Past, Present and Future of the Aral Sea - A Review of its Fauna and Flora before and during the Regression Crisis

© 2023 Academia Sinica, Taiwan Open Access Past, Present and Future of the Aral Sea - A Review of its Fauna and Flora before and during the Regression Crisis Igor S. Plotnikov1, Nikolai V. Aladin1, Lubov V. Zhakova1, Jens Mossin2, and Jens T. Høeg3,* 1Zoological Institute, Russian Academy of Sciences, Universitetskaya nab. 1, St. Petersburg 199034, Russia. E-mail: Igor[email protected] (Plotnikov); [email protected] (Aladin); Lubov[email protected] (Zhakova) 2Gentoftegade 76, DK-2820 Gentofte, Denmark. E-mail: [email protected] (Mossin) 3Section of Marine Biology, Department of Biology, University of Copenhagen, Universitetsparken 4, DK-2100 Copenhagen, Denmark. *Correspondence: E-mail: [email protected] (Høeg). Phone: +45 28 75 12 47 Received 21 July 2022 / Accepted 28 January 2023 / Published 12 May 2023 Communicated by Benny K.K. Chan We review the past, present and possible future of the Aral Sea system in context of the human caused regression crisis that resulted in the drying out of the larger part of this original brackish water sea. The results are put into the context of other threatened saline lakes and the general water crisis in the world due to overexploitation of water resources and climate change. We cover the geographic history and hydrology from the origin of the sea 17,000 years ago to the present. The original biota including animals, higher plants and algae are covered in full detail, and tracked through the regression crisis. We put special emphasis on fish and fisheries because of their economic importance for the surrounding populations. We also review the side effects of the regression in terms of human health and changes to the terrestrial environment and local climate. We explain the dramatic improvements to the fauna in the northern Small Aral Sea following the construction of dams to retain its waters and discuss future options to further improve this restored water basin. We contrast this with the progressing hypersalinization of the remnants of the southern Large Aral Sea, which faces conditions that will eventually render a “Dead Sea” condition hostile to all metazoan life. We end by highlighting the partial restoration of the Small Aral Sea as an example of how much restoration can be achieved for relatively little financial expense and in a short period, when good ideas, kind hearts and hard work operate together for the benefit of the environment and our human society. Key words: Global Change, Ecology, Saline lake, Fisheries, Irrigation, Agriculture. BACKGROUND Water bodies in arid areas around the world are facing serious problems, mostly due to increased diversion of water for human purposes such as irrigation of agricultural fields. This situation is now exacerbated by ongoing global warming, which has caused water bodies to experience decreased levels of precipitation and water flow in rivers (see e.g., López-López 2021). Almost all affected water bodies, whether rivers, freshwater lakes or saline lakes, have a high biological value in terms of biodiversity and ecosystem services, including the economy and lifestyle of local human societies (Nature Editorial 2023). Prominent examples are the The Great Salt Lake (Oren 2018; Kintisch 2022; Derouin 2017), Colorado River (Stokstad 2021; Fleck and Udall 2021), Lake Tchad (Nour et al. 2020; PhamDuc et al. 2020), Lake Urmia in Iran (Oren 2018; Radmanesh et al. 2022; Davarpanah et al. 2021; Hobbins and Barsugli 2020), the River Jordan (Katz 2022; Givati Citation: Plotnikov IS, Aladin NV, Zhakova LV, Mossin J, Høeg JT. 2023. Past, present and future of the Aral Sea - a review of its fauna and flora before and during the regression crisis. Zool Stud 62:19. doi:10.6620/ZS.2023.62-19. Zoological Studies 62:19 (2023) doi:10.6620/ZS.2023.62-19 1 © 2023 Academia Sinica, Taiwan et al. 2019) and Lake Balkhash and the Aral Sea in Central Asia (Mischke 2020). The most serious case is the virtual desiccation of the larger part of the Aral Sea (Figs. 1, 2). This crisis has been well documented in specialized journals and easily ranks as the most serious local area environmental disaster in recent times (e.g., Deliry et al. 2020; Loodin 2020; Micklin 2007 2016; Micklin et al. 2014 2020). It has even been subject to personal visits by two General Secretaries of the United Nations, Ban Ki-Moon and Antonio Guiterres, the former calling the shrinking of the Aral Sea “one of the planet’s worst environmental disasters”. (The Telegraph 2010; UN News 2010; Agency of IFAS 2022). Yet, unlike the vanishing rain forests or damage to the coral reefs, the Aral Sea crisis has yet to gain full international attention. To illustrate this, a search in the Web of Science (title field; core collection; last five years) offered 260 hits for the Colorado River but only 144 for the Aral Sea. Even more critically, limiting the search to just the journals Nature and Science offered only a single hit for the Aral Sea but five for the Colorado River. Diversion of water from its only two inflowing rivers caused the Aral Sea to regress in surface area from 67,499 km2 in 1960 to only 39,734 km2 in 1990. This entailed increasing salinity and a gradual disappearance of most of the original biota (Aladin and Potts 1992; Aladin et al. 2019). By 1990 the original Aral Sea had separated into a northern Small Aral and several isolated water bodies in the south, all the latter rapidly becoming hypersaline and unliveable for most metazoan life forms. Commercial fisheries, formerly an important occupation and source of food protein in the area, virtually ceased (White 2014). Adding to this, the vanished sea caused the local climate to become more violent and unhealthy (Deliry et al. 2020). Altogether, the Aral Sea crisis entailed a multitude of interrelated problems that seriously affected the economy, health and livelihood of the surrounding population. In 1992, a first primitive dam was constructed to Fig. 1. The position of the Aral Sea in Central Asia. The only water inflow is from the southern Amu Darya and the northern Syr Darya rivers, which originate in the Pamir and Tien Shan Mountains. The Aral Sea is depicted with its present, highly reduced extension. Modified from www.earthmaps. org and Plotnikov et al. (2021b). page 2 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan retain water in the northern Small Aral. When damaged due to bad weather, The World Bank financed a much more robust replacement that, since 2005, has ensured a positive water balance in this water body. Soon after, many animals reappeared in the Small Aral and fisheries rebounded (Micklin 2016; Micklin et al. 2020; Plotnikov et al. 2016). By contrast, the southern Large Aral developed into several more or less water isolated bodies that all became hypersaline and increasingly hostile to most life forms. Here we review the history, hydrology and biology of the Aral Sea before, during and after the human caused regression crisis. We cover all relevant fauna and flora elements, including invertebrates, fish, waterfowl, microalgae and macrophyte vegetation in the sea and along the shores. We put special emphasis on the commercial fisheries due to their economic and social importance in the area. We also discuss the human health issues and effects on terrestrial ecosystems that were intrinsically linked to the former Aral Sea. MATERIALS AND METHODS General aspects of the Aral Sea Location, topography and climate The original Aral Sea was a large body of saline water located in an area below sea level in Central Fig. 2. The Aral Sea before the regression and now. Light blue shows the original sea; dark blue is the present extension. The variable shorelines of the hypersaline Large Aral is shown in yellow. The inset at upper left shows the existing Kokaral Dam and plans for an additional dam to restore even more of the Small Aral Sea. Based on Plotnikov et al. 2021b. page 3 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan Asia (Fig. 1). Its topographical parameters are given in figure 2. Before its modern regression, it was the second largest lake in the