Seasonal and inter-annual sedimentation in meromictic Lake Shira (Siberia, Russia) during disturbance of meromixis

Abstract

Detailed studies of processes of sediment record formation are necessary for accurate sediment-derived paleo-environmental reconstructions. In order to understand the formation of sediments in meromictic Lake Shira (southern Siberia, Russia), we conducted the multi-year seasonal sediment trap study of sediment fluxes of chemical elements, organic and inorganic carbon, total nitrogen, and photosynthetic pigments for a period from 2012 to 2017. In parallel, in 2013, 2016, 2017, and 2018, we estimated the contents of photosynthetic pigments in uppermost sediment layers frozen in situ with a freeze-corer. In 2015 and 2016, a rare event of transition from meromixis (i.e. long-term hypolimnetic anoxia) to holomixis was observed, which was accompanied by the temporary disappearance of hydrogen sulfide from the water column in spring and a decrease in hydrogen sulfide in other seasons compared to the meromictic state. Using the freeze-corer, we observed that sediment structure changed from laminated to homogeneous after holomixis. The content of algal photosynthetic pigments slightly increased in uppermost sediments in 2017 and 2018, which was probably caused by an increase in lake primary production resulting from the release of nutrients during the mixing events in 2015 and 2016. The content of okenone, a specific carotenoid of purple sulfur bacteria, decreased during the holomictic state both in traps and in uppermost sediments and did not considerably increase two years after the meromixis restoration. The sedimentation flux of molybdenum reflected the seasonality of hydrogen sulfide content in the water column, and it was maximal in winter, in contrast to all other sediment components. Other components reflected the seasonality of organic production and terrigenous inputs, with the maximum in late summer and autumn. An increase in the sedimentation rate of terrigenous elements in the autumn of 2012 was supