Dissolved methane in the residual basins of the Aral Sea

E S Izhitskaya,E V Yakushev,A S Izhitskiy,A V Egorov,P O Zavialov

doi:10.1088/1748-9326/ab0391

Abstract

Due to catastrophic desiccation, today’s Aral Sea consists of a few separate residual basins, characterized by different ecological conditions (the Large Aral, Lake Tshchebas, the Small Aral). This study is the first report on dissolved methane concentrations in these basins. Overall, 48 water samples were obtained and analyzed for methane content. High values of dissolved methane in the anaerobic layer of the Large Aral Sea, including the Chernyshev Bay, are apparently caused by damping of vertical mixing and decomposition of abundant organic matter in anoxic conditions. The estimated methane flux from the surface of the Large Aral Sea is actually higher than that from many other lakes in the world. For the anoxic layer of the Large Aral, certain relations between distributions of methane and other hydrochemical parameters, including dissolved oxygen and hydrogen sulfide, were found. In the brackish Small Aral Sea, methane content was moderate. Lake Tshchebas exhibits intermediate conditions between the Large and the Small Aral seas in terms of salinity and methane concentration. The observed differences of methane content and distributions in separate residual basins are linked with the differences of their mixing and oxygenation regimes.

Highlights

In the mid-20th century, the Aral Sea, an endorheic brackish lake in the Central Asia, was the World’s fourth largest inland water body by area
The observed differences of methane content and distributions in separate residual basins are linked with the differences of their mixing and oxygenation regimes
Methane in surface waters of the Large Aral Sea The values of methane concentration in the surface layer of the central part of the Large Aral Sea ranged from 37–47 nmol l−1 in September 2012, and from 124–234 nmol l−1 in October 2013 (figure 2(a))

Summary

Introduction

In the mid-20th century, the Aral Sea, an endorheic brackish lake in the Central Asia, was the World’s fourth largest inland water body by area. The diverging water budget conditions of the residual basins (restriction of river flow) led to the formation of a new hydrological structure of waters specific to each reservoir These changes caused differences in the physical, chemical and biological properties of each basin. The residual basins (except the Small Aral Sea) are characterized by extremely high salinity, especially in the lower layers. Such density stratification conditions damped vertical mixing process in the lake and caused anoxia in the bottom layers (Zavialov 2005). The upper layers are rich in zooplankton and organic matter (Mirabdullaev et al 2004, Arashkevich et al 2009) This combination results in sulfide contamination of the lower portion of the water column. The objective of the present study was to evaluate dissolved methane concentration in different parts of the Aral Sea and to identify factors that may influence dissolved methane content in the lake

Objectives

Results

Discussion

Conclusion