world by area (Plotnikov et al. 2021a). The Aral Sea has no effluents, and the only inflowing water comes from the Amu Darya in the south and the Syr Darya in the north. These two rivers originate in the Tien Shan and Pamir mountains (Fig. 1). Like the Caspian Sea and many smaller water bodies in Central Asia, the Aral Sea was brackish with an original mean salinity at 10.3 grams/kg. It is noteworthy that its composition differed from sea water in being enriched in divalent ionic forms (Table 1). The Aral system is located in an arid continental area with an original seasonal temperature range of -18 to 35°C in the north (Aralsk) and -12 to 34°C in the south (Muynak). The large surface area of the original sea acted as a buffer on seasonal temperature oscillations. Still, the lake became ice covered in the winter, while evaporation was substantial during summers, but until the mid 20th century the water balance was nonetheless fairly stable. Topographically the Aral Sea consisted of a northern Small Aral and a southern Large Aral, these being connected by two narrow straits on either side of the Kokaral Island. There were several other variously sized islands, especially those forming the Akpetkinskyi Archipelago at the southeastern end. Habitats The original Aral Sea offered considerable diversity in habitat, thus promoting biodiversity. The sea at large was brackish, but there were large areas close to the two river deltas that were connected to the pure fresh water in the rivers themselves. Alongside the lower part of the rivers, there were also numerous, essentially isolated lake systems with almost fresh water (Plotnikov et al. 2021b). In contrast, there were also shore locations that sustained a permanent or temporarily increased level of evaporation, thus causing a higher salinity than in the sea at large. Hence, the entire Aral Sea offered a salinity range from saline over brackish to fresh water and from lacustrine to riverine habitats. The habitat diversity was utilized by the native ichthyofauna. Many species migrated to the shore or into the rivers for breeding, since their fry could not tolerate the saline waters. Finally, close to the sea there were also smaller hypersaline water bodies, isolated from but faunistically connected with the Aral Sea itself due to the spread of organisms by animal or wind transport (Plotnikov et al. 2021b). Both the river deltas and their associated lake systems became pivotal refugia that enabled many species to survive the height of the regression crisis and eventually enabled repopulation of the restored Small Aral. Origin of the Aral Sea The modern view of the history of the Aral Sea is different from how it was presented in the past (Boomer et al. 2009; Svitoch 2010; Burr et al. 2019; Krijgsman et al. 2019). The present sea arose ca. 17,000 years ago from water that flowed into a dry depression. During the Pliocene, a drain-less depression already existed in the place of the modern Aral Sea. It was formed in an arid climate as a result of deflation, i.e., the process of wind blowing loose particles off rock surfaces. In the late Pliocene, during the transgressions of the Caspian, the Aral Sea depression was filled with waters first of the Akchagyl Sea and later of the Apsheron Sea (Fig. 3; Table 1. Ionic composition of the Aral Sea before the recent regression. Comparison with the World Ocean and the Caspian Sea. Note the relatively high values of divalent cations (Ca2+, Mg2+) and anions (SO4 2-, CO3 2-) in both the Aral Sea and the Caspian Sea. Data from IFAS (Agency of IFAS for implementation of the Aral Sea basin) Ions World Ocean World Ocean Caspian Sea Caspian Sea Aral Sea 1952 Aral Sea 1952 g/kg % of total g/kg % of total g/kg % of total Na+10,556 30,69 3,156 24,61 2,96 27,16 K+0,38 1,1 0,1 0,78 0 Ca2+ 0,4 1,16 0,334 2,6 0,48 4,4 Mg2+ 1,272 3,7 0,74 5,77 0,54 4,95 Cl-18,98 55,18 5,347 41,7 3,55 32,57 Br-0,065 0,19 0,007 0,05 0 SO4 22,649 7,7 3,038 23,69 3,21 29,45 CO3 20,071 0,21 0,1 0,78 0,16 1,47 H3BO30,026 0,08 0 0 Total 34,399 100 12,822 100 10,9 100 page 4 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan Zonn 2009). The vast Akchagyl Sea (or Basin) covered both the present Caspian and Aral Sea and an extensive area between the lower Volga and Ural rivers. The Akchagyl Sea was a deep water, saline and cold basin with a water level 100–150 m above the present Caspian Sea. Some of its fauna intruded into the Black Sea with which it was connected by the Manych-Kerch Strait (Fig. 3). The later Apsheron Sea existed during late Pleistocene and was smaller than the Akchagyl Sea, but larger than the modern Caspian Sea (Krijgsman et al. 2019). Eventually losing the connection with the Black Sea, the Apsheron Sea became shallower, warmer and also less saline (salinity similar to the present Caspian Sea). These events were important for the fauna of the present Caspian Sea, but not for the Aral Sea, which dried up completely during a following continental period that lasted almost until the end of the Pleistocene (Burr et al. 2019; Aladin and Plotnikov 1995; Svitoch 2010). The waters of the Syr Darya started flowing into Fig. 3. A and B, Geological history and topographic details of the Aral Sea from late Pliocene to present; the extensions of the present Black Sea, Caspian Sea and Aral Sea are shown in outline. The large Akchagyl Sea (or Basin) (~3.2–2 mln. years BP) covered a vast area and connected to the Black Sea by the Manych-Kerch Pillway. The later Apsheron Sea (~1.8–0.7 mln. years BP) was smaller, but still larger than the present Caspian Sea. C, The position of the present Aral Sea in its extension before the modern regression. D, Topographic and hydrological details of the Aral Sea as of about 1960. Original figure partially based on Krijgsman et al. (2019). page 5 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan the Aral Basin from the Late Pleistocene. Subsequently, Amu Darya also turned to flow into the Aral Sea rather than to the Caspian. Accordingly, the Aral Sea biota consist of invaders that entered it at different times and from different faunal provinces. From its formation 17,500 years ago, the Aral Sea has experienced repeated regressions and transgressions (Leroy et al. 2007). The water level and salinity of this drainless basin were influenced only by climate and the precise course of the Syr and Amu Darya. Climate, drier or wetter, determined both loss to evaporation and water flow in the rivers from their sources in the Pamir and Tien Shan mountains. But while the waters of the Syr Darya always flowed into the Aral Sea, the Amu Darya could at times flow into Lake Sarygamysh to the south-west of the Aral and further along its ancient channel – Uzboy – into the Caspian Sea (as was the case in the Pliocene). Alternatively, it could also flow simultaneously into both reservoirs. Eventually, the amount of inflowing water was also affected by the emergence and development of irrigated agriculture. During the Khwarazmian Dynasty (1077 to 1231) people could actually shift the flow of the Amu Darya from the Aral to the Caspian, or vice versa, but such control could only be maintained during periods of relative social affluence and stability. Social upheavals and wars in the region, such as the Mongolian invasion, lead to the loss of control over the river. Protective dams and irrigation systems were destroyed, and then, by chance, the flow of the Amu Darya turned in one direction or another (Aladin and Plotnikov 1995). The variability in size of the Aral has been welldocumented, although until recently the exact water levels have been debated (Boomer et al. 2009). Even in medieval Arabic documents there are references to changes in water level and direction of flow of the Amu Darya. When inflow decreased, the Aral Sea would sometimes break up into separate lakes filled with highly mineralized water, while near the river mouths there were floodplains with freshened shallow waters (Svitoch 2010). Dating of the regressions and transgressions over the past 2000 years are based on data from geology, geomorphology, archaeology and on fossilized remains of aquatic organisms in bottom sediments (Boomer et al. 