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

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.