Five years of field hydrographic research in the Large Aral Sea (2002–2006)

Journal of Marine Systems 76 (2009) 263–271

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Journal of Marine Systems j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / j m a r s y s

Five years of field hydrographic research in the Large Aral Sea (2002–2006) P.O. Zavialov a,⁎, A.A. Ni b, T.V. Kudyshkin c, A.K. Kurbaniyazov d, S.N. Dikarev a a b c d

P.P. Shirshov Institute of Oceanology, 36, Nakhimovskiy Prospect Ave., Moscow, 117997, Russia Institute of Geology and Geophysics, 49, Khodzhibayeva Str., Tashkent, Uzbekistan Research Hydrometeorological Institute, 72, Makhsumova Str., Tashkent, Uzbekistan Nukus Institute of Education, 104, Dustnazarova Str., Nukus, Uzbekistan

a r t i c l e

i n f o

Article history: Received 20 December 2006 Received in revised form 8 March 2007 Accepted 12 March 2008 Available online 8 August 2008 Keywords: Aral Sea Hydrographic regime Thermohaline structure Circulation

a b s t r a c t The paper is intended to provide a summarizing account of hydrographic research activities on the Large Aral Sea since 2002. To date of the writing (August, 2006), 6 field surveys have been accomplished, some of which consisted of separate legs covering different parts of the lake. The 3D thermohaline structure was investigated by means of surface-to-bottom profiling from motor boats at various hydrographic stations. In a part of the cruises, direct observations of water velocity and lake surface height were also conducted from mooring stations at specific locations. The measurements were accompanied by water sampling and collection of meteorological data. Over the period of the observations, the surface salinity in the western, relatively deep basin has increased from about 82 g∙kg− 1 in November 2002 to over 99 g∙kg− 1 in March 2006. Because the lake surface level remained nearly constant during this period, spanning only slightly between 30.1 and 30.7 m above the ocean level, and the fluvial and groundwater inputs of salts are likely negligible at the temporal scale of a few years, this continuing salinity build-up in the western basin must be associated with water exchanges with the shallow and saltier eastern basin. In the latter, salinity has decreased from up to 160 g∙kg− 1 as previously reported for 2002 [Mirabdullaev, I.M., I.M. Joldasova, Z.A. Mustafaeva, S. Kazakhbaev, S.A. Lyubimova, and B.A. Tashmukhamedov, 2004. Succession of ecosystems of the Aral Sea during its transition from oligohaline to polyhaline waterbody. J. Marine Systems, 47(1–4), 101–108.] to only about 130 g∙kg− 1 in 2005. The denser eastern basin water sinking down the slope of the western trench is responsible for enhanced vertical stratification observed in 2002 and 2003. Such stratification greatly damped vertical mixing, which resulted in hypoxia and sulfide contamination in the bottom layer. However, the water column was ventilated in spring of 2004, presumably, following deep winter convection, thus suggesting that stratification and hypoxia are intermittent rather than permanent features. A recently formed deep (up to 7 m) channel has been discovered in the otherwise shallow strait between the two basins, presumably associated with erosion of the silty bottom by currents. This means that the expectations of imminent separation of the two basins must be revisited. Water velocity values 20–50 cm∙s− 1 have been observed in the channel. The lake surface level in the western basin responded energetically to wind-controlled inter-basin flow through the strait. © 2008 Elsevier B.V. All rights reserved.

1. Introduction

⁎ Corresponding author. Tel.: +7 495 1245994; fax: +7 495 1245983. E-mail address: [email protected] (P.O. Zavialov). 0924-7963/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmarsys.2008.03.013

Up to the early 1960s, the Aral Sea, a terminal lake situated in the Central Asian part of the former Soviet Union, was the fourth largest inland water body of the planet whose area exceeded 66,000 km2. At the time, the lake volume was

