Comparative Study of the Stability of Stratification and the Food Web Structure in the Meromictic Lakes Shira and Shunet (South Siberia, Russia)

Lakes Shira and Shunet (South Siberia, Russia) are saline ectogenic meromictic lakes. This meromixis is annually sustained by ice formation in winter. In spring due to melting of ice the second halocline is formed in the near-surface layers of mixolimnion that prevents the lakes from full circulation. During our observations (1998–2014) the mixolimnion in both the lakes was monomictic and undergone full circulation in autumn. Purple and green sulphur bacteria are abundant in the chemocline ; the phytoplankton diversity is relatively low; the major constituents of the zooplankton community are several genera of ciliates, calanoid copepod Arctodiaptomus salinus, rotifers Brachionus plicatilis and Hexarthra sp.; Paleolimnological study of the content of organic matter in sediments and the concentration of okenone demonstrated that Lake Shira was holomictic about 90 years ago when the water level was low. It became strongly meromictic just after the water level increase in the 1940s.