2009). The first regressions are dated approximately from the 1st century BC to the 4th century BC. Regressions during the last millennium have been dated more precisely and were also documented in contemporary historical records (Krivonogov 2014; Krivonogov et al. 2010 2014; Yang et al. 2014). The data of Boomer et al. (2009) indicate that the Aral Sea experienced minima in area and level between AD 900–1350, AD 1500–1650 and 1790 to the present. Nevertheless, all records indicate that a large scale and very rapid regression started during the early 1960s and was almost exclusively due to large-scale diversion of upstream waters from the Amu Darya for agricultural purposes (Boomer et al. 2009; Aladin and Potts 1992; Micklin 2007). This eventually resulted in the present state, where the Aral Sea has been reduced to only a tiny remnant of its former size. In summary, the Aral Sea is a comparatively young system, whose inflowing rivers varied in water volume and also changed course since the Ice Ages, thus together affecting its area, water level and salinity. It always remained isolated from any other large water bodies such as the Caspian Sea. This entailed a low biodiversity, and its young age also means that it contained few endemic forms. We emphasize that despite variations in the past, the Aral Sea never experienced any condition remotely resembling the severe, human caused regression that occurred during the latter half of the 20th century and eliminated most of the original sea and its biota. Investigations of the Aral Sea The Aral system was an early target for detailed biological investigations, starting with the efforts of Berg (1908). The importance of this multidisciplinary scientific study of both the Aral and other areas in Central Asia can hardly be underestimated (Goaravetisyan 2021). During the latter half of the 20th century, it was subjected to detailed monitoring of physico-chemical and biological parameters. These took place regularly and at fixed stations by local researchers and staff from the Zoological Institute, Russian Academy of Sciences (ZIN RAS). As a result, both the original state and the entire period of regression have been very well documented (details in Plotnikov et al. 2021b). The monitoring even included screening of cores taken from the dried out lake bed (thanatocoenoses) that allowed establishing time series of organisms that left hard identifiable parts, such as Ostracoda (Aladin 1991). RESULTS Original biodiversity We here review the native aquatic fauna and flora and their decline during the regression period. We include important invertebrate taxa, fish, water birds, microalgae and macrophytes, including both macroalgae and flowering plants in the sea and along the shores. The native fauna has previously been surveyed by Aladin and Potts (1992) and also reviewed page 6 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan briefly in Keith et al. (2013), but here we add new and more detailed information. The crustacean and fish fauna was previously treated in detail (Ermanakhov et al. 2012 2013; Plotnikov et al. 2021b). According to our most recent estimate, the native aquatic fauna of the Aral Sea comprised 20 species of fish, 195 species of free-living invertebrates and 71 species of parasites (Fig. 4). For the ichthyofauna, the most important food items were benthic and planktonic crustaceans and the benthic bivalves and chironomid larvae. In addition to the aquatic fauna, there were a number of terrestrial animals, especially birds, closely associated with the Aral Sea. The diversity of microalgae was impressive with more than 640 species. They served as food for invertebrates both in the plankton and on bottom sediment and plant surfaces. The flora of macrophytes contributed to habitat formation both along the shores and in deeper waters. It comprised 24 species of Fig. 4. Aral Sea Fauna. Important native and introduced species in the Aral Sea fauna. Mesocyclops leuckarti and Arctodiaptomus salinus were important members of the zooplankton, but the latter was replaced by the introduced Calanipeda aquaedulcis. In the benthos Dikerogammarus aralensis was displaced by the introduced shrimp Palaemon elegans. Bivalves of the genus Adacna, Cerastoderma and the introduced species Abra segmentum were important food for fish. The same is true for the introduced polychaete Hediste diversicolor. The fish depicted were all valuable commercial fisheries. Chironomid larvae and pupae were important food items for fish. The Aral Sea Trout and the Aral Sea Sturgeon have both gone extinct. Further details in text. page 7 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan angiosperms, six species of charophytes and about 40 other species of macroalgae. Some macrophytes were also very important as food items for omnivorous fish and some of the waterfowl. For invertebrates, our main focus is on crustaceans and molluscs, which were the principal food items for the fish. Invertebrates Crustaceans and molluscs were the most important invertebrates in the original Aral Sea, being primary food items for most native fish (Aladin et al. 2022). In table 2, data are given for all native species of Mollusca. The Crustacea were previously given detailed attention (Aladin et al. 2021; Plotnikov et al. 2021b) so information on this group is only summarized at higher taxon level (Table 2). Native crustaceans comprised species with varying degrees of salinity tolerance, which affected their distribution. Nine species of pelagic cladocerans were present in the plankton, but they were always restricted to areas with low salinity. The plankton also contained two species of calanoid copepods and 14 species of cyclopid copepods. Especially important as fish food were the euryhaline Arctodiaptomus salinus and Halicyclops rotundipes, which were both found throughout the sea. On or associated with the bottom were 15 species of Harpacticoida and 11 species of Ostracoda, again with varying degrees of salinity tolerance. The euryhaline gammarid Dikerogammarus aralensis was the only native malacostracan. Finally, there were five species of parasitic copepods hosted by the native fish fauna. Native Mollusca were poorly represented compared to Crustacea, but they still provided an important food source for fish (Aladin et al. 2022). The literature has been troubled with incorrect naming of species, but here we follow the most recent and authoritative account by Wesselingh et al. (2019). The Aral Sea contained two species of Gastropoda (Ecrobia grimmi, Theodoxus pallasi) and seven species and subspecies of Bivalvia, including two species of Cerastoderma, three subspecies of Dreissena and two subspecies of Adacna. Among the bivalves, Adacna minima minima lived throughout the sea even down to 30 m while other forms were most numerous at shallow depths. The two species of Cerastoderma differed in distribution, with the saltwater tolerant C. glaucum occurring in the more saline areas. The other species was previously identified as C. rhomboides, but is here called C. sp. A. (Wesselingh et al. 2019), occurring only in the lower salinity waters. All the bivalves reproduced during the summer, at which time their larvae were the most numerous component of zooplankton. (Lukonina 1960; Kortunova 1975). Initially the fish intensively ate only the two widely distributed subspecies of Adacna minima, while consumption of Cerastodema spp. and Dreissena spp. of older ages were limited due to their thick shells. After being introduced in 1960–1963, the Mediterranean-Atlantic mollusk Abra segmentum, which also has a thin-walled shell, became an additionally valuable food source for the benthivorous fish (Karpevich 1960 1975; Yablonskaya 1960). Chronomid larvae were a significant element in the deep water bottom area, and provided an important food item for fish when they swarmed to the surface and depupated into imagos. The strict seasonal availability of this food item together with the generally low density of free water crustaceans explains why none of the native fish were exclusive plankton feeders. Meiobenthos have not traditionally been studied in the Aral Sea, but recently living samples