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approximately 1020 km3, and the maximum depth was 63 m. The mean elevation of Aral's surface above the ocean level was 53.5 m. Like any significant lake, the Aral Sea had a moderating effect on the regional climate. Such influence was evident in the riparian zone up to about 100 km from the shore (e.g., Muminov and Ignatova, 1995). Amu-Darya and Syr-Darya rivers feeding the lake provided annual freshwater discharge of 56 km3 on the long-term average. The Aral Sea was brackish—its salinity spanned around 10 g∙kg− 1 and never exceeded 12 g∙kg− 1. As a rule, the water column was well mixed and fully ventilated (sometimes, except the areas immediately adjacent to the river mouths). The conditions were favorable for biological communities running to hundreds of species. The Aral Sea was responsible for a considerable part of fisheries in the former USSR, for example, about 13% of sturgeon catches. The turnover of cargo navigation on the lake was also significant, totaling to about 200,000 tonnes per year (Bortnik and Chistyaeva, 1990). In the early 1960s, the water budget of the Aral Sea that had been close to equilibrium for centuries became deficient. The primary cause triggering the desiccation was the anthropogenic impact in the form of large-scale diversions of river runoff for agriculture. Many researchers believe, however, that natural climate change has also played a considerable role, and the lake surface level would have dropped (although less dramatically) even without an increase of anthropogenic water diversions (e.g., Bortnik and Chistyaeva, 1990). According to these authors (see also Volftsun and Sumarokova, 1985), natural climate variability is

responsible for about ¼ of the observed level drop, while the remaining ¾ are associated with human impacts. To date of this writing, the overall level drop is about 23 m. In 1988–1989, the northernmost, relatively small part of the lake known as the Small Sea has detached from the main water body. To preserve the Small Sea and prevent its water from spilling into the principal part of the lake, i.e., the Large Sea, the local authorities have built a temporary and, later (2005), a permanent dam. In consequence, because of considerable residual inflow from Syr-Darya, the level drop and salinity increase in the Small Sea ceased, and the present ecological situation there is relatively favorable. In this paper, we restrict ourselves to the Large Sea, where the ecological crisis is manifested to the largest extent. At present (2006), the Large Aral Sea has shrunk in the area by over 75% and almost split into two separate parts, namely, relatively deep (41 m) western basin and shallow (∼3 m), but broad eastern basin. The overall mean depth of the Large Aral Sea is 5.9 m. The two basins are connected through a narrow (1–3 km) strait, see Fig. 1. The desiccation resulted in profound changes of ecosystems of the lake and the surrounding terrain. Salinity has attained values around 100 g∙kg− 1 in the western basin and 135 g∙kg− 1 in the eastern basin (Table 1). The salinization was accompanied by massive precipitation of calcium and magnesium carbonates at the initial stage, and gypsum and mirabilite thereafter, which led to significant modification of the salt composition (e.g., Ni et al., 2005) and corresponding alterations of basic physical properties of the water (e.g., Zavialov, 2005). Few biological species in benthic

Fig. 1. Map of the Aral Sea and locations of the hydrographic stations (black) and moored instruments (white).

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Table 1 Field surveys accomplished to date and types of measurements Cruise 1 2 3 4 4 5 5 5 6

Leg

1 2 1 2 3

Date

Location

CTD

November 2002 October 2003 April 2004 August 2004 August 2004 October 2005 October 2005 October 2005 March 2006

Western basin Western basin Western basin Western basin Strait Western basin Strait Eastern basin Western basin

SBE19plus, 11 stations SBE19plus, 20 stations SBE19plus, 4 stations YSI6600, 16 stations SBE19plus, 6 stations SBE19plus, 8 stations SBE19plus, 7 stations SBE19plus, 7 stations SBE19plus, 2 stations

and plankton communities managed to survive in the hyperhaline environment (e.g., Mirabdullaev et al., 2004, see also Arashkevich et al., 2009-this issue; Sapozhnikov et al., 2009-this issue; Mokievsky, 2009-this issue). The Large Aral Sea has been relatively stable and no significant further level drop has been observed since 2002 until the date of this writing (August, 2006). On the one hand, this may be attributed to a certain increase of Amu-Darya runoff that took place during this period. For example, according to the estimates by the meteorological service of Uzbekistan, the residual river inflow into the lake was as large as 6 km3 in 2003 and 8 km3 in 2005 (Gorelkin and Kudyshkin, 2006). On the other hand, the shrinking of the lake area in the preceding period has led to a strong decrease of the total evaporation from its surface. In consequence, the lake's water budget tends to be equilibrated by even relatively small river and groundwater discharges. Moreover, in a sense, the shallow, flat-bottom eastern basin in its present state acts as a stabilizing “damper”: any further level drop, even small, results in a great reduction of the basin area and, hence, corresponding decrease of the net evaporation, and vice versa. This intrinsic negative feedback prevents the system from abrupt level changes. It can be said that at present, the Large Aral Sea should be close to equilibrium. For detailed analysis of the water budget, see (Zavialov, 2005). The Aral Sea's shrinking caused a spectrum of social and economic consequences. The collapse of fisheries and navigation resulted in unemployment and severe damages to the regional economy. Deterioration and salinization of potable water used by local households, as well as frequent salt and dust storms transporting up to 40 million tonnes of salt per year (Rubanov and Bogdanova, 1987) from the newly dry bottom for up to 500–800 km (Micklin, 2004), represent a serious threat to the health of the population. Another consequence of the desiccation is the aridization of the regional climate. However, the importance of studying the ongoing changes of the Aral Sea is not limited to the applied, regional aspects. The lake can be thought of as a natural “extreme model” of the response of a large inland water body on anthropogenic interventions through diversions of the river runoff. Similar impacts, although manifested in less dramatic forms, are also characteristic for other inland seas and lakes all over the World. This is why the Aral Sea crisis has attracted attention from the international scientific community. On the other hand, most works published after the early 1990s—i.e., during the period of the most profound changes of the ecosystem—

Velocity

Potok-2M, 2 moorings

Potok-2M + tidal gauge, 1 mooring Potok-2M + tidal gauges, 2 moorings

Nr. of water samples analyzed 8 11 4 12 3 6 3 5 2

were confined to either modeling or remote sensing, while the direct field observations in the lake were extremely sparse. In part, this was because of the well-known political and economic troubles following the disintegration of the USSR. In addition, by the early 1990s, navigation in the Aral Sea had ceased completely, and the shoreline had gone far away from all roads and infrastructure, leading to significant logistic difficulties one inevitably encounters while organizing the field work. As a result, at the beginning of the millennium, many basic characteristics of the rapidly changing Aral Sea environment were practically unknown. In 2002, the Shirshov Institute of Oceanology of the Russian Academy of Sciences launched a long-term programme of field research and monitoring of the Aral Sea. The programme is conducted in collaboration with the Hydrometeorological Centre of Russia, the Academy of Sciences of Uzbekistan, the National University of Uzbekistan, the Nukus State Teacher Institute (Uzbekistan), the Karakalpak State University, the Central Asian Research Hydrometeorological Institute (Uzbekistan), and the Kazakh-Turkish International University (Kazakhstan). Some biological, chemical, and meteorological results of the programme are presented in the other articles of this special issue. In this paper, we give a summarizing account of the results of hydrographic research within the programme obtained to date. The data collected in the surveys of 2002, 2003, and 2004 have been partly published previously (Zavialov et al., 2003a,b, 2004a; Zavialov, 2005). Here we recall them briefly, mainly focusing on the results of 2005 and 2006 presented for the first time. 2. Materials and methods A summary of the hydrographic field work accomplished to date of this writing is given in Table 1, see also the map of stations in Fig. 1. Six expeditions were realized from November, 2002, through March, 2006. Some of them were divided into separate legs in different parts of the lake (Table 1). Until 2004, the field campaigns were limited to the western basin, which is technically and logistically easier to work in. However, the measurements were extended onto the strait area in August, 2004, and then the eastern basin in October, 2005. The total of 81 hydrographic stations have been occupied from motor boats delivered to the site by allterrain vehicles. In addition, 5 mooring stations equipped with rotational Potok-2M current meters and, sometimes, pressure gauges have been deployed during this period. The locations of the deployments are shown by the white bullets

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in Fig. 1. A technical description and specifications of the current meters can be found in (Lozovatsky et al., 2003). Each of the moorings remained in operation for a few days following the deployment, recording the data every 20 min. Simultaneously, the basic meteorological parameters (wind speed and direction, air temperature, absolute and specific humidity, atmospheric pressure) were continuously registered as 20-min averages by a portable automatic meteorological station HeavyWeather installed on the shore, at the distance of a few kilometers from the moorings. In addition, a complete set of the surface meteorological data were recorded every 6 h at the Aktumsuk meteorological station of the Uzbekistan Hydrometeorological Service, located on Ustyurt Plateau, 8 km west of the lake in the central part of the western basin. Finally, the absolute elevation of the Aral Sea surface above the ocean level was determined during most of the cruises through direct geodesic leveling using a triangulation point located near Aktumsuk cape at the western bank of the western basin. At all hydrographic stations, surface-to-bottom CTD profiling was done using a manual winch, normally, accompanied by water sampling from standard depth levels (0, 10, 20, 30, 40 m) by Sea Bird's 5-liter Niskin bottles. At some stations, similar 5-liter Molchanov bottles were used instead of Niskin bottles. The Sea Bird's SBE19plus CTD profiler was used in all expeditions, except the one of summer 2004. In the latter, both salinity and temperature of water were expected to be high and the electric conductivity could be therefore beyond the range of the SBE profiler, so we opted for Yellow Springs YSI6600 instrument designed for a broader range of conductivity. A major problem with interpreting CTD data collected from the Aral Sea is linked with the salt composition of the water, which is significantly different from that of the ocean water. In consequence, the relation between the electric conductivity and salinity is also different. Moreover, the empirical relation once used for the pre-desiccation Aral (e.g.,

Sopach, 1958), is no longer valid because of the ongoing precipitation of salts and corresponding changes in salt composition (Zavialov, 2005). No known explicit relation is available at present. Therefore, we applied the following procedure to infer the salinity from the CTD data. First, the true salinities Strue of the collected water samples were obtained chemically in laboratory of Abdullaev Institute of Geology and Geophysics, Uzbekistan, using the dry residue method (Ni et al., 2005). Then, the corresponding “pseudo salinity” values Sctd, i.e., those computed through the standard oceanic relation, were extracted from the CTD data, and linear regression between the chemically obtained and CTD-derived salinity values was constructed. The linear relation obtained thereby was then used to convert the entire set of CTD data to the “true” salinity. This conversion was done on an individual basis for each cruise (except for some cruises where the number of analyzed water samples was too small, namely, those of April 2004 and March 2006, and the regressions derived from the preceding cruise were used). As an example, we present here the regression based on 14 chemically analyzed water samples collected in October, 2005: Strue ¼ 2:047 Sctd −44:8;

ð1Þ −1

where the units of Strue are g∙kg , and those of Sctd are psu. The regression yielded R2 N 0.94, however, the rms deviation was nearly 2 g∙kg− 1. Fortunately, the range of spatial variability characteristic for today's Aral Sea (up to 12 g∙kg− 1 between the surface and the bottom, 30 g∙kg− 1 and more between the western and the eastern basins) is, typically, much larger than the uncertainty of the conversion procedure. 3. Results and discussion Typical examples of vertical profiles of temperature and salinity in the western part of the lake are shown in Fig. 2. A summary of principal hydrographic characteristics in

Fig. 2. Typical vertical profiles of temperature and salinity observed in the western trench.

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different basins is given in Table 2 (the density values in the Table were obtained through laboratory measurements as described by Zavialov, 2005). First of all, we note the continuous growth of salinity in the western basin. The only documented exception occurred in the spring of 2004, when the salinity temporarily dropped, probably because of the admixture of snowmelt water. It is known from satellite altimetry data that snow melting often results in spring upraise of the lake level by 30–50 cm (e.g., CNES/LEGOS, 2006), and this is consistent with the observed salinity drop. Apart from this localized decrease, the surface values generally grew from 82 g∙kg− 1 in 2002 to about 99 g∙kg− 1 in the early 2006, which constitutes a notable increase of over 20%. The lake surface level has dropped only insignificantly (less than 30 cm, or only about 5% of the mean depth of the lake, see Table 2) during the same period. Therefore, the progressive salinization observed in the western basin cannot be explained by the lake volume shrinking. Theoretically, the inputs of salts from the residual river discharge and groundwater inflow into the terminal lake may have contributed to the salinity growth. However, although little or no quantitative data on these factors is presently available, they are more than likely to be negligible at the temporal scales of our interest. Indeed, according to the most recent available figures for 1981–1985 (Akhmedsafin et al., 1983; Chernenko, 1983; Bortnik and Chistyaeva, 1990), the fluvial and groundwater fluxes of salt into the Aral Sea are estimated as 2.2 million tonnes per year and 0.7 million tonnes per year, respectively. We note that the average salinity values used for these estimates, i.e., 1.3 g∙kg− 1 for Syr-Darya, 1.8 g∙kg− 1 for Amu-Darya, and 7 g∙kg− 1 for groundwater, appear to be rather similar to those characteristic for the early 2000s (Umarov and Karimov, 2000). Given that the total mass of salts in the today's lake is nearly 6 billion tonnes (Zavialov, 2005), the above fluxes together could be responsible for annual increase of the salt content by about 0.05%. This means that it would take at least 400 years for the river and groundwater inputs alone to account for the 20% increase of salinity, actually observed during only 5 years between 2002 and 2006. Hence, the observed salinization of the western basin must be mainly due to the water exchanges with the saltier, shallow eastern basin. In contrast, the salinity in the eastern basin apparently gradually decreased from nearly 160 g∙kg− 1 reported by (Mirabdullaev et al., 2004) for 2002 to about 134 g∙kg− 1 we observed in 2005. This salinity drop may have been partly associated with relatively high residual river

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discharges reaching the basin. On the average, during this period, the eastern basin was receiving annually about 3.9 km3 of fresh water from Amu-Darya (Gorelkin and Kudyshkin, 2006). In addition, about 9.0 km3 per year of Syr-Darya water reached the Small Sea, and an unknown part of this volume eventually spilled into the eastern basin of the Large Sea. However, the exchange with the fresher western basin through the connecting strait also must have been a cause of the salinity decrease in the eastern basin. In the course of the inter-basin water exchanges, upon entering the western part of the Aral Sea, the relatively dense waters of the eastern basin sink along the northern slope of the trench until they eventually reach their isopicnal level in the near-bottom layer (Zavialov, 2005). This situation results in enhanced haline and, therefore, density stratification of the western basin. In 2002–2003, the typical difference between the bottom and upper layer salinity values was as large as up to 12 g∙kg− 1 (which corresponds to the density difference of 9–10 kg∙m− 3) across the halocline whose thickness was about 20 m, see also (Zavialov et al., 2003a). This factor largely damps vertical mixing and greatly reduces the exchanges between the bulk of the water column and the atmosphere. One illustrative consequence is the temperature inversions seen in the autumn profiles (Fig. 2). But the most striking manifestation of the lack of vertical exchanges is the anoxic conditions and the presence of hydrogen sulfide at high concentrations up to 80 mg∙l− 1, (e.g., Zavialov, 2005). On the other hand, neither enhanced stratification nor anoxic conditions were observed in the spring of 2004. In April, the entire water column was nearly uniform in salinity at 86 to 87 g∙kg− 1. Therefore, the stratification and the anoxia are intermittent rather than permanent features, and their variability should be associated with the wind-controlled inter-basin water exchanges through the strait, and also, possibly, the severity of winters. Moreover, in August, 2004, the haline stratification was unstable, with the salinity decreasing from 91 g∙kg− 1 in the upper mixed layer to only 87 g∙kg− 1 at the bottom. The column stability was maintained by the thermocline, where the temperature drop between the base of the mixed layer, approximately at the depth 13 m, and the bottom (40 m) was as large as 23 °C (!). The elevated salinity near the surface is a natural manifestation of intense evaporation. The excessive salt released thereby stays within the upper mixed layer as long as the thermocline is sufficiently steep. Hence, a locally unstable salinity stratification is imminent in summer and early

Table 2 Summary of hydrographic state of the lake during the surveys Cruise #

Date

Location

Lake level, m

1 2 3 4(1) 4(2) 5(1) 5(2) 5(3) 6

Nov 2002 Oct 2003 Apr 2004 Aug 2004 Aug 2004 Oct 2005 Oct 2005 Oct 2005 Mar 2006

West basin West basin West basin West basin Strait West basin Strait East basin West basin

30.47 30.50 – 30.71 – 30.12 – – 30.20

Salinity, ppt

Temperature, °C

Density σt, kg/m3

Surface layer

Bottom layer

Surface layer

Bottom layer

Surface layer

Bottom layer

82 87 86 91 100 98 132 130 99

94 96 87 87 100 101 132 134

10 14 5 25 23 18 17 15 −2

15 2 1 2 23 4 17 15 –

57 60 61 64 71 68 84 84 70

67 69 62 65 71 70 84 87 –

H2S

Yes Yes, c = 80 mg/l No No No Yes, c = 5 mg/l No No Unknown

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autumn unless there is some concurrent mechanism tending to increase the salinity below the mixed layer—for example, advection of denser waters from the eastern basin. Therefore, the unstable profile of salinity observed at the time can be hypothetically attributed to the lack of wind conditions favoring the intrusions of the eastern basin water in the preceding period. In August, 2004, the hydrographic measurements were conducted not only in the deep western basin, but also in the strait connecting the two basins. The water column in the strait was fully mixed, and its salinity was vertically uniform at 100 g∙kg− 1, slightly increasing eastward. In October, 2005, measurements quasi-synoptically covered for the first time all the 3 principal parts of the lake, namely, the western basin, the strait, and the northern portion of the eastern basin. In the western basin, the overall stratification was again stable, the salinity in the upper mixed layer was 98 g∙kg− 1, and the bottom salinity was about 101 g∙kg− 1. The temperature in the mixed layer was slightly above 17 °С. However, the vertical thermohaline structure of the basin was rather complex (Fig. 3). Near the western slope of the trench at the depth of 35 m, there was a maximum of both temperature (8 °С) and salinity (up to 102 g∙kg− 1), forming a significant picnocline in the layer above it. In the bottom layer, salinity and temperature decreased downwards. Hydrogen sulfide was observed again in the lowermost few

meters. Its concentration, however, was only 5 mg∙l− 1, i.e., much lower than that observed in 2002–2003. Near the eastern slope of the trench, the hydrographic situation was opposite: a local minimum of both salinity (about 94 g∙kg− 1) and temperature (5 °С) was seen at the depth about 25 m. The temperature minimum served to maintain the column stable. Below it, salinity increased towards the bottom, accompanied by a temperature inversion. The salty and warm near-bottom “lens” at the western slope may have resulted from an intrusion of the eastern basin water in the preceding summer or early fall. Its location at the western side of the basin corresponds to the action of Coriolis force on a water mass propagating southward from the inter-basin strait. Similar salty lenses associated with the western slope were also observed in some previous cruises (Zavialov, 2005). On the other hand, the eastern part of the basin is not much affected by this inflow, and the relatively fresh and cold intermediate water at the eastern slope is close in salinity to the mixed layer water as observed in summer of 2004. CTD profiling at a section across the northern part of the eastern basin (Fig. 1), from the eastern entrance to the strait to the former Barsakelmes Island, revealed that the eastern basin, despite being very shallow, was significantly stratified (Fig. 4). Salinity varied vertically from about 129–131 g∙kg− 1 at the surface to up to 133 g∙kg− 1 (and even over 134 g∙kg− 1 at

Fig. 3. Vertical distributions of salinity (upper panel) and temperature (lower panel) along a zonal cross-section of the western basin from Aktumsuk cape. October, 2005.

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Fig. 4. Vertical distributions of salinity (upper panel) and temperature (lower panel) along a zonal cross-section of the eastern basin from the eastern outlet of the strait to former Barsakelmes Island. October, 2005.

the eastern extremity of the section) at the bottom (∼ 3 m). The vertical distribution of temperature throughout the section was characterized by a moderate inversion, with surface values between 15.2 and 15.6 °C to up to 15.8 °C at the bottom. Horizontally, salinity increased eastward from below 129 g∙kg− 1 near the strait to almost 132 g∙kg− 1 near Barsakelmes. The bulk of water in the strait itself was fully mixed at 132 g∙kg− 1. The survey of early March 2006 was limited to Aktumsuk site in the western basin, and, for technical reasons, only the coastal waters up to the depth of 16 m were sampled. In this shallow region, the water column was mixed through at 99 g∙kg− 1 (i.e., the highest salinity ever observed at the surface in the western basin) and −2 °C. We note that at present, the freezing temperature for the western basin water is about −4.5 °C (Zavialov et al., 2004b). As exposed above, the water exchange through the strait between the western and eastern basins of the Aral Sea is likely to be a major factor controlling the thermohaline regimes of either basin. On the other hand, until quite recently, the quantitative characteristics of the flow in the strait, and even the very depth of the latter, were unknown. According to the available “old” bathymetric maps of the 1960s, the strait should have actually dried up completely for today's lake level standing. Given this fact, and considering that no direct depth measurements were conducted in this area, at least during the last 20 years, it has been tacitly assumed in a number of

previous publications that the strait was very shallow (b 1 m) and its closure was imminent in the near future. However, already our first survey of the strait in 2004 has unexpectedly revealed a relatively deep (up to 7 m!) channel in the otherwise shallow strait. This channel must have been produced by erosion of the silty bottom by intense currents in the strait. More systematic bathymetric mapping of the strait was undertaken in the survey of 2005. The depth values were determined using a motor boat mounted echo sounder Humminbird with horizontal intervals 100 m. A bathymetric profile along a cross-section of the strait in its narrowest spot (the coordinate of the northern extremity of the cross-section is 45°44.82'N, 59°12.96'E) is shown in Fig. 5. A deep “fairway” along the southern bank of the strait is clearly seen. This means that the expectations of imminent disappearance of the strait and separation of the two basins must be revisited. Direct velocity and level measurements conducted in the strait during the survey of 2005 along with collecting meteorological data give some insight into the dynamical response of the system to atmospheric forcing. The current meter and the level gauge at the mooring station were deployed at the depth 4 m, about 1.5 m above the bottom in the deep channel at the western outlet of the strait. The meteorological station was installed at the northern bank of the strait, some 3 km north of the mooring. The data collected during the 76 h observation period are exhibited in Fig. 6. The period was characterized by the

