Ecosystem Changes in the Diminishing Aral Sea Observed by Satellites

The diminishing of the Aral Sea is “one of the worst environmental disasters in the world” (from United Nations Secretary-General Ban Ki-moon). Total coverage maps since 1981 show constant decline of the Aral Sea. The coverage dropped from ~4.7–4.8×10⁴ km² in 1981 to about ¼ of the value in recent years. Twelve-year MODIS-Aqua observations (2002–2013) are used to quantitatively assess the water property changes in the Aral Sea. We used radiance ratio nL_w(555)/nL_w(443) as a surrogate to characterize the spatial and temporal variations of chlorophyll-a (Chl-a) in the Aral Sea. Both seasonal variability and significant interannual changes were observed when the Aral Sea desiccated between 2002 and 2013. All three regions of the Aral Sea show increased nL_w(555)/nL_w(443) ratio and the diffuse attenuation coefficient at the wavelength of 490 nm (K_d(490)) during the fall season. Of the three regions, the North Aral Sea has had the least interannual variability, while South-East (SE) Aral Sea experienced drastic changes. Waters in the SE Aral Sea are the most turbid with significantly higher K_d(490) than those in the other two sub-regions. K_d(490) gradually increased from ~2 m^–1 in 2002 to ~3.5 m^–1 after 2008 in the SE Aral Sea. In comparison, both radiance ratio nL_w(555)/nL_w(443) and K_d(490) were relatively stable for the North Aral Sea. In the South-West (SW) Aral Sea, however, nL_w(555)/nL_w(443) values reached peaks in the fall of 2007 and 2010. A possible link between the Aral Sea water property change and the regional climate variation is also discussed.