg
10–203 ± 0.60.45 ± 0.450.35 ± 0.35
20–503 ± 0.70.58 ± 0.220.56 ± 0.22
50–1003 ± 10.28 ± 0.280.26 ± 0.26
100–2004 ± 10.71 ± 0.210.58 ± 0.24
200–3004 ± 10.61 ± 0.280.53 ± 0.25
300–4004 ± 0.010.77 ± 0.230.76 ± 0.21
Mean4 ± 0.40.47 ± 0.350.42 ± 0.33
Table
Table 10. Comparative characteristics of the zooplankton structure in the eastern section of the Middle Caspian.
Table 10. Comparative characteristics of the zooplankton structure in the eastern section of the Middle Caspian.
Indicator2013201420152016
MayMaySept.AprilMay
Total number of taxa81413119
Number, thousand. ind./m33.1 ± 0.76.8 ± 2.05.3 ± 0.51.2 ± 0.118.1 ± 10.6
Dominant group by abundanceCopepodaCopepodaCopepodaCopepodaAcartia tonsa
Biomass, mg/m338.2 ± 8.8771.9 ± 186.864.1 ± 7.917.0 ± 1.8392 ± 257
Dominant group by biomassCladoceraCladoceraCopepodaCladoceraAcartia tonsa
Number of Mnemiopsis leidyi, ind./m3008100
Shannon–Weaver index, bit/ind.-2.471.901.19 ± 0.030.47 ± 0.35
Shannon–Weaver index, bit/mg---1.57 ± 0.030.42 ± 0.33
Average individual weight of the specimen, mg 0.1150.0120.0176 ± 0.0011-
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Kurochkina, T.F.; Nasibulina, B.M.; Bakhshalizadeh, S.; Popov, N.; Kuanysheva, G.; Fazio, F.; Ali, A.M. Plankton Community Structure and Biomass in the Eastern Middle Caspian Sea. Water 2023, 15, 138. https://doi.org/10.3390/w15010138

AMA Style

Kurochkina TF, Nasibulina BM, Bakhshalizadeh S, Popov N, Kuanysheva G, Fazio F, Ali AM. Plankton Community Structure and Biomass in the Eastern Middle Caspian Sea. Water. 2023; 15(1):138. https://doi.org/10.3390/w15010138

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Kurochkina, Tatyana Fedorovna, Botagoz Murasovna Nasibulina, Shima Bakhshalizadeh, Nikolai Popov, Gulnur Kuanysheva, Francesco Fazio, and Attaala Muhaysin Ali. 2023. "Plankton Community Structure and Biomass in the Eastern Middle Caspian Sea" Water 15, no. 1: 138. https://doi.org/10.3390/w15010138

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