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Fig. 5. Bathymetric profile of the strait along its cross-section in the narrowest spot (the coordinate of the northern extremity of the cross-section is 45°44.82'N, 59°12.96'E).

conditions of moderate to strong (τ b 0.08 N∙m− 2) easterly winds. This wind is rather close to the climatic average for the region. The bulk current at the depth 4 m in the strait was westward at up to 27 cm∙s− 1 (in fact, higher velocity values up to 50 cm∙s− 1 were observed at the surface). The water velocity was correlated with the wind stress at about 0.5, and the best

correlation corresponded to the phase delay of the current of 5–6 h. This constitutes a notable decrease of the time lag between the wind and the current compared with the predesiccation period when the delay was typically 8–12 h (Bortnik and Chistyaeva, 1990). During the period of the measurements, estimated 0.1–0.2 km3 of water has been

Fig. 6. From top to bottom: air temperature; atmospheric pressure; relative humidity; zonal and meridional components of wind stress; zonal and meridional components of current velocity; and lake surface height anomaly during 76 h period of observations near the western outlet of the strait. October, 2005.

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transported from the eastern basin into the western basin. In consequence, the lake surface level at the western edge of the strait has increased by about 40 cm over this period (Fig. 6). This must lead to a compensating barotropic pressure gradient and, eventually, a countercurrent in the lowermost portion of the strait. It is this barotropic mechanism that must be responsible for equilibrating the inter-basin water exchanges in the case of mainly unidirectional wind forcing. 4. Conclusions In spite of relatively stable lake surface level during the period 2002–2006, in the western basin, salinity increased from 82 g∙kg− 1 in November, 2002, to 99 g∙kg− 1 in March, 2006 (these values correspond to the lake surface). This increase must have been a result of the water exchanges with the saltier, shallow eastern basin. As direct velocity and level measurements in the strait between the basins have demonstrated, strong (up to 30– 50 cm∙s− 1) wind-controlled currents are common there, and the characteristic volume transport through the strait during such events is of order of 0.1 km3 per day. Surface salinity of the eastern basin was about 160 g∙kg− 1 in 2002 (Mirabdullaev et al., 2004) but decreased to 130 g∙kg− 1 in summer of 2005. Even if the lake level remains stable, the growth of salinity in the western basin and its decrease in the eastern basin will continue for some time because of the inter-basin exchanges, until the values eventually come to dynamical equilibrium. During the study period, the inter-basin exchanges and the difference in salinity between the basins were the principal factors responsible for the enhanced vertical stratification of the western trench: entering the western basin through the strait, the salty and dense eastern basin water slipped downslope and accumulated in the bottom layer of the trench, thus producing elevated vertical gradients of salinity and density in the latter. The corresponding reduction of vertical mixing was the primary cause of anoxic conditions and sulfide contamination. In the future, if the tendency towards the decrease of the inter-basin salinity difference persists or the exchanges through the strait diminish as the lake shallowing progresses, the probability of strongly stratified, anoxic and H2S-contaminated conditions in the western basin should decrease accordingly. An important finding of the recent field research is the discovered “self-deepening” of the strait between the western and the eastern basins, i.e., the formation of a channel whose depth is over 6 m, associated with the erosion of the bottom by currents. This means that even if the lake's level drop continues, the strait is unlikely to dry out in the near future and the two parts of the Large Aral Sea should remain connected and interacting. The unique physical, chemical, and biological systems of the lake are presently undergoing profound changes. Obtaining scientific insight into these changes and the underlying mechanisms is important, both in the context of regional mitigation actions and in a broader global perspective. Further international, interdisciplinary efforts aimed at research and monitoring of the Aral Sea are needed. Acknowledgments The authors gratefully acknowledge the support from the Division of Earth Sciences, Russian Academy of Sciences,

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