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International Journal of Salt Lake Research 7: 301–343, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands.

Changes in the structure and function of biological communities in the Aral Sea, with particular reference to the northern part (Small Aral Sea), 1985–1994: A review N.V. ALADIN1, A.A. FILIPPOV1 , I.S. PLOTNIKOV1, M.I. ORLOVA1 and W.D. WILLIAMS2 1 Zoological Institute of Russian Academy of Sciences, 199034, St. Petersburg, Russia; 2 Department of Zoology, University of Adelaide, South Australia, 5005

Abstract. The construction of a dam between the Small and Large Aral Sea in 1993 has had an effect on biological communities. From field and laboratory investigations and analysis of the literature, the structure and function of the main plant and animal communities in the northern (Small) Aral Sea are described and discussed, first, for the period before 1985, and, second, between 1985 and 1994. A prognosis for future changes is provided. Key words: biological communities, Aral Sea, salinisation, salt lakes

1. Introduction The period from the end of 1980s to the beginning of the 1990s was significant for the northern Aral Sea for two reasons. First, from 1989, the connection between the Large (southern) and Small (northern) Aral disappeared (Figure 1); Berg’s Strait, a branch of the Syrdarya Delta, was reduced to a small stream (Aladin, 1989). Since 1989, the Large and Small Aral began to develop separately. Until 1989, their common feature was progressive salinisation, with similar rates in both basins (Figure 2). During this period, the main aquatic communities characteristic for a given salinity developed lake-wide (Aladin et al., 1996; Rusakova, 1994). In 1991, however, a number of natural and man-made events caused the Syrdarya’s outflow to increase and this led to a widening of Berg’s Strait. Some authors (Aladin et al., 1996) suggested that the widening would lead to the discharge of almost all of the outflow of the Syrdarya into the Large Aral and the desiccation of the Small Aral. Concern for this possibility, especially because of the relatively high human population density in the northern part of the Aral Sea region, led to plans to build dams to prevent the outflow of water from the Small to

302 the Large Aral Sea (Micklin, 1991; Aladin et al., 1996; Bortnic, 1978, 1980; L’vovich and Zigelnaya, 1978; Chernenko, 1983). These dams were regarded as one way to rehabilitate the Small Aral Sea. A dam across Berg’s Strait was started in 1993 and repaired after damage in 1994. This dam now prevents the loss of Syrdarya water from the Small Aral Sea and, despite occasional damage, has already resulted in increasing the water-level of the Small Aral Sea by 2–2.5 m. The second significant event in the period from the end of the 1980s to the beginning of the 1990s was a consequence of the first event: hydrological changes in the Small Aral Sea. Thus, in 1992–1994, certain parts of the dry lake bed flooded and there was a reduction in salinity (Figure 2, Table 1). By autumn 1993 and spring 1994, the water-level had risen 1.2–2 m and Bolshoi Sarychaganak Bay was recreated [henceforth referred to as Sarychaganak Bay]. This had been the largest bay of the Aral Sea and had had numerous coastal settlements, including shipping ports and the city of Aralsk. So far, reflooding and decreased salinity has had no obvious effect on the structure and function of biological communities, but such events are clearly prerequisites if at least some elements of the fauna and flora that existed before the regression of the water-level, and now existing only in refuges in deltaic areas and small freshwater terminal lakes, are to be restored. The increase in the water-level has led to the appearance of a wide belt of temporary, coastal water-bodies having considerable salinity gradients (Table 1). They are a source from which the biodiversity of the northern Aral Sea can be rehabilitated. Contemporary investigations of any part of the Aral Sea are now complicated by at least two problems: the absence of ports, and the inability of land expeditions to reach the water’s edge (for both direct and financial reasons). Thus, almost all hydrobiological investigations in 1985–1994 concentrated on coastal waters down to depths of 3–5 m, or occasionally deeper. Data were obtained during short-term expeditions without repetition during other seasons. Nevertheless, despite their fragmentary nature, the data are representative for most coastal regions (Figure 1) and, to some extent, can be extrapolated to the whole of the northern basin since most of this has depths of 3–6 m, apart from the central part and the middle of Shevchenko Bay. They also offer the chance to evaluate the nature of changes in biological communities resulting from the conservation actions in the northern Aral. With some caution, they may usefully serve for long-term monitoring of the status of the northern Aral. Since most of the literature on the Aral Sea is in Russian and not easily accessible to western scientists, part of this review considers the results

303

Figure 1. Study areas. 1, near Barsakelmes Island; 2, Tsche-Bas Bay; 3, Tastubek Cape (or Kockturnack Peninsula); 4, near Bugun’ settlement (east coast); 5, Shevtchenko Bay; 6, areas near the Syrdarya mouth and Berg’s Strait; 7, Butakov Bay; 8, Bolshoi Sarychaganack Bay. Dotted outline indicates the site of the coastline in 1960.

304

Table 1. Some environmental characteristics of the study sites (table continues on next page) District

Barsakelmes Is. Syrdarya St. 1–3

Syrdarya mouth Pre-mouth Bay St. 1 St. 1 St. 2 Berg’s Strait Near Bugun’ St. 1 St. 2 St. 3 St. 4

Date

Surface water Temperature (◦ C)

Transparency (m)

Depth (m)

Salinity

‰

Seston Total weight g m−2

15–25.05.90 24.05.92 24.05.93 22.05.92 24.05.93 25.05.92 22.05.93

– 20 17 19 17 16 17

– 0.3 0.1 0.3 0.1 0.7 0.65

– 0.6–0.7 2 (6–7) 0.6–0.7 2 1.5 1.4

30 1.8∗ 1.8∗ 1.8∗ 1.8∗ 7 18.5

– 22.4 329 – 376 19.4 27

– 0.74 3.42 – 5.67 4.31 21.86

27.05.92 22.05.93 27.05.92 27.05.92

16 16 20 –

>bottom >bottom >bottom –

1.2 1.4 2 0.8

22–23 22 25 –

9.1 – 10 10.2

0.67 – 0.93 1.02

30.05.93 31.05.93 1.06.93 1.06.93

17 18 18 20

>bottom >bottom 1.3 1.7

0.4 1.5 3 3

24 20 21 (surf.) 11.5 (0.5) 15.5 (bott.) 22

∗ After Bortnik (1990); ∗∗ Stations in temporary water-bodies at coast.

25 39 60 (surf.) 16.5 (0.5) 14 (bott.) 72.5

POM (in carbon) g m−2

4.73 1.66 1.51 (surf.) 2.04 (0.5) 3.59 (bott.) 19.36

Chlorophyll “a” µg L−1 – – 39.84 – – 13.06 – – – – – 11.05 5.04 41.72 11.02

Table 1. Continued. District

Butakov Bay Shevchenko Bay Tshe-Bas Bay Near Tastubek Cape St. 1 St. 2 St. 3 St. 4 St. 5 Sarychaganak Bay St. 1∗∗ St. 2∗∗ St. 3∗∗ St. 4 St. 5 St. 6

St. 7

Date

Surface water Temperature (◦ C)

Transparency (m)

Depth (m)

‰

21 12 15

>bottom >bottom >bottom

– 2–2.5 1.8–3

36 29.5 41

10.09.93 10.09.93 18.09.93 12.09.93 16.09.93

18 17 18 19.5 17.5

>bottom >bottom 0.6 1.8 1.8

0.5 1.3 3 5 6

– 17 17 25 25

22.06–26.06 22.06–26.7 22.06–26.8 23.06–26.06 23.06–26.06 25.06

18–35 19–35 20–35 19–30 25 23

>bottom >bottom >bottom >bottom >bottom >bottom

0.35–0.6 0.35–0.5 0.5–0.15 0.2–0.45 0.2–0.45 1.2

23–24 24–31 34–43 19–23 19–23 20

25.06

24

>bottom

1.4

20

1.06.92 09.92 09.92

Seston Total weight g m−2

POM (in carbon) g m−2

– – –

3.61 – –

177 63.75 48 48 50.75

2.20 2.13 2.11 2.19 2.19

– – – – – – – – – – –

70.05 101.89 125.95 27.72 12.94 (surf.) 9.94 (0.5) 7.47 (bott.) 8.84 (surf.) 8.22 (0.5) 12.21 (bott.) 10.84

Chlorophyll “a” µg L−1 – – – 14.33 11.64 3.66 9.02 2.09 11.82 184.61 145.97 12.18 11.06 (surf.) 20.39 (0.5) 11.47 (bott.) 31.23 (surf.) 40.25 (0.5) 32.08 (bott.) 34.62

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∗ After Bortnik (1990); ∗∗ Stations in temporary water-bodies at coast.