were collected in Large Aral Sea in 2003 and 2004 at depths of 0 to 39 m (Mokievsky 2009; Mokievsky and Miljutina 2011). In the now hypersaline waters, the near-bottom salinity at the sampling sites varied from 88 to 109‰, but there was still a diverse meiobenthos consisting of nematodes, harpacticoids, ostracods, turbellarians and foraminiferans. The density showed significant spatial variation, with nematodes predominating in most samples. The maximal abundance of free-living nematodes (1440 specimens/10 cm2) was recorded in 2003 at a sampling site at 10 m depth, 89 ppt salinity and 13.6°C. In 2004, the maximal abundance of nematodes was 750 specimens/10 cm2 at about the same depth and salinity, but at 24.5°C. The highest value for harpacticoid copepods was 116 specimens/10 cm2 at 1 m depth. The high spatial variation in meiobenthos density is to a considerable extent related to the sediment characteristics at the sampling stations. Meiobenthos is now often used for biomonitoring purposes and is normally dominated by nematodes in terms of specimen numbers (Semprucci et al. 2015). For comparison with the Aral Sea, Huys et al. (1992) sampled meiobenthos all over the North Sea and found that harpacticoids were almost always less frequent than nematodes, whose density ranged from 61–4167 individuals/10 cm2. The density of meiobenthos in the hypersaline parts of the Aral Sea agrees well with other studies. In marine habitats, conditions can sometimes also reach hypersaline levels, but if they exceed 100 ppt, all macroinfauna vanishes, while meiofauna remains at relatively low densities. In tidal areas off Zanzibar, Olafsson et al. (2000) found salinities as high as 89–160 ppt in sediment pore waters and specimen densities in the meiobenthos at 271 to 656 specimens/10 cm2, the majority of these (58–87%) being nematodes. page 8 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan Table 2. Occurrence of selected invertebrate taxa at fixed stations during the regression crisis. Percentage of total stations where taxon was found are indicated (See Plotnikov et al. 2021b for details). For Crustacea, a complete species list appeared in Plotnikov et al. (2021b) so data is only summarized for genera or higher taxa Taxon/year 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 PELAGIC Branchipopoda Moina mongolica 48.2% 52.3% 16.7% 8.3% 0% 1.2% Cercopagis 21.2% 4.7% 54.8% 25.0% 27.7% 33.3% Freshwater Cladocera 5.9% 3.5% 3.6% 1.2% 9.2% 0.0% Copepoda Cyclopoida 100% 100% 98.8% 100% 89.2% 46.9% Calanipeda 0% 22.1% 100% 100% 100% 100% Arctodiaptomus 70.6% 70.9% 4.8% 6.0% 1.5% 0.0% BENTHIC Crustacea Dikerogammarus 52.0% 29.5% 22.8% 5.6% 1.1% 2.4% 4.6% 1.2% 1.2% 2.3% 0% 0% Bivalvia Abra segmentum 0% 0% 0% 0% 1.1% 5.9% 10.3% 18.6% 30.6% 55.8% 82.1% 80.2% Adacna spp.76.5% 60.0% 47.5% 38.9% 40.0% 51.8% 31.0% 24.4% 37.6% 18.6% 16.1% 32.1% Cerastoderma spp.58.2% 50.5% 45.5% 45.6% 60.0% 70.6% 57.5% 27.9% 55.3% 60.5% 75.0% 82.7% Dreissena spp.87.8% 74.3% 69.3% 68.9% 77.9% 82.4% 43.7% 25.6% 38.8% 15.1% 12.5% 27.2% Theodoxus pallasi 55.1% 44.8% 52.5% 37.8% 44.2% 30.6% 9.2% 8.1% 7.1% 3.5% 7.1% 11.1% Polychaeta Hediste diversicolor 0% 3.8% 11.9% 20.0% 30.5% 34.1% 48.3% 76.7% 84.7% 84.9% 94.6% 95.1% Insecta Chironomidae larvae 75.5% 74.3% 71.3% 56.7% 54.7% 48.2% 52.9% 29.1% 20.0% 9.3% 1.8% 0% Taxon/year 1975 1976 1977 1980 1981 Trend Reason PELAGIC Branchipopoda Moina mongolica 0.0% 0.0% 0.0% 0.0% 0.0% decline displacement Cercopagis 20.8% 7.6% 7.0% 8.5% 0.0% decline salinity Freshwater Cladocera 0.0% 0.0% 0.0% 0.0% 0.0% decline salinity Copepoda Cyclopoida 17.0% 7.6% 10.5% 0% 46.8% decline salinity Calanipeda 100% 100% 100% 100% 100% increase introduction Arctodiaptomus 0.0% 0.0% 0.0% 0.0% 0.0% decline displacement BENTHIC Crustacea Dikerogammarus 0% 0% 0% decline displacement Bivalvia Abra segmentum 83.3% 86.4% 81.4% increase salinity Adacna spp.28.8% 6.1% 11.9% decline salinity Cerastoderma spp.78.8% 77.3% 83.1% variable Dreissena spp.34.8% 24.2% 28.8% decline salinity Theodoxus pallasi 6.1% 13.6% 11.9% decline salinity Polychaeta Hediste diversicolor 93.9% 97.0% 98.3% increase introduction Insecta Chironomidae larvae 0% 0% 0% decline salinity & displacement page 9 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan Table 6. Macrophytobenthos of the original Aral Sea Taxon/ Species Author Common name Depth and Habitat Important fish food Comment GREEN ALGAE Cladophoraceae Chaetomorpha linum (O.F. Müller) Kützing 1845 not common or rare Cladophora comatula Kützing not common or rare Cladophora dalmatica Kützing 1843 [= Cladophora conglobata (Kützing) Rabenhorst 1847] not common or rare Cladophora flexuosa (O.F.Müller) Kützing 1843 rare Cladophora fracta (O.F. Müller ex Vahl) Kützing 1843 common; sandymuddy bottoms Cladophora glomerata (Linnaeus) Kützing 1843 common, near shore Cladophora gracilis Kützing 1845 common, near shore Cladophora rupestris (Linnaeus) Kützing 1843 not common or rare Cladophora sericea (Hudson) Kützing Kützing1843 [= Cladophora glaucescens (A.W. Griffiths ex Harvey) Harvey 1841] not common or rare Rhizoclonium hieroglyphicum (C. Agardh) Kützing 1845 rare Ulvaceae Ulva compressa Linnaeus 1753 [= Enteromorpha compressa (Linnaeus) Nees 1820] sea lettuce Ulva intestinalis Linnaeus 1753 [= Enteromorpha intestinalis (Linnaeus) Nees 1820] sea lettuce Ulva prolifera O.F. Müller 1778 [= Enteromorpha prolifera (O.F. Müller) J. Agardh 1883] sea lettuce Charophycea Chara aculeolata Kützing 1832 [= Chara polyacantha A. Braun, nom. inval. 1862] deeper waters yes seriously declined or absent Chara tomentosa Linnaeus 1753 deeper waters yes seriously declined or absent Lamprothamnium papulosum (K. Wallroth) J. Groves 1916 deeper waters yes seriously declined or absent Nitella hyalinа (De Candolle) C. Agardh 1824 deeper waters yes seriously declined or absent Nitellopsis obtusa (Desvaux) J. Groves 1919 deeper waters yes seriously declined or absent BLUE-GREEN ALGAE Xanthophyceae Vaucheria dichotoma (Linnaeus) C. Martius 1817 deep water "Vaucheria" forests yes RED ALGAE Rhodomelaceae Carradoriella denudata (Dillwyn) Savoie et G.W. Saunders 2019 [= Polysiphonia variegata (C. Agardh) Zanardini 1842] Carradoriella elongella (Harvey) Savoie et G.W. Saunders 2019 [= Polysiphonia elongella Harvey 1833] Herposiphonia tenella (C. Agardh) Ambronn 1880 Leptosiphonia fibrata (C. Agardh) A.M. Savoie et G.W. Saunders 2019 [= Polysiphonia fibrata (Dillwyn) Harvey 1833] Polysiphonia dichotoma Kützing 1843 Polysiphonia ornata J. Agardh 1842 Polysiphonia vinosa Kützing Vertebrata byssoides (Goodenough et Woodward) Kuntze 1891 [= Brongniartella byssoides (Goodenough et Woodward) F. Schmitz 1893] Vertebrata fruticulosa (Wulfen) Kuntze 1891 [= Polysiphonia fruticulosa (Wulfen) Sprengel 1827] Vertebrata fucoides (Hudson) Kuntze 1891 [= Polysiphonia violacea (Roth) Sprengel 1827] page 16 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan Taxon/ Species Author Common name Depth and Habitat Important fish food Comment FLOWERING PLANTS Alismataceae Sagittaria trifolia Linnaeus 1753 threeleaf arrowhead shore, common Butomaceae Butomus umbellatus Linnaeus 1753 flowering rush shore, common Hydrocharitaceae Najas marina Linnaeus 1753 spiny water nymph shallow underwater meadows Potamogetonaceae shallow underwater meadows Potamogeton crispus Linnaeus 1753 curled pondweed shallow underwater meadows Potamogeton lucens Linnaeus 1753 shining pondweed shallow underwater meadows Potamogeton nodosus Poiret 1816 longleaf pondweed shallow underwater meadows Potamogeton perfoliatus Linnaeus 1753 clasped pondweed shallow underwater meadows Potamogeton pusillus Linnaeus 1753 small pondweed shallow underwater meadows Stuckenia filiformis (Pers.) Börner 1912 [= Potamogeton filiformis Pers. 1805] fineleaf pondweed Stuckenia macrocarpa (Dobrocz.) Tzvelev 1999 [= Potamogeton macrocarpus Dobrocz. 1951] loddon pondweed shallow underwater meadows Stuckenia pectinata (Linnaeus) Börner 1912 [= Potamogeton pectinatus Linnaeus 1753] sago pondweed or fennel pondweed Zannichellia palustris Linnaeus 1753 horned pondweed shallow underwater meadows Zannichellia pedunculata Reichenbach 1830 horned pondweed shallow underwater meadows Ruppiaceae Ruppia cirrhosa (Petagna) Grande 1918 spiral ditchgrass shallow underwater meadows Ruppia maritima Linnaeus 1753 ditch grass shallow underwater meadows Zosteraceae Zostera noltei Hornemann 1832 [= Zostera nana Roth 1827] dwarf eelgrass very common, shallow water Poaceae Phragmites australis (Cav.) Trin. ex Steud. 