Salinity

306

Figure 2. Water-level and salinity in the Aral Sea from 1961 to 1994. After Orlova et al. (1998).

of investigations undertaken when the lake was in a quasi-stable state and during the first stages of the modern regression. Some of this literature is in English and German (e.g., Aladin, 1991; Williams and Aladin, 1991; Keyser and Aladin, 1991; Aladin and Potts, 1992; Aladin et al., 1993; Aladin and Williams, 1993; Aladin et al., 1996; Orlova et al., 1998). Most of this literature simply reviews data originally published in Russian. The review concentrates on those areas considered most representative for the the northern Aral Sea, including areas reflooded in 1993–1994, as well as some temporary water-bodies in the flooded areas of the dried bed. Major environmental data on the areas investigated in 1990–1994 are given in Table 1. The main criteria for selection of areas were the presence of different environmental gradients, particularly salinity. Brief descriptions of the methods used are given by Orlova et al. (1998, Table 2).

2. Nature of the biological community during quasi-stable conditions and subsequent desiccation and salinisation From the first investigations of the Aral Sea (Butakov’s expedition in the last century and Berg’s in the early forties) to the early 1970s, the lake was regarded as having an extremely low biodiversity, even when compared to the nearest large saline water-bodies, Lake Balkhash and the Caspian Sea. Moreover, most species were derived from groups of freshwater origin.

307 The first noticeable changes in the Aral ecosystem were observed after intensive acclimatisation of new species of aquatic organisms (Karpevich, 1975). Acclimatisation aimed to improve the biological productivity and usefulness of the basin for human needs, and community restructuring in the 1970s mainly involved the zooplankton and zoobenthos. Changes in species composition, abundance and community structure followed the establishment of introduced invertebrates and predator pressure from introduced fish. A further series of ecosystem changes followed lake regression from 1960 onwards. Three periods can be distinguished in these changes. The first period, a critical one, occurred in 1971–1975 as salinity rose to 12–14 g L−1 . During this period, most freshwater species became extinct. Then, in 1976– 1985, at a salinity of 14–22 g L−1 , there was a period of relative stability with a depressed biota. Finally, from 1985 to the present, only those communities able to exist in conditions of polyhalinity (salinity >23–24 g L−1 ) remain. In addition to salinity, two other significant factors have influenced the structure of biological communities: the fall in water-level and changes to inflows and the balance of nutrients. For the development of bottom communities, one further factor was important – an increase of areas covered by mud. The extension of areas with favourable light conditions to most areas in the northern Aral has also been important for the development of plant communities. The distinctiveness of the biota meant that all anthropogenic and natural environmental changes (e.g., introductions and desiccation with associated environmental impacts) rapidly gave rise to marked changes in the structure and function of biological communities. 2.1 Plant communities There are no definitive lists of plant species in the Aral Sea. According to Husainova (1958a, b), there were 12 species of higher plants and 82 species of algae, including 67 in the phytoplankton and 26 in the phytobenthos. Zenkevich (1963) noted 39 species of phytoplanktonic algae and 32 species of macrophytes. In a further study of the phytoplankton, the number of algal species and subspecies has increased to >300 (Pichkily, 1970, 1981; El’muratov, 1981, 1988). Almost all higher plants, except Zostera, were of freshwater derivation. During early regression (1971–1975), falling water-levels and changes in the nutrient balance, as well as the increase in salinity, greatly impacted plant communities. Unfortunately, regular and comprehensive investigations of plant communities were not undertaken during that time. Only intermittent studies were made. Even so, it seems that there was a rapid substitution of freshwater and oligohaline forms by mesohaline and halo-

308 phytic ones of marine and moderately saline origin, followed by a decrease in species diversity and quantitative abundance (Rusakova, 1994; Pichkily, 1981; El’muratov, 1988). The total biomass of all aquatic plants in the Aral Sea was estimated as 8.3 million tons (Karpevich, 1975) or 10 million tons (according to Berwald quoted in Karpevich, 1975). Of this, 90 per cent was contributed by benthic plants with a mean biomass of 80–100 g m−2 . The form and structure of planktonic and benthic producers in the Aral Sea was one of the most important features of the trophic structure of the biota in the quasi-stable period. It determined why benthic detritus trophic chains prevailed over planktonic ones. In turn, its structure was determined by the morphometric characteristics of the sea bed and the nutrient regime (Blinov, 1956; Karpevich, 1975; Novozhilova, 1973). There are no data on the impact of introduced species on the nature of benthic plant communities. Since lake recession, the main negative effect has been the decreased water-level, the drying of coastal areas, and the transformation of bottom sediments from sand to mud along all southern, eastern and northern coasts. The rapid and complete disappearance of former shallow areas and the degradation of all communities in and near deltas led to the extinction of all macrophyte species (Aladin and Kotov, 1989). From the beginning of the 1980s until the present, benthic plant communities are represented only by sea-grass (Zostera and Ruppia), green filamentous algae, and Bacillariophyta. These can settle on all types of modern sediments and can tolerate extensive changes in salinity. 2.1.1 Phytoplankton The first descriptions of planktonic algae were from material obtained by early expeditions (collected by Alenitzin in Butakov’s expedition and identified by Borshchov). The total number of species during the quasi-stable period differs according to author, as noted, but all investigators characterised the Aral Sea phytoplankton as having both low diversity and quantitative abundance (Karpevich, 1975; Zenkevich, 1963; Pichkily, 1981; Yablonskaya, 1964). The main algal groups were the Bacillariophyta, Dinophyta, Chlorophyta and Cyanophyta. Species diversity decreased from areas of reduced salinity to the Aral Sea proper. In the sea itself, only Bacillariophyta was abundant. Phytoplankton associations were dominated by several taxa, but Actinocyclus ehrenbergii var. crassa was the most abundant (Zenkevich, 1963). Phytoplankton biomass varied from 0.5–2.6 g m−3 (Pichkily, 1970).

309 2.1.2 Macrophytes Before the lake began to recede, extensive reed communities occurred in coastal waters where salinity was reduced. Apart from dense stands of semi-aquatic freshwater and oligohaline plants (according to Berwald (1964) quoted in Karpevich (1975)), fully aquatic communities of Potamogeton (P. perfoliatus, P. lusens, P. natans), Myriophyllum and Ceratophyllum were also well developed in the extensive and well-lit coastal zone. In fish spawning areas, eggs were often observed attached to P. crispus (Karpevich, 1975). Sea-grass was abundant on muddy sands in the northern-western parts of the Large Aral. Among the macro-algae, Vaucheria (13 per cent of total biomass), Cladophora (Chlorophyta) and Polysiphonia (Rhodophyta) were present in significant amounts. In many benthic communities, various species of Charophyta dominated and comprised 75 per cent of total biomass (Karpevich, 1975). 2.1.3 Primary production Direct measurements of photosynthetic rates and indirect estimates of primary production in the 1960s (Karpevich, 1975; Novozhilova, 1973; Yablonskaya and Lukonina, 1962) indicated that phytoplanktonic productivity was extremely low for all areas of the Aral Sea. They corresponded to production levels in oligotrophic waters (Novozhilova, 1973). However, delta areas appeared to have been mesotrophic. In general, daily total primary production was 50–55 mgC m−2 in the sea itself, and 275–650 mgC m−2 near deltas. Daily decomposition rates in the water column exceeded total primary production by a factor of 1.5–2. Thus, the role of allochthonous organic matter in the water column (e.g. organic input from rivers, the products of phytobenthic decomposition) was extremely important in ecosystem processes. The main causes for the low primary phytoplanktonic production has been attributed to the structure of the first trophic level; this led to low nutrient turnover rates. The structure and function of plant communities were also connected to the limited inflow of nutrients from rivers and other sources during the stable period. At this time, the concentration of soluble phosphorus was only 1–4.2 µg L−1 and dropped with depth to 0 (Novozhilova, 1973). However, at the beginning of the regression, 1961 to 1977, significant amounts of phosphorus were recorded; these came from fertilisers leached from fields into river waters following irrigation (Alekin and Liakhin, 1984). Additionally, a reduction in the areas of macrophytes increased nutrient turnover rates in the water column. Thus, the concentration of dissolved phosphorus became rather stable, with mean annual limits of 14–30 µg L−1 (Zizarin, 1991; Anon., 1990; Table 2).

310 Table 2. Mean phosphorus concentrations, 1960–1985. Data as PO4 µg L−1 in surface waters for several years. Site

Period

PO4

Syrdarya

1911–1960 1961–1970 1971–1980

6–11 48–93 12–42

Amydarya

1911–1960 1961–1970 1971–1980

10–13 14–20 11–22

Small Aral

1961–1965 1971–1975 1981–1985

3–5 5 15–20

Large Aral

1961–1965 1971–1975 1981–1985

3 5 20–55

Karpevich (1975) predicted that a significant restructure of the primary producer community, together with changes in the nutrient regime, would lead to changes in the productivity of the phytoplankton and an increase in it (but with some decrease in the early phases of regression). Unfortunately, however, there are no direct measurements of photosynthesis for the period 1970 to early 1980s. Even so, an increase in the transparency of the Aral Sea was noted (Aladin and Kotov, 1989; Anon., 1990). This provides some evidence of a decrease in the amount of seston, including phytoplankton, and suggests some decrease in the photosynthetic rate in the water column following low densities of phytoplankton. Investigations of transparency and depth (Bul’on, 1985) are relevant in this connection (see also Table 1). In July–August 1980, data on primary production at 28 stations (Table 3) indicated values lower than the 1960s (Aladin and Kotov, 1989). From 1976 to 1980, other investigators observed a relative decrease in the concentration of dissolved oxygen, especially in delta areas, and this may relate to a decrease in the photosynthetic activity of the phytoplankton (Anon., 1990). Considering all available evidence including that in the literature (Pichkily, 1981; Kravtzova, 1989), it seems that an increase of salinity from 8–10 to 13–20 g L−1 induced a succession of phytoplankton communities, different in species content and quantitative abundance, and a decrease in primary production for the period. Note that the barrier 12–14

311 g L−1 is also a “critical” one for the development of faunal communities in most inland waters (Karpevich, 1975; Aladin and Kotov, 1989; MordukhaiBoltovsky, 1972; Andreev and Andreeva, 1981). From 1980 until 1989, no further studies of primary productivity were carried out. Data on the relative contribution of the phytobenthos to total primary production are based only on phytobenthic and phytoplanktonic biomass values. Overall, it seems that the phytobenthic contribution was about 80–90 percent (Karpevich, 1975; Yablonskaya, 1964). 2.2 Animal communities During the quasi-stable period, 195 species of free-living (Yablonskaya, 1964) and 71 parasitic invertebrates (Dogel and Bychovsky, 1934) and 20 species of fish (Nikolsky, 1940) were recorded. These records are neither exact nor, for many higher groups in particular, complete. The number of species has also changed as taxonomic knowledge developed. Additionally, some authors (Husainova, 1958a; Zenkevich, 1963) did not consider the inhabitants of less saline and delta areas as Aral Sea species, even though these areas were extensive in southern coastal parts of the Aral Sea. According to Yablonskaya (1960), 17 per cent of free-living invertebrates were of Caspian origin, 18 per cent originated from inland waters that were fresh or mildly saline, and 5 per cent were of Mediterranean and Atlantic origin. According to Nikolsky (1940), the fish fauna included 3 complexes: remnants of an upper Tertiary fauna, representatives of an Aral-Caspian fauna (45 per cent), and representatives of a northern Siberian fauna. Most, however, were species of freshwater origin and also found in the Caspian Sea. 2.2.1 Zooplankton The zooplankton (Table 4) was first studied at the beginning of the present century when material was collected by an expedition of the Geographical Society under Berg in 1900–1902 and 1906. It provided data not only on zooplankton species composition, but also on their distribution in relation to salinity (Berg, 1908; Meisner, 1908; Zhernov, 1903). No further studies were made until the 1920s when some additional data were obtained by Karzinkin (1924) and Virketis (1927). All investigations only provided data on the qualitative characteristics of the zooplankton. Its abundance and biomass remained unknown. A new stage in investigations began in the 1930s when wider hydrobiological studies commenced under Behning. These provided not only a complete list of zooplankton species, but also the first data on their abundance (Behning, 1934, 1935). After Behning, a further significant contribution was

312

Table 3. Plankton primary production and decomposition at study areas in northern Aral in 1980 and 1989–1994.

District

Stations Season (total)

Near 282 Barsekelmes Is.1 1 Butakov Bay 2 1 Syrdarya 3 2 Syrdarya mouth 1 1 Pre-mouth Bay 2 1

Daily values Primary production Aopt A (µgC/L) (µgC/m2 )

0.7–0.8.1980 2.7–324.6 07.1989 177 09.1990 300 09.1992 332 05.1992 1440 05.1993 0–476 05.1992 134 05.1993 208 05.1992 401 05.1993 336

34.3–649.2 166.52 310–6502 830 – 0–47.6 93.8 135.2 521 470.4

Decomposition P D (µgC/m2 ) (µgC/m2 ) – – abs – abs abs abs 68 abs

A/D ratio DAN Source (µgC/µgChla)

– – 151.5 1.1 – – 1117 0.7 – – 900–8400 Near 0 128 0.7 176.9 0.8 434 1.2 603.4 0.8

30 – – – – 0–15 – 14 – –

Aladin and Kotov (1989) Dorbrinin et al. (1990)3 Dobrinin and Koroliova (1991)3 Orlova (1993) Orlova (1993) Orlova (1995) Orlova (1995) Orlova (1995) Orlova (1995) Orlova (1995)

Table 3. Continued.

District

Shevchenko Bay Tshchebas Bay The Bay near Bugun’ settle. Tastubek Cape Sarychaganack Bay

Stations Season (total) 1 2 24 2 24 3 35 24 2

09.1992 09.1992 05.1993 05.1993 09.1993 09.1993 06.1994 06.1994 06.1994

Daily values Primary production Aopt A (µgC/L) (µgC/m2 )

Decomposition P D (µgC/m2 ) (µgC/m2 )

A/D ratio

DAN Source (µgC/µgChla)

323 646 545 101 6.4 – 84–581 252–872 48–746 204–126 1.2–6.9 – 4289–883 2534–222 2313–95 221–127 11.5–1.7 372–175 3215–324 1142 abs 287 0.4 25–29 3054–3696 1525–4850 131–814 1462–3747 1.0–1.3 20–370 1416–2000 3501–64192 abs–2202 4110–5110 0.5–1.5 – 62–1496 15–1025 abs–169 367–1095 0.01–1.46 0.36–8 440–880 132–5282 abs–176 220–352 0.6–1.5 20–51 545–1496 352–12322 abs–600 528–572 0.7–2.15 12–31

Orlova (1993) Orlova (1993) Orlova (1993) Orlova (1993) Orlova and Rusakova (1995) Orlova and Rusakova (1995) Orlova (unpubl. data) Filippov et al. (in press)

A: total value of daily primary production. Aopt : total value in optimal light conditions. P: net value of daily primary production (P = A − D). D: daily value of decomposition. DAN: daily assimilation number. 1 Values from measurements based on depth and transparency by method of Bul’on (1985). 2 Integral values from curves of vertical distribution of primary production. Values without integral values by formula A = A ∗S (S, opt transparency: Bul’on, 1985). 3 Radiocarbon method used for determination of primary production; recorded value lies between total and net production. 4 Stations near shoreline. 5 Stations in temporary water-bodies connected to shoreline.

313

314 Table 4. Composition of the zooplankton. Modified from Andreev (1989) and updated. Zernov, Meissner, Virketis, Behning, Lukonina Atlas Andreev, Aladin, Plotnikov 1900 1901–1902 1925 1932–1933 1976–1981 1981–1988 1990–1994 Rotaria: Eosphora ehrenbergi Weber Trichocerca marina (Daday) Synchaeta vorax Rouss. S. cecilia Rouss. S. gyrina Hood S. tremula (Müller) S. neapolitana Rouss. Synchaeta sp. Brachionus plicatilis Müller B. quadridentatus Herm. B. calyciflorus Pallas Keratella cochleari s (Gosse) K. tropica (Apstein) K. quadrata (Müller) K. valga (Ehrenb.) Notholca squamula (Müller) N. acuminata (Ehrenb.) Lecane (Monostyla) lamellata (Daday) Colurella adriatica Ehrenb. C. colurus (Ehrenb.) Hexarthra oxyuris (Zernov) Collotheca mutabilis (Hudson) Cladocera: Diaphanosoma brachyurum Lievin Alona rectangula G. Sars Ceriodaphnia reticulat a (Jurine) Moina mongolica Daday Cercopagis pengoi aralensis M.-Bolt. Evadne anonyx G. Sars Podonevadne camptonyx (G. Sars) P. angusta (G. Sars) P. trigona (G. Sars)

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Copepoda: Arctodiaptomus salinus (Daday) + Calanipeda aquaedulcis Kritch. Cyclops vicinus Uljanin Cyclops oithonoides G. Sars Thermocyclops crassus (Fisher) Acanthocyclops viridis (Jurine) A. bisetosus (Rehb.) Mesocyclops leuckarti (Claus) + Halicyclops rotundipes aralensi s. Bor. Schizopera aralensis Bor. S. jugurta (Blanch. et Rich.) Cletocamptus confluens (Schmail) C. retrogressus Schmank. Halectinosoma abrau (Kritch.) Nannopus palustris (Brady Mesochra aestuarii Gurney Nitocra lacustris (Schmank.) N. hibernica (Brady) Onychcampus mohammed (Blanch. et Rich.) Canthocamptus sp. + Limnocletodes behning i Bor. Enhydrosoma birsteini Bor. Leptocaris brevicornis (Van Douwe) Paraleptastacus spinicauda trisetosus Noodt Copepoda parasitica: Ergasilus sieboldi Nordmann Paraergasilus sieboldi (Mark.) Caligus lacustris St. et Lut. Lamproglena pulchella Nordm. Lernea esocina (Burm.)

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Branchiura: Argulus foliaceus (Linne) Mollusc larvae: Dreissena sp. Hypanis sp. Syndosmya segmentum Recluz Cerastoderma sp.

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+ +

+ +

315 made by Lukonina and Kortunova; Lukonina (1960) was the first to obtain data on zooplankton biomass and seasonal dynamics, and, together with Kortunova, she found that the cause of the sudden changes in zooplankton at the end of the 1950s and the beginning of the 1960s was caused by introduced species (Kortunova and Lukonina, 1970). Also of interest in this connection is the work of Guseva (1961). In the 1960–1970s, changes caused by salinisation were investigated by Andreev (1989). He studied changes in species composition, distribution, abundance and biomass as salinity increased. In the 1980s, zooplankton studies became irregular. During that period, Aladin investigated the summer zooplankton near Barsakelmes Island (Aladin, 1989) and changes due to greatly increased salinity in some isolated and desiccating bays and gulfs of the Sea (Aladin, 1990). 2.2.2 Benthic macroinvertebrates Our first knowledge of benthic macroinvertebrates was gained in the 1840s. Lehman and Basiner collected material from coastal terraces and material from the sea itself came from Butakov’s expedition. Several species of bivalves and gastropods were recorded by Helmersen in 1845, Basiner in 1848, and Lehmann in 1852 (all quoted by Berg, 1908; Husainova, 1951; Butakov, 1853). The later investigations of Fedchenko and Alenizin (quoted after Behning, 1934, 1935; Husainova, 1971; Grimm, 1881; Beklemishev, 1922, 1923) expanded the list of known benthic macroinvertebrate species. MordukhaiBoltovsky (1974) summarised all available information. According to him, there were 44 species of macroinvertebrates in 16 families, 6 classes, and 3 higher groups. In the period 1970–1980s (Table 5), largely as a result of systematic studies of the gastropod Caspiohydrobia starob (Starobogatov and Andreeva, 1981; Issatullaev, 1987; Andreeva, 1987), the list was enlarged by some 20 species. Thus, by the end of the 1980s, 64 species of macroinvertebrates were known to have been present in the Aral Sea during the previous 100 years. Most were species of freshwater origin, including the Oligochaeta (10 species) and insect larvae (27 species). Species of Caspian origin (9) included two species of Dreissenia (Bivalvia) and Hypanis (Bivalvia), Theodoxus pallasi (Gastropoda), Gammarus aralensis (Amphipoda), and three introduced species (Mysidacea). A group of species of Mediterranean origin included two species of Cerastoderma (earlier regarded as a single species, Cardium edule) and four introduced species: Nereis diversicolor (Polychaeta), Palaemon elegans (Decapoda), Rhithropanopeus harrisii tridentata (Decapoda), and Syndosmya segmentum (Bivalvia). Species of Caspio-

316 Table 5. Composition of the benthic macroinvertebrate community. Data for 1971, 1976–1977 and 1980 after Andreeva (1991). Species

1971

1976–1977

1980

1990

Annelida Nereis diversicolor O.F. Muller Nais elenguis Muller Limnodrilus helveticus Piguet Potamothrix bavaricus (Oesch.) Psammorychtides albicola (Mich) Lumbricillis lineatus (Mull)

+ + + + + +

+ − − − − −

+ − − − − −

+ − − − − −

Arthropoda Paramysis intermedia (Czern.) P. ullskyi (Czern) P. lacustris (Czern.) Dikerogamarus aralensis (Ulljan) Palaemon elegans Rathke Rhitropanopeus harrisi tridentatus (Maitl.) Agrypnetes crassicornis Mcl. Oecetis intima Mcl. Pelopia villipennis Kieff. Procladius ferrugineus Kieff. Corynoneura sp. Tschern. Cricotopus gr. silvestris F. Tanytarsus gr. lobatifrons Kieff. T. gr. gregarinus Kieff. T. gr. lauterborni Kieff. T. gr. exiguus Joh. Polypedilum gr. scalaemun Schr. Cryptochironomus supplicans Meig. Cr. gr. defectus Kieff. Cr. gr. conjugens Kieff. Cr. gr. viridulus F. Limnochironomus nervosus Staeg. Chironomus behningii Goetgh. Ch. halophilus Kieff. Ch. salinarius Kieff. Glyptotendipes glaucus Mg. Gl. gripekoveni Kieff.

+ + + + + + + + + + + + + + + + + + + + + + + + + + +

+ − + − + + − + − − − − − − − − − − − − − − − + + − −

− − − − + + − − − − − − − − − − − − − − − − − + + − −

− − − − + + − − − − − − − − − − − − − − − − − − − − −

317

Table 5. Continued. Species

1971

1976–1977

1980

1990

Mollusca Dreissena polymorpha aralensis (Andr.) Dr. p. obtusecarinata (Andr.) Dr. caspia pallasi (Andr.) Cerastoderma rhomboides rhomboides (Lam) C. istmicum Issel Hypanis vitrea bergi Star. H. minima sidorovi Star. H. mimima minima (Ost.) Syndosmya segmentum Recluz Theodoxux pallasi Ldh. Caspiohydrobia covexa (Logv. et Star.) C. conica (Logv. et Star.) C. husainivae Star. C. kazakstanika Star. et Andreeva C. aralensis Satar et Andreeva C. obrutchevi Star. et Andreeva C. parva (Logv. et Star.) C. dubia (Logv. et Star.) C. curta (Logv. et Star.) C. gemmata (Kol) C. nikolskii Star. et Andreeva C. bergi Star. et Andreeva C. oviformis (Logv. et Star.) C. subconvexa (Logv. et Star. C. grimmi (Gless. et V. Dyb.) C. chrysopsis (Kol.) C. cylindrica (Logv. et Star.) C. behningii Star. et Andreeva C. sidorovi Star. et Andreeva C. nikitinskii Star. et Andreeva C. pavlovskii Star. et Issat. C. tadzhikistanika Star. et Izzat. C. sogdiana Star. et Izzat.

+ + + + + + + + + + ? + + ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

− − + − + − − − + + + + + + + + + + + + + + + + + + + + + + + + +

− − − − + − − − + − − − + + + + − − − + − − + + − − − + + − − − −

318 hydrobia comprised a separate group considered as originating from saline water-bodies of Central Asia’s arid zone (Andreeva, 1989). The year 1930, when Nikitinsky (1993) first obtained data on zoobenthic abundance and biomass, may be considered as the beginning of quantitative studies. Further studies (Behning, 1934, 1935; Husainova, 1951, 1958b; Yablonskaya, 1960) clarified the quantitative development, distribution and seasonal dynamics of individual representatives during the quasi-stable period. The data from these studies showed that until the 1960s benthic communities were rather monotonously distributed and not abundant. Average biomass from 1930–1957 was 22 g m−2 (Husainova, 1958b; Yablonskaya, 1960). Bivalves dominated, contributing 67 per cent of total biomass. A second important group was insect larvae, mainly Chironomidae (32 per cent of total biomass). Molluscs of marine origin and Oligochaetes contributed little (Yablonskaya et al., 1970). Investigations in 1960–1980 showed that freshwater species became extinct as the salinity increased (Figure 3) (Husainova, 1968, 1971; Yablonskaya et al., 1970; Gavrilov, 1970; Mordukhai-Boltovsky, 1972; Markova and Proscurina, 1974; Karpevich, 1975; Proscurina, 1979; Andreev and Andreeva, 1979, 1981, 1991; Aladin and Kotov, 1989; Andreev et al., 1990). At the same time, original euryhaline forms and introduced invertebrates of marine origin began to spread and form dense communities. In the 1970s, a constant increase of total macrozoobenthic abundance and biomass was observed (Andreeva, 1989; Andreev and Andreeva, 1979). By 1980, total biomass had increased to 196 g m−2 , 8.5 times more than the average annual value during the quasi-stable period (Andreev, 1989). In the 1980s, a further decrease in benthic species diversity and some stabilisation of abundance occurred. However, hydrobiological investigations in the main part of the Aral Sea practically ceased. Those that did occur were confined to occasional investigations in coastal areas near Barsakelmes Island (Aladin, 1989). They showed that by the middle of the 1980s, when average salinity was >23 g L−1 , the species diversity and abundance of the Ostracoda have decreased greatly. However, the total biomass of benthic macroinvertebrates was still gradually increasing and had reached 207 g m−2 by 1984. 2.2.3 Fish Of the 20 fish species inhabiting the Aral Sea before recent changes, 12 were significant for the fishing industry. The annual catch was 43,000–44,000 tons, 3–5 times less than that of the Caspian and Azov Seas. Benthivorous fish were the most important part of the catch (Karpevich, 1975). Accord-

319

Figure 3. Composition and biomass of benthic macroinvertebrates between 1954 and 1991. 1, freshwater species; 2, brackish-water species; 3, hypersaline species; 4, marine species; 5, Chironomidae; 6, Polychaeta; 7, Gastropoda; 8, Bivalvia. Data for 1954–1980 from Andreeva (1989); data for 1989 from Andreev et al. (1990); data for 1990, 1991, original data.

320 ing to all investigators, the low productivity of fish was because spawning areas for most species was limited and food was inadequate for the young stages. Adults had enough food only in shallow waters, of depth 400 g m−2 , and chlorophyll values in a thin upper layer of the bottom sediments were >1000 mg m−2 (Orlova, 1995) [cf 77 near Bugun in May 1993]. Thus, in most coastal areas over the period 1985–97 (spring–early summer and autumn seasons), the plankton dominated both photosynthetic processes and organic decomposition (Table 6). In delta areas, the water column and benthic oganisms contributed about equally to these processes. The situation in the recently flooded Sarychaganack Bay was quite different. There, bottom macrophytes had developed extensively and covered about 30 per cent of the bay itself and, in temporary embayments, about 60–70 per cent. The biomass of some patches exceeded that for all other areas studied and sometimes reached 4,000 g m−2 (Filippova et al., in press). Plankton biomass, however, was 90 per cent of zooplankton abundance and biomass. While considerable changes in the composition have occurred, the principal groups remain copepods and larval bivalve molluscs, as in the past. Originally, the fauna comprised the copepod Arctodiaptomus salinus and larvae of the bivalves Dreissena and Hypanis. These elements have been replaced by marine euryhaline species. The main element since the 1970s is the copepod Calanipeda aquaedulcis. In spring and summer, significant

328 Table 7. Zooplankton abundance and biomass in the Aral Sea, 1992–1994. A: abundance as 103 ind/m3 ; B: biomass as mg/m3 Site

Large Sea Tsche-Bas Gulf Small Sea Northern coast Eastern coast Shevchenko Gulf Syrdarya Syrdarya Berg’s Strait Butakov Bay Sarychaganak Bay Sarychaganak Bay Sarychaganak Bay Sarychaganak Bay

Season Rotifers Crustaceans A B A B

Larval molluscs Total A B A

09.92 09.93 05.93 09.92 05.92 05.93 05.92 05.92 05.93 09.93 06.94 09.94

0.2 0.4 0.1 0.3 23.8 52.9 0.0 0.0 70.9 156.1 18.2 40.0 3.3 7.2 43.2 95.1 33.3 73.2 0.5 0.0 14.6 32.0 0.2 0.4

1.4 0.1 0.8 2.8 0.6 0.4 0.0 4.6 1.5 0.4 0.7 0.8

2.4 8.5 36.6 0.2 4.4 18.6 1.4 45.6 100.7 5.0 2.6 7.7 1.1 172.2 547.9 0.8 31.7 68.2 0.1 60.0 158.7 8.3 9.3 29.6 2.7 304.4 649.5 0.8 27.0 94.1 1.3 29.9 62.5 1.5 1.7 5.3

10.0 4.6 70.0 5.0 243.7 50.6 63.3 57.2 304.8 27.9 45.2 2.7

B 39.4 19.3 155.0 12.7 705.0 109.0 166.0 132.9 725.4 95.6 95.8 7.1

numbers of larval Syndosmya segmentum (=Abra ovata) and Cerastoderma isthmicum also occur because these species reproduce at that time, with larval S. segmentum dominant, reflecting the importance of this species in the zoobenthos (Andreev, 1989; Filippov, 1991, 1993a, b, 1994; Andreev and Andreeva, 1991; Andreev et al., 1990, 1992; Plotnikov, 1995). Sometimes, mollusc larvae are more important than crustaceans. Additionally, rotifers are usually present, though not an important element; most are species of Synchaeta, mainly S. vorax. In all areas of the Small Aral Sea investigated and in the Tsche-Bas Gulf of the Large Aral Sea, Halicyclops rotundipes aralensis occurs, though not usually in large numbers. In the Small Aral Sea near the channel in Berg’s Strait and in the salinised Butakov Bay, remnant populations of the cladoceran Podonevadne camptonyx survive. All other species in the zooplankton occur rarely. Although the zooplankton is broadly similar in both parts of the Aral Sea, diversity is somewhat higher in the Small Aral Sea. The higher salinity of the Large Aral Sea now precludes the existence of Caspian Podonidae there. However, in spring, the zooplankton of the Small and Large Aral Sea differs in quantitative composition. In the Small Aral Sea, larvae and adult crustaceans dominate, but, at least in the Large Aral Sea near Barsakelmes Island, bivalve larvae dominate. Because of its higher salinity, Butakov Bay is similar to the Large Aral Sea in this respect (Table 7). The principal seasonal changes in the zooplankton during the first half of the 1990s were similar to those of the 1970s. In spring, C. aquaedulcis

329 (sometimes with Halicyclops rotundipes aralensis in the Small Aral Sea) and larval bivalves dominated. In September, copepods still dominate but as mollusc reproduction ceases the number of bivalve larvae falls significantly until they disappear altogether (Table 7). Spatial differences in species composition and quantitative abundance occur, reflecting differences in local hydrological and hydrochemical conditions. (i) The Large Aral Sea near Barsakelmes Island. In spring and summer 1990–1991, the zooplankton of the coastal waters near Barsakelmes Island was dominated by larval S. segmentum and C. isthmicum. They comprised 77–98 per cent of total zooplankton abundance. The rest consisted of C. aquaedulcis and S. vorax (Table 7). The present zooplankton biomass is similar to that of the 1980s (Aladin, 1989), the main difference being the absence of any contribution from P. camptonyx. (ii) Large Aral Sea; Tsche-Bas Gulf. At the end of September 1992, the zooplankton mainly consisted of adult C. aquaedulcis and copepod nauplii. There were extremely few mollusc larvae. The zooplankton was similar throughout the Gulf except in the centre where rotifers were present in higher abundance and biomass (Table 7). (iii) The Small Aral Sea near Kokturnak Peninsula. In autumn 1991, the zooplankton had more mollusc larvae than copepods, an uncommon feature for this season. Zooplankton biomass (135 mg m−3 ) was lower than in August 1991 (533 mg m−3 ) (Andreev et al., 1990). In autumn 1993, C. aquaedulcis and H. rotundipes aralensis dominated the zooplankton and the abundance of larval molluscs was low, as is usual for this season. Zooplankton abundance and biomass increased significantly eastwards to Mt Trekhgorka and Sarychaganak Bay (Table 7). (iv) The Small Aral Sea near its eastern coast at Bugun. In May 1993, the quantitative characteristics of the zooplankton differed spatially according to their depth and distance from the shore. Abundance and biomass were higher above shoals (203,300 ind. m−3 and 437 mg m−3 ) than at depths of 4–6 m (13,700 ind. m−3 and 75 mg m−3 ). Mean values were (70,000 ind. m−3 and 155 mg m−3 ) (Table 7). (v) The Small Aral Sea at Shevchenko Bay. In autumn 1992, zooplankton composition was typical for the season. However, the abundance of Synchaeta vorax (2800 ind. m−3 ) was very high. Because of this, total zooplankton biomass was not high (22 mg m−3 ) (Table 7). (vi) The Small Aral Sea near the delta of the Syrdarya. This area is of special interest because of its particular hydrological regime. Apart from its shallowness, it is also subject to variable inputs of fresh water from

330 the Syrdarya. A stable salinity gradient is present, ranging from that of fresh water to that of the open Small Aral Sea. The Syrdarya also contributes biogenic elements, suspended organic matter and minerals. In May 1992, zooplankton abundance and biomass was very high, reaching a mean of 244,000 ind. m−3 and 705 mg m−3 . In May 1993, values were significantly lower: 50,600 ind. m−3 and 109 mg m−3 (Table7). Because of difficulties in determining site positions, however, sampling sites were not exactly the same in 1992 and 1993. Differences, therefore, may be attributable not only to changes in zooplankton development and structure, but also to local differences in these changes. In the past, this part of the Aral Sea was one of those with the highest zooplankton abundance and biomass (Lukonina, 1960; Andreev, 1989; Kortunova, 1975). (vii) The Small Aral Sea at the former site of Berg’s Strait. Zooplankton from shoal regions of the Small Aral Sea south of the Syrdarya delta wash out through the channel in Berg’s Strait. Here, zooplankton abundance and biomass were high. Composition differed from that in the adjoining part of the Small Aral Sea near the Syrdarya delta. Copepods were dominant and, in May 1992 and 1993, zooplankton consisted almost entirely of C. aquaedulcis and copepod nauplii. This probably reflects peculiarities of this region’s benthic fauna, since no bivalves occur on the bottom. Larval bivalves in the plankton are clearly carried by currents from elsewhere. (viii) Isolated bays of the Small Aral Sea; Butakov Bay. Butakov Bay is one of the few remaining bays and gulfs of the Aral Sea. All forms found in its zooplankton are euryhaline and are typical for the modern Aral Sea fauna (Andreev, 1989; Aladin, 1990; Andreeva, 1989; Andreev et al., 1990). Zooplankton abundance and biomass in September 1990 was very low and bivalve larvae were unusually dominant (more than 90 per cent) for this season (Andreev, 1989). At the end of September 1991, the zooplankton was more typical and quite different (Table 7). Perhaps the zooplankton here is strongly influenced by local factors, including hydrological ones. However, insufficient data preclude a more certain explanation. (ix) Isolated bays of the Small Aral Sea; Sarychaganak Bay. In spring 1993, by which time the level of the Small Aral Sea had risen by >1 m after Berg’s strait had been dammed, the southern part of Sarychaganak Bay refilled with water. This gulf had become isolated from the Aral Sea in 1987 and had largely dried up except for a few hypersaline remnants. It is now a relatively large water-body with depths up to 2 m and connected to the Small Aral Sea by a narrow opening. Restoration of planktonic and benthic communities is now taking place.