1841 common reed shore, very common as dense reeds Cyperaceae Schoenoplectus litoralis ssp. kasachstanicus (Dobrochotova) Sojak 1979 [= Scirpus kasachstanicus Dobrochotova 1950] club-rush, bulrush shore, very common as dense reeds Schoenoplectus tabernaemontani (C.C. Gmelin) Palla 1888 [= Scirpus tabernaemontani C.C. Gmelin 1805] great bulrush Schoenoplectus triqueter Linnaeus (Palla) 1888 [= Scirpus triqueter Linnaeus 1767] streambank bulrush Typhaceae Typha angustifolia Linnaeus 1753 cattail, lesser bulrush shore, common Haloragaceae Myriophyllum spicatum Linnaeus 1753 Eurasian watermilfoil shallow underwater meadows Menyanthaceae Nymphoides peltata (S.G. Gmelin) O. Kuntze 1891 fringed water lily, yellow floatingheart shallow underwater meadows Table 6. (Continued) page 17 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan colloids. The resulting nutrient poor waters of the Aral Sea therefore had and still have a paucity of planktonic microalgae. Moreover, most particulate matter carried by the rivers is sedimented near the deltas, and the overall effect is therefore very transparent waters that favor the development of benthic macrophytes (such as Chara) and benthic microalgae. Altogether, the nutrient poor waters reduced the primary production available to food webs of the Aral Sea (Karpevich 1975). Phytoplankton is devoured by planktonic crustaceans, principally copepods and cladocerans, but the paucity of these algae explains why there are no fish from the open water that depend entirely upon zooplankton. Both species diversity (128 spp.) and biomass was highest in the freshened area (salinity 5–7 ppt) in front of the Syr Darya delta in the Small Aral Sea. This area was and is still favorable for freshwater and fresh-brackish water species (Rusakova 1995) due to sufficient warming, a relatively low salinity and a somewhat higher amount of biogenic elements than in the sea at large. The species diversity of phytoplankton in the Tshche-Bas (Large Aral) and the Shevchenko and Butakov Bays (Small Aral) were much lower than in the freshened area of the sea, primarily due to the almost complete loss of green algae. Benthic microalgae. The paucity of plankton algae meant that microalgae on surfaces were an important food source. In the bottom mud invertebrates fed largely on diatoms coating the silt particles, although detritus also formed part of their diet (Behning 1935). Important components in this fauna were chironomid larvae, mollusks, the now extinct amphipod, nematodes and oligochaetes all of which were again dietary components of fish. It is likely that various prokaryotes may also cover macrophytes in the near-shore reed beds, where they could be an important food item for small, grazing animals (Mossin 1988), but there is no data on these organisms. Macroflora and habitats In the original Aral Sea the range in depth and salinity and other variable conditions enabled the existence of a variety of plant communities in deeper waters, along the shores and in the delta regions. Several macrophytes occurring commonly in deeper waters, e.g., Chara spp. and Vaucheria dichotoma were at various periods important as food for fish. These were mainly roach but also white-eyed, bream, aral barbel, shemaya and sabrefish (Behning 1935; Pankratova 1935). As for animals, most changes to vegetation can clearly be attributed to the increasing salinization after 1960, but some occurring prior to that time still warrants a satisfactory explanation. In deeper waters the benthic macrophytes could form dense vegetative mats as deep as 30 m (Table 6), and they greatly increased oxygen levels of the bottom layers. At the start of the 20th century characean algae formed extensive thickets at a 18–23 m depth (Alenitsyn 1875; Berg 1908). Yet, by the 1930s these characean were no longer recorded in deeper waters, but mainly on black silt in more or less isolated bays (Behning 1935). It is not known exactly when and why the deepwater characeans disappeared. They are known to prefer clear waters with a low content of nutrient salts and organic matter, and it is entirely possible that human caused changes to the waters may have accelerated their demise. By the 1950s the characeans had definitely been replaced by the yellow-green alga Vaucheria dichotoma, which formed dense aggregations called “Vaucheria forests” on gray silts. In the Small Aral Sea, these tickets occurred down to 26 m and were important food items for several fish species (Zhakova 2013). In shallower waters and before the modern regression crisis, small but constant fluctuations in salinity in the bays and along the coasts were favorable for the existence of both freshwater and brackish water vegetation, resulting in a diversity of plant communities forming belts of hydrophytes and helophytes. Helophytes are perennial marsh plants with buds overwintering underwater. In the sea, they formed clumps and border thickets located in a continuous or discontinuous strip from 1 to 100 m wide. They were more significant on the south and east coasts than on the north and west. Thickets of Phragmites australis dominated everywhere. The maximum height of these reed beds reached 4.5 m, with a density of up to 300 plants/m2 and they produced 2.8 million tons of organic matter per year. In the northern part of the Aral Sea, behind the reed zone, there was often a band of Schoenoplectus litoralis kasachstanicus (club rush), forming thickets up to 4 m high at depths from 1.5 to 3.5 m and a density of up to 25 plants/m2 (Behning 1935; Bervald 1964; Dobrokhotova 1971; Yablonskaya 1964; Zhakova 2013). Other helophytes (Butomus, Sagittaria, Typha) did not form such significant thickets. Hydrophyte communities were more abundant in the northern part of the Aral Sea as various associations formed vast underwater meadows, and comprised species of Potamogeton, Zannichellia, Ruppia, Zostera, Myriophyllum, Najas and Nymphoides (Table 6). In the seaward part and in open bays dwarf eelgrass (Zostera noltei) dominated on sandy bottoms at depths of 3–11 m, where it formed continuous or discontinuous thickets. The productivity of the Z. noltei communities was so high that the layer of leaves thrown ashore reached 0.8–1 m in thickness and 2–3 m in width along a considerable length of the coastal strip. This resembles page 18 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan conditions that existed in Z. marina communities in many areas of Western Europe, before the arrival of the “Zostera wasting disease” that affected this species dramatically. Interestingly, this disease seems not to have affected Z. noltei although this slime mold has been identified in its tissues (Vergeer and Hartog 1991). This apparent “immunity” may well have spared the Aral Sea communities of an early and major ecological change. According to Zhakova (2013) Z. marina is not present in the Aral Sea. In Bolshoy Sary-Chaganak bays, the macrophytes consisted of mostly non-Alismatales species forming small associations of one to three species: 1. Zostera noltei + Ruppia cirrhoza + Chaetomorpha linum (salinity 21–26 ppt); 2. Ruppia cirrhoza (salinity 21–26 ppt); 3. Chaetomorpha linum + Cladophora glomerata + Cl. fracta (salinity 20–30 ppt) (Zhakova 1995). Salinized, closed bays were dominated by charophytes (Bervald 1964; Dobrokhotova 1971; Yablonskaya 1964). Fauna disturbances by introduced species Both planned and accidental introductions of animal species occurred during the latter half of the 20th century (Table 7, Aladin et al. 2019). The planned introductions were intended to increase commercial fish stock and involved both non-native fishes and invertebrates considered suitable as fish food. These efforts were unrelated to the increasing salinization crisis (Figs. 6–8). The euryhaline polychaete worm Table 7. Planned (P) and accidental (A) species introductions to the Aral Sea. Planned introductions were all performed to improve fisheries, but only the European Flounder was a success. All introduced species, except Paramysis baeri, became established, but some eventually declined to very low abundance. Further information in text and in Plotnikov et al. (2021b) Common name Latin name Reason Effect Comment Baltic herring (P) Clupeus harengus membras (Linnaeus) improve fisheries caused serious decline of zooplankton initially abundant followed by serious decline European flounder (P) Platichtys flesus (Linnaeus) improve fisheries success remains a fisheries asset Big sand smelt (A) Atherina boyeri caspia Eichwald accidental caused serious decline in zooplankton not commercial, initially abundant followed by serious decline Bubyr goby (A) Pomatoschistus caucasicus Berg accidental competitor for native fishes not commercial and not abundant Sand goby (A) Neogobius fluviatilis pallasi (Berg) accidental competitor for native fishes not commercial and not abundant Tubenose goby (A) Proterorhinus marmoratus (Pallas) accidental competitor for native fishes not commercial and not abundant Round goby (A) Neogobius melanostomus affinis (Eichwald) accidental competitor for native fishes not commercial and not abundant Bighead goby (A) Neogobius kessleri gorlap Iljin accidental competitor for native fishes not commercial and not abundant Syrman goby (A) Neogobius syrman eurystomus (Kessler) accidental competitor for native fishes not commercial and not abundant Grass carp (P) Ctenopharyngodon idella (Valenciennes) improve fisheries success commercial but not abundant Silver carp (P) Hypophtalmichthys molitrix (Valenciennes) improve fisheries success commercial but not abundant Spotted silver carp (P) Aristichtys nobilis (Richardson) improve fisheries success commercial but not abundant Black carp (A) Mylopharyngodon piceus (Richardson) improve fisheries success commercial but not abundant Snakehead (A) Channa argus warpachowskii Berg accidental success commercial but not abundant Mysids (P) Paramysis lacustris (Czerniavsky) P. intermedia (Czerniavsky) P. baeri Czerniavsky fish food little effect Bivalve mollusc (P) Abra segmentum Récluz fish food positive effect Polychaete (P) Hediste diversicolor (O.F. Müller) fish food positive effect Calanoid copepod (P) Calanipeda aquaedulcis Kritchagin fish food positive effect, replaced native species Shrimp (A) Palaemon elegans Rathke accidental displaced native gammarid species Mud crab (A) Rhitropanopeus harrisii (Gould) accidental only in the Large Aral Copepod fish parasite Actheres percarum Nordmann accidental none? page 19 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan Hediste diversicolor was introduced in 1960–1961. In 1973–1974 it had spread around the Aral Sea and become a valuable food for the fish. The euryhaline bivalve Abra segmentum was introduced in 1960–1963, and by the mid 1970s had spread to the whole Aral Sea and become a valuable food for the fish (Fig. 8). In contrast, almost all introductions of fish and crustaceans had either negative effects or no effects at all on the fisheries. Accidental introduction of the shrimp Palaemon elegans led to displacement of the native gammarid Dikerogrammus aralensis (Fig. 4). The planned introduction of the euryhaline and productive copepod Calanipeda aquaedulcis provided better food for the fish. Unfortunately, it displaced the already declining native species Arctodiaptomus salinus (Figs. 4, 7). Three species of mysids were also introduced, but only one, P. intermedia, became established and abundant in non-salinized areas (Plotnikov et al. 2021b). The planned introductions of fish were mostly unsuccessful or had catastrophic effects on the ecosystem (Ermakhanov et al. 2012). In the 1950s, attempts to introduce commercially valuable mullets (2 species), resulted in accidental introductions of the Caspian atherine (Atherina boyeri caspia) and several species of gobies (Pomatoschistus caucasicus, Proterorhinus marmoratus, Neogobius fluviatilis pallasi, N. melanostomus affinis, N. kessleri gorlap, N. syrman eurystomus). Simultaneously, Baltic herring (Clupea harengus membras) were also introduced. The mullets did not become established. The herrings and young atherine are pure planktivores and not typical of Aral Sea species. Their voracious feeding resulted in a dramatic decline in planktonic crustaceans, which again caused serious starvation of the fish. Thus the abundances of herring and atherine subsequently decreased to very low levels. In 1960–1961, there were introductions of introduced commercial fishes from the Far East (Ermakhanov et al. 2012; Plotnikov et al. 2016): the herbivorous grass carp (Ctenopharyngodon idella); fish consuming phytoplankton, zooplankton and detritus such as silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis) and also the predatory snakehead (Channa argus warpachowskii). They are now numerous, occurring in the Aral near the mouth of the rivers. The only real success for fisheries was the introduction of the European flounder, Platichthys flesus from 1979 to 1987. This took place under scientific and practical guidance from experienced Danish fishermen (Plotnikov et al. 2021b), and the result was that these bottom foragers became and remain a permanent asset for the fisheries in the Aral Sea. DISCUSSION Regression and salinization The modern regression crisis commenced at the middle of the 20th century due to the accelerated diversion of water in the tributary rivers for human purposes, mainly irrigation (Figs. 7, 9). Historically, water has long been used from the Amu Darya and Syr Darya to sustain agriculture, and this enabled the sprawling cultures located here in the Middle Ages (See e.g., Frye 2011). Despite this, the salinity of the Aral Sea remained rather stable, inasmuch as these advanced cultures were able to economize water usage (see below). From around 1960, water was increasingly used for irrigating new water demanding crops, principally cotton, and also to sustain the increasing population. The reduced inflow resulted in an increasingly negative water balance, whence the area of the Aral Sea started to shrink and, accordingly, the salinity to increase (Figs. 2, 6; Table 8). Effects on invertebrates During the first years of area regression, the biodiversity was largely unaffected by the salinity increase itself, although disturbances were caused by the planned or accidental introduction of alien species. Over time the various types of organisms met their upper salinity tolerance and decreased to minimal levels or vanished completely (Table 7; Figs. 7, 8). Following Plotnikov et al. (2021b) we divide the events into three crises. The (1) first crisis occurred from 1971–1976, when salinity reached levels (12–13 ppt) that caused true freshwater crustaceans to disappear. Sharp declines were also seen for the bivalves, where Adacna and Dreissena became less common and Cerastoderma sp. A disappeared entirely by 1976 (Table 7, Fig. 8). Following the first crisis, conditions remained relatively stable for a time, allowing both brackish water and euryhaline species to thrive or even to increase. Thus, the saltwater tolerant bivalves C. glaucum and A. segmentum spread to the entire sea (Figs. 4, 8). The halophilic gastropod Ecrobia grimmi also increased in abundance (Andreeva 1989). But irrespective of this, the entire molluscan fauna eventually declined due to the increasing salinization. The (2) second crisis occurred from 1987–1990, when salinity reached 27–32 ppt, causing a sharp decline in species. For crustaceans, most native species, including cladocerans, completely disappeared. Only the introduced calanoid copepod Calanipeda aquaedulcis, remained in the plankton (Table 7). The (3) final crisis happened around 1990. At this page 20 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan time, the original continuous sea had become separated into a southern Large Aral and a northern Small Aral. The large Aral became several hypersaline bodies of water, a western deeper part and a more shallow eastern part. All fish and euryhaline invertebrates have vanished in these remnants of the Large Aral and only hypersaline