331 In 1993 and 1994, the zooplankton comprised C. aquaedulcis and H. rotundipes aralensis, P. camptonyx, larvae of S. segmentum and C. isthmicum, and S. vorax. Zooplankton abundance and biomass in May 1993 was almost ten times that of the open Small Aral Sea (Table 10). The accumulation of biogenic elements in terrestrial plants when the gulf dried, and their subsequent release after reflooding, may explain this (Kuznetsov et al., 1993; Orlova, 1995). Such conditions would favour the intensive development of zooplankton, especially the phytodetrivorous C. aquaedulcis. In September 1993, however, aside from the usual decrease in the abundance of larval molluscs, zooplankton abundance and biomass as a whole sharply decreased. In 1994, zooplankton in the gulf was different from that in 1993. In June, abundance and biomass were near those of September 1993, and almost one third of both abundance and biomass consisted of bivalve larvae. By autumn 1994, abundance and biomass had decreased tenfold. This decrease, at least in spring, may be explained by the low biomass of phytoplankton (unpublished data of O.M. Rusakova) and relatively low primary production. Most primary production during this period was by microphytobenthos and patches of Zostera and Ruppia covered by periphyton (unpublished data, M.I. Orlova, O.M. Rusakova, L.V. Zhakova). From these observations it seems that the behaviour of the biota in this recently refilled part of the Small Aral Sea is analogous to that when reservoirs first fill: an initial bloom followed by decreased abundance and biomass. Returning to the lake as a whole, of the 8 species of rotifers originally found in the zooplankton of the open parts of the Aral Sea (Andreev, 1989, 1990), only 6 remain (Table 4), of which only S. vorax and (in part) S. cecilia are widespread. The diversity of cladocerans has also decreased; 5 species occurred at the end of the 1970s (Andreev, 1989, 1990), 4 species disappeared during the 1980s (Aladin, 1989; Plotnikov et al., 1991; Orlova, 1995; Aladin et al., 1991), and now just P. camptonyx remains (only in the Small Aral Sea) probably as the most halotolerant. Of the 5 species of Copepoda (excluding Harpacticoida) recorded at the end of the 1970s, only 2 species remain now, but only the introduced C. aquaedulcis is found in large quantities (Plotnikov, 1995; Andreev, 1989, 1991). Species diversity of the meroplankton did not change much during the 1980s because benthic macroinvertebrate species composition remained stable (Filippov, 1991, 1993a, 1994; Andreev, 1990; Andreev et al., 1990). During the ‘crisis’ of the second part of the 1980s, nearly all Caspian-derived species became extinct because a salinity level of 23–25 g L−1 was not tolerable (Plotnikov et al., 1991; Aladin, 1990; Aladin

332 et al., 1991; Husainova, 1961). In the Small Aral Sea, where mean salinity has not exceeded this value because of hydrological stabilisation, elements of the Caspian-derived fauna persist; on the other hand, in the Large Aral Sea, where salinity exceeded this limit and continues to increase, this fauna is now absent. From the end of the 1950s to the beginning of the 1990s, the changes to the Aral Sea zooplankton as a whole occurred because of progressive salinisation and successful introductions. The original fauna, consisting of fresh and brackish-water species, had disappeared by the beginning of salinisation and replaced by local or introduced species of marine origin, or halotolerant forms from saline continental waters of the arid zone. All Caspian-derived species except P. camptonyx had become extinct (Table 4). Except for the meroplankton, the zooplankton of the Aral Sea now has only a fifth of the species originally present. During 1991–1994, diversity remained unchanged. Thus, it is now stable in the Large Aral Sea again, but remains unstable in the Small Aral Sea where salinity is at a critical level of 23–25 g L−1 . 3.2.2 Benthic macroinvertebrates Detailed changes in the nature of the zoobenthos during most of the 1980s can only be based on investigations of benthic communities near Barsakelmes Island. These indicated a further decrease in the species diversity of benthic invertebrates in the Aral Sea. In 1984, only the following species were found: Abra ovata and C. isthmicum, Caspiohydrobia spp. and Theodoxus pallasi, Nereis diversicolor, P. elegans, R. harrisi tridentata and Cyprideis torosa, Amnicythere cymbula, Tyrrhenocythere amnicola donetziensis and Limnocythere elegans. By 1987–1988, almost all ostracods, except C. torosa, had become extinct. By the end of the 1980s, T. pallasi had also disappeared (Aladin, 1989). By the mid-1980s, zoobenthic biomass reached 207 g m−2 , stabilised, and then begun to decrease gradually. By the end of the 1980s, values for average biomass near Barsakelmes Island were 121 g m−2 (Aladin, 1989). Less detailed investigations of benthic macroinvertebrates elsewhere in the Large and Small Aral Sea indicated that changes were generally similar to those near Barsakelmes Island (Andreeva, 1981, 1983, 1984; Andreeva and Andreev, 1987; Andreev and Andreeva, 1991; Filippov, 1991; Lim and Ermakhanov, 1986; Filippov et al., 1993a, b). In 1989, after an interval of ten years, investigations of the open waters of the Aral Sea recommenced (Andreev et al., 1990). These showed that the total biomass of the zoobenthos in the Large Aral Sea was 108 g m−2 , and in the Small Aral Sea, 247 g m−2 . In benthic communities, only bivalves, gastropods (excluding T. pallasi), N.

333 diversicolor, P. elegans, and R. harrisii tridentata were present. Larvae of Chironomidae were extremely rare. More or less regular expeditions to the Small Aral Sea took place from the beginning of the 1990s (Filippov, 1991, 1993a,b; Filippov, 1993; Lim and Ermakhanov, 1986). These aimed to determine species composition and abundance, and the spatial distribution of benthic invertebrates. Few changes in the nature of the zoobenthos occurred in 1990–1993 (Table 5). All benthic communities present at the end of the 1980s were found (Aladin and Kotov, 1989). This lack of significant change, despite a marked increase in salinity (from 27 g L−1 in 1989 to 41 g L−1 in 1993), is noteworthy. The investigations also revealed the extremely monotonous character of benthic communities. Species composition was the same in almost all areas investigated. In the Small Aral Sea, it comprised S. segmentum and C. isthmicum, Caspiohydrobia spp., N. diversicolor and P. elegans. In the Large Aral Sea, R. harrisi tridentatus also occurred. Syndosmya segmentum usually had the greater biomass, but Caspiohydrobia spp sometimes formed dense populations also (Figure 3, Table 5). Of all areas investigated, the former Berg’s Strait and the delta were noticeably different: bivalves were almost absent, polychaetes contributed most of the biomass and abundance, and total biomass was low. These features evidently reflected the reduced salinity, the greater amounts of suspended material (Table 1), and the unstable hydrological regime. Elsewhere in the Small Aral Sea, notwithstanding strong salinity gradients (20–40 g L−1 ), the composition and structure of the benthic fauna did not differ significantly. Like plant and zooplankton communities, the benthic fauna of Sarychaganak Bay was significantly different from elsewhere in the Small Aral Sea. The difference is connected to the Bay’s former isolation and the complete destruction of all communities when the Bay was dry or highly salinised. For the two years after the Bay’s reflooding, benthic communities have been in a stage of development. By 1994, all typical elements of the Small Aral Sea fauna were present, but bivalve populations had not completely reestablished. Overall, the benthic macroinvertebrate biomass of the Small Aral Sea was higher than in the Large Aral Sea, evidently a consequence of higher rates of primary production and larger inputs of organic matter from the Syrdarya (Tables 1 and 3). Studies of vertical distribution showed that the abundance of all species and groups was dependent on depth. Total biomass was generally least in the shallowest and deepest parts, and greatest at intermediate depths (in the Small Aral Sea, 2–8 m, in Butakov Bay, 1 m, in the Large Aral Sea, 2.5–15 m).