tolerant metazoan species survived. The brine shrimp Artemia began to spread in these waters. The bivalve Fig. 6. Salinity and inflow of water for the Aral Sea. A, Total inflow to the Aral Sea during the regression and caused by diversion for irrigation purposes. B, The resulting increase in salinity; after 1990 the waters of the Large and Small Aral were separated and evolved independently of each other; the inserted maps show the decrease in area. After 2000 also the eastern and western parts of the Large Aral became separate water bodies, but both rapidly hypersalinizing. The construction of the dams that kept water in the Small Aral, had almost immediate effects. On the axis is indicated key events for the fauna in the system; further details in text. Modified and extended with new data from Plotnikov et al. (2021b). A B page 21 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan Fig. 7. Zooplankton recorded at fixed stations during the crisis. The percentages indicate the number of fixed sampling stations where the taxon was recorded. Native cladocerans, cyclopoids and calanoids all declined. The introduced calanoid Calanipeda aquaedulcis initially spread throughout the sea. Detailed methodology explained in Plotnikov et al. (2021b). page 22 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan Fig. 8. Zoobenthos collected at fixed stations during the crisis. The percentages indicate the number of fixed sampling stations where the taxon was recorded. Detailed methodology explained in Plotnikov et al. (2021b). page 23 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan Cerastoderma glaucum disappeared by 2001 and Abra segmentum by 2004. By the 2000s the salt water tolerant gastropod Ecrobia grimmi also vanished in the Large Aral (Aladin and Plotnikov 2008; Plotnikov 2013). At present, the water bodies of the original Large Aral are steadily increasing in salinity, and it can be predicted that they will ultimately become like the Dead Sea with no metazoan life at all (Oren 2018; Plotnikov et al. 2021b). Effect on ichthyofauna Aside from the effects of species introductions, the successive extirpation of fish species was almost entirely due to the increasing salinity (Ermakhanov et al. 2012 2013). There is no historical data on standing fish stock but figure 10 shows a clear correlation between the decline in commercially caught fish and increasing salinity. Simultaneously, the number of fish species decreased from the original 22 to only the Fig. 9. The Aral Sea regression crisis. A, The desert left by the vanished sea is slowly being populated by drought and salt resistant plants. B, The first, primitive dam over the Berg Strait. C, Local fishermen look from the north at the final Kokaral Dam over the Berg Strait. D–E, ESA Spot satellite images of the Small Aral Sea soon after the dam construction and showing the increase in area already during the first year. F, Commercial fishing from small boats in the reconstituted Small Aral Sea. page 24 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan species of introduced flounder by 1990. Since almost all native fish had a wide diversity in their diet, they were minimally affected by changes in the available food species, just shifting to other food assets. This lack of specialized diet was also crucial to their survival in localized habitats, and part of why they could eventually repopulate the partially restored Small Aral Sea (see below). The decline in fish stock and species diversity was in some part due to their reproductive behaviour. For species spawning in freshened parts of the sea, the fry were much less salinity tolerant than the adult fish. The increasing salinity in the general sea may also have negatively affected the maturation of reproductive products in the adults, although this issue is still debatable. Effect on birds It is obvious that the disastrous reduction in area and coastline of the Aral Sea must have had a significant effect on the birds associated with these habitats, but we are unaware of detailed studies. Migratory birds often depend on very specific resting grounds, and if these are damaged or disappear, it may have catastrophic effects because no alternative sites may exist (Reneerkens et al. 2005; Pennisi 2015). However, it can be hoped that the reconstituted Small Aral and the areas around the river deltas will still furnish enough habitats for the birds to avoid this kind of catastrophy. Just like mammalian wildlife, birds are an important asset for ecotourism, which may benefit the local economy and assist efforts at habitat conservation, although increased tourist flow by itself also poses a threat (Kumar and Sheryzasdanova 2021). For the flamingo, the present availability of brine shrimp may be an asset, but this food source is threatened by the increasing hypersalination. Fortunately, there are smaller saline lakes to the northeast of the Aral Sea (Lake Shalkar and Lake Tengiz) that offer alternative grounds. Flamingos are now even flocking in the small Maly Taldylkol lake in the center of Kazakhstan’s capital Astana, formerly called Nur Sultan (UN News 2021). Nevertheless, although some of these lakes are part of nature reservations, these localities are also threatened by human activities. Effects on macrophytobenthos The regression salinization also led to a catastrophic reduction in the biodiversity of macrophytes and the demise of most biocenoses just as was the case for the fauna. Of the flowering plants, the first extirpations due to salinization were the freshwater and freshwaterbrackish-water hydrophytes. Within a few years, freshwater pondweeds (Potamogeton) disappeared, followed by the more resistant Myriophyllum spicatum and comb pondweed Stuckenia pectinata. By the end of the 1970s, a few euryhaline species became the dominant species. During this time, the cover of reed thickets had become reduced to half their former coverage. They were at first restricted to a vast near shore zone out of the water but then disappeared completely in the 1980s (Zhakova 2013). Studies performed on the salinity gradient of the Karabayli archipelago (Dengina 1959) showed that reeds developed normally at a salinity of up to 18.5‰ but died at 24‰. Reed thickets of Schoenoplectus litoralis kasachstanicus disappeared at 16‰ salinity. New and rapidly salinizing shallow-water biotopes were rapidly overgrown with the halophilic annuals such as Zanichellia spp., Ruppia spp. and the characean Lamprothamnium papulosum. With a further increase in salinity above 25–26‰, these species also disappeared (Dengina 1954 1959; Husainova 1960). By the end of the 1980s there only remained Ruppia spp., Cladophora fracta, C. glomerata, Chaetomorpha linum, Rhizoclonium hieroglyphicum, Ulva intestinalis, and U. prolifera, all of which are able to withstand high salinity (Zhakova 2013). In the Small Aral, where salinity began to decrease in the late 1990s, the predominating macroalgae were Table 8. Water Balance of the Aral Sea (km3/year). Data from Agency of IFAS (Agency of IFAS for implementation of the Aral Sea basin) Time period Rivers inflow Gain from precipitation Loss to evaporation Water balance 1911–1960 56,0 9,1 66,1 -1,0 1961–1970 43,3 8,0 65,4 -14,1 1970–1980 16,7 6,3 55,2 -32,2 1981–1990 3,9 6,2 43,7 -33,6 1991–1994 21,0 4,6 33,6 -8,0 1995–2002* 4,81** 3,5 28,6 -20,29 *Estimations of the Institute of Geography of the Academy of Sciences of Kazakhstan. **number indicates inflow to the Small Sea, only. page 25 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan water body. Another plan is to increase the height of the existing dam, thus retaining a single uniform water body but with the benefit of an increase in area and water volume. Both these plans have clear consequences for the resulting biodiversity, including commercially important fish, and it is important that biological insight go together with socio-economic deliberations to arrive at the best possible solution (Plotnikov et al. 2021b). Aside from the retention and managing of water levels, the Small Aral may also face dangers from the inflowing waters. The intensive agriculture results in run off that contains both fertilizers and possibly unwanted levels of pesticides. General wash out of compounds, such as rare earth elements naturally present in the arid soil, may also be a problem. The extent to which this poses a problem is presently largely unknown. For the Large Aral Sea plans for improvement would be more costly and difficult to carry out. They would involve channelling waters from the Amu Darya, and ideally also from the Small Aral outflow, to selected areas to restore at least a limited and stable water body (Micklin 2016). Aside from partially restoring the size of the Aral Sea, such efforts would also have beneficial effects upon the climate in the area (He et al. 2022). Unfortunately, prospects are currently unfavourable for an impartial discussion, not to mention any direct action for improving the Small or Large Aral Sea. The present political situation makes this a very secondary issue to most decision makers. To this discussion, some voices opine that the Aral Sea crisis represents effects of climate variations rather than being caused by human activity. While this is unsubstantiated by scientific data, the opinion stems from recent unwillingness to criticize actions occurring during USSR rule. This problem is exacerbated by some local groups who do not welcome any outside interference. Finally, the entire Aral Sea situation involves several countries, both those directly bordering the original sea (Kazakhstan, Uzbekistan) and those through which the two tributary rivers flow and originate (Turkmenistan, Tajikistan, Kyrgyzstan and the People’s Republic of China). As for the World in general, any long-term plans for the water economy of the entire Aral Sea region can only be solved by scientifically informed cooperation and in close association with the needs and hopes of the local populations (Mukherji 2022). Table 9. Major changes to the Aral Sea fauna during the modern regression. Adapted and extended from Plotnikov et al. (2021b) Time Period Physical and chemical factors Main changes to invertebrates and fish fauna Pre 1961 Before regression Native semi-stable state Introduction of alien crustaceans (Mysida, Calanoida, Caridea) Introduction of planktivorous fish (smelt and herring) Introduced herring caused disastrous decline of all planktonic crustaceans) Massive deaths of planktivorous fish due to starvation 1961-1971 Initial salinization Slow salinity increased to 11.5 ppt Changes to crustacean fauna mostly due to introduced species Introduced caridean spread and displaced native amphipod Introduced calanoid displaced native calanoid Freshwater cladocerans started to decline Native fish species declined to only 11 spp 1971-1976 First crisis Salinity passed 12-13 ppt All freshwater and brackish water crustaceans disappeared Freshwater bivalves decline or disappeared 1976-1987 Relative stabilization Salinity continues to increase Many crustacean species declined and disappeared Euryhaline copepods, cladocerans and introduced shrimp now dominated Saltwater tolerant bivalves spread throughout entire sea Native fish declined to 6 spp. (by 1980) Plaice introduced and became only valuable species for fisheries 1987-1990 Second crisis Salinity passed 27-32 ppt Almost all crustaceans declined and disappeared Introduced calanoid only widespread planktonic crustacean Introduced mud crab and caridean shrimp only benthic crustaceans Commercial fisheries virtually ceased Post 1990 Separation of Large and Small Aral Large Aral became hypersaline Small Aral partially restored due to dam Small Aral: many native invertebrates and fish reappeared Commercial fisheries rebounded Large Aral: Artemia only common crustacean; all bivalves and gastropods disappeared; all fish disappeared page 32 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan CONCLUSIONS The regression of the Aral Sea was primarily caused by the ill advised diversion of water for irrigation with no long term considerations of the ecological effects. The resulting crisis involved a complex set of factors that disastrously affected the entire hydrology, biota and even climate of the region, and severely affected the surrounding human population, in terms of both health and economy (Table 9, Fig. 12). This should have served as a warning (UN News 2010), but unfortunately many saline ecosystems in world, most notably the Great Salt Lake, now face a situation with many of the same problems that affected the Aral Sea, including serious threats to human health (Oren 2018; Kintisch 2022; Derouin 2017). Yet it is encouraging that the partial recovery of the Small Aral Sea, by rather modest economical expenditure, almost immediately resulted in a rebounding fishery and improved economy. Although the topology of the Aral Sea was an important factor enabling the re-establishment of a healthy Small Aral, it is nevertheless clear that significant environmental improvement can sometimes be obtained by rather simple means, and have almost immediate Fig. 12. Causes and effects in the Aral Sea crisis. Hatched outlines show the extension of the sea after the regression had split it into two main bodies in the south and the Small Aral Sea in the north. (1) Diversion of water for irrigation resulted in (2) disastrously reduced inflow to the sea from ca.1960 onwards (3) Almost simultaneously planned species introductions seriously disturbed the native fauna (4) The reduced water inflow caused area regression, increased salinity and therefore serious reduction in biodiversity (5) Almost all fish disappeared with serious economic consequences for the human population (6) Sediment form the dried out sea bottom was blown into the air, causing very serious health problems for the human population (7) At the height of the area regression in 1990, the northern Small Aral Sea was connected to the vanishing Large Aral only through eastern Berg Strait, since the western Auzy-Kokaral Strait had dried out; a dam constructed in 2005 across the Berg Strait now retains inflowing water from the Syr Darya (8) Water level in the Small Aral Sea rapidly increased and salinity decreased; the fauna was reconstituted from refugial populations and fisheries recovered, where it again became an important commercial asset. (9) The larger part of the original Aral Sea continues to degrade into hypersaline water bodies and dried out salt flats with little or no metazoan life (10) The surrounding arid steppe now suffers from more extreme weather oscillations, since Aral waters no longer act as a buffer to temperature (11) In the future, climate caused, decreased precipitation at the sources of the two rivers will add to the shortage of water. page 33 of 38Zoological Studies 62:19 (2023) © 2023 Academia Sinica, Taiwan effects. At a recent event organized and held by a civil society organization “Turan lowland – Aral Sea,” further improvements of the Aral Sea area, such as those outlined above, were discussed (Ecomarathon 2022). At present, there is no final decision on specific actions, but we hope that plans can be agreed upon across national boundaries for further improvements of the Aral Sea system and its two tributary rivers to the benefit of these ecosystems, including the human population. Acknowledgments: This work was supported by the theme of the State assignment for 2022–2024 “Systematization and study of the dynamics of biological diversity and the functioning of ecosystems of continental water bodies under the conditions of anthropogenic impact and climate change” 122031100274-7. We are indebted to Prof. Brian Tsukimura for checking the English language. JTH is grateful to Prof. Carsten Rahbek for many fruitful discussions on biodiversity and conservation issues, without which he would never have ventured into this study. Authors’ contributions: All authors contributed equally to all aspects of this work. Competing interests: We declare NO competing interests. Availability of data and materials: The work is a “review”, so data availability is not an issue. Consent for publication: YES agreed by all authors. Ethics approval consent to participate: YES agreed by all authors. REFERENCES Agency of IFAS. 2022. Crisis of the Aral Sea. 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