334 The distribution also probably reflects morphometric characteristics, which in turn determine wave strength in shallow areas. Thus, hydrological and biotic factors, as well as salinity, will affect future zoobenthic communities. 3.4 Microbial communities 3.4.1 Bacterioplankton Modern studies of the bacterioplankton began in the years 1989–1992 (Dobrinin et al., 1990; Dobrinin and Koroliova, 1991; Sulalina and Smurov, 1993). In the Small Aral Sea, bacterioplankton is mainly represented now by rod-like forms and to a less extent by coccoid (no more than 10 per cent of total number), filamentous, and spiral forms (Sulalina and Smurov, 1993). In relation to salinity, the bacterial community comprises forms of different salinity resistance. Obligate freshwater forms have been isolated as well as halophiles which grow on substrates to which has been added 200 g L−1 of NaCl. According to various authors (Dobrinin et al., 1990; Dobrinin and Koroliova, 1991), both freshwater and halophilic forms are allochthonous and originate from the numerous temporary freshwater to hypersaline pools at the coast. Most bacteria grow in media of salinity close to the environmental salinity. Bacterial numbers are more or less similar in all areas investigated (Table 12), except near the mouth of the Syrdarya. At present, total numbers in the Aral Sea itself and the isolated Bay (Butakov) vary from 0.7 to 2.4 × 106 ml−1 , but tend to increase near the bottom. These values are greater than those obtained by Novozhilova (1973) during the quasi-stable period. At the mouth of the Syrdarya, bacterial numbers were positively correlated with the concentration of particulate organic carbon (POM) (Sulalina and Smurov, 1993). Comparing these data with those for the Caspian Sea shows that bacterioplankton of the Small Aral Sea and the northern and eastern areas of the Caspian Sea has similar features. In the 1970–80s, these areas were characterised as the most productive (Salmanov, 1987). The numbers of heterotrophic bacteria have also increased since the 1960s. Their distribution in the Sea itself was also more or less homogeneous, but with some increase near Barsakelmes Island (Dobrinin et al., 1990). At the mouth of the Syrdarya, numbers were again strongly dependent on POM concentration (Sulalina and Smurov, 1993). Absolute values for daily bacterial production in the Small Aral Sea are now also higher than they were in the 1960s (Novozhilova, 1973; Sulatina and Smurov, 1993). At the same time, the P/B ratio (P production; B biomass) is significantly less than mean P/B at the end of the 1960s, with some decrease in bacterial production.

335 4. Discussion and prognosis During prehistory, the salinity and water-level of the Aral Sea were primarily controlled by regional climatic factors. These caused changes to river discharge. In historical times, however, human actions, mainly irrigation, war, and economic and political decisions, have become the dominant factors controlling salinity and water-level fluctuations. Even so, recent environmental events in the Aral Basin are not new; similar events have occurred several times previously. The only really “new” event is chemical contamination of water and land resulting from the use of chemical fertilisers, pesticides and defoliants. During the last regression, mainly the result of anthropogenic activity and also characterised by rapid salinisation and desiccation, the biota of the Small Aral Sea underwent comprehensive destruction and transformation during certain critical periods. Since the 1980s, however, new communities have appeared which are adapted to the polyhaline environment. Now, local morphometric differences and their effects on sediments, relief, depths, and wind and wave action are more important than salinity. In the 1960s, environmental changes were accompanied by significant changes in the structure of plant communities. During the progressive salinisation of the 1970–1980s, salinity was a major determinant of both planktonic and benthic plant communities, which at the beginning consisted mainly of freshwater forms. In most areas except the delta, freshwater forms were gradually replaced by halotolerant forms (euryhaline species, oligomesohalobes and halophilobes) in both planktonic and benthic communities. Most species and groups in the phytoplankton are now also resistant to variations in nutrient concentrations. As a result, present phytoplankton diversity and biomass is higher, as well as its contribution to total plant biomass following the mass development of groups resistant to rapid environmental impact. In the development of benthic plant communities, sediment redistribution, as well as salinity, was an important factor too. Macrophyte species diversity and mean total biomass have fallen compared with the quasi-stable period. Summarising, changes in the structure of the first trophic level were clearly determined by salinity during the initial stages of regression, but later by changes in nutrient concentration, light conditions, coastline formation, and the redistribution of bottom sediments. Experiments show that primary production in the water column is now greater than during the quasistable period and period of progressive salinisation. A marked contribution to primary production comes from coastal areas down to a depth of 5 m, and especially from formerly desiccated areas. Almost everywhere, however, the phytoplankton species are the main primary producers.

336 The zooplankton and benthic fauna also comprise new communities adapting, or already adapted to, high salinity. They are characterised by extremely low and stable species diversity, and a high tolerance to changes in salinity (and probably other environmental factors). Present benthic communities contain large numbers of animals, especially in the Small Aral Sea, clearly reflecting high amounts of allochthonous and autochthonous organic matter. Planktonic crustaceans and rotifers are characterised by low species diversity and abundance. Salinity is not now the main environmental factor for the zooplankton and zoobenthos. Important factors now include water depth, transparency, and the nature and extent of wind and wave action, i.e. local morphometric characteristics (especially in isolated bays), and climatic conditions. The position in the recently reflooded Sarychaganak Bay, however, is different. Here, communities have not yet fully developed after desiccation, as indicated by variations in abundance and a biodiversity lower than in the Aral Sea itself. With regard to the bacterioplankton, comparison of data from the 1960s with later data indicates great changes in the nature of the bacterial community over the past 25–30 years and points to its increased ecological role. So far as a prognosis is concerned, at least three main future scenarios can be envisaged for the biological communities in the Small Aral Sea following the commencement of the present conservation program. First, should the construction of the dam be unsuccessful, further desiccation of the Small Aral Sea will occur since Syrdarya flows (Figure 2) will discharge to the Large Aral Sea. Plant diversity will decrease following the extinction of all remaining oligohaline and mesohaline species. Only halophilic algae now present in the basin (Table 6) would persist at a salinity >40 g L−1 in hypersaline, temporary, coastal water-bodies. There have been some suggestions that the present hypersaline flora could persist at salinities >42 g L−1 to saturation levels (Aladin, 1991). Experiments on invertebrates (Plotnikov, 1995; Filippov, 1994; Aladin, 1995), and information on the distribution of invertebrates in hypersaline basins (Kchlebovich et al., 1989; Kinne, 1971), indicate that organisms of marine origin will keep their dominant role up to 60–70 g L−1 . Further salinisation will result in their replacement by halophilic organisms of inland origin. At salinities >70 g L−1 , only a few species will survive, mainly larvae of Chironomidae and Ephydridae (Zeeb, 1982; Hammer, 1986; Timms et al., 1986), as already found in hypersaline coastal pools in the Small Aral Sea. These groups will enter before salinity reaches 80–90 g L−1 . Caspiohydrobia spp. may be able to survive to 100– 110 g L−1 . Further changes in animal communities will be determined by the adaptive abilities of the halophilic fauna. However, even halophilic organisms

337 are generally not able to tolerate salinities >200 g L−1 , and are found only rarely at salinities of 200–300 g L−1 (Kinne, 1971; Hammer, 1986). A second scenario involves the conservation of water-levels similar to those found at the start of the 1990s. In this case, the present communities of the Small Aral Sea will persist. A third scenario involves the rehabilitation of the Small Aral Sea after the construction of major dams to prevent uncontrolled loss of water from the basin into the Large Aral Sea. In this case, the water-level may be expected to increase and salinity to decrease. Field and laboratory experiments and field observations, at a variety of natural environments, point to the restoration of all communities (Karpevich, 1953, 1975; Husainova, 1959; Kiseleva, 1960; Murdukhai-Boltovsky, 1960; Zeeb, 1961; Zenkevich, 1963; Kchlebovich and Ardab’eva, 1989; Kravtzova, 1989; Andreev et al., 1990; Filippov, 1994). Recolonisation by a freshwater and oligohaline fauna and flora, presently inhabiting refuges in the delta region and on the coast, could commence only after salinity had decreased to 15–17 g L−1 . This process could be hindered by competition with euryhaline organisms now present. Only after salinity had reduced to 10 g L−1 , might a moderately halotolerant and freshwater biota gradually replace marine elements.

Acknowledgements The authors are grateful to O.M. Rusakova, L.V. Zhakova, O.A. Smurov and D.D.Piriulin for information on phytoplankton and phytobenthos, bacterioplankton and the insect fauna in temporary water-bodies. Most expeditions since 1993 were possible only by the direct financial support of BMFT of Germany and the Environmental program of UNESCO. We are also indebted to the representatives of these programs, Drs Dietmar Keyzer and Vefa Moustafaev. Scientific research and manuscript preparation were supported by the Russian Fundamental Fund (Project No. 96-04-48-114) and by INTAS (Project 93-1491). Mr Bob Lewis, University of Adelaide, assisted with preparation of the manuscript and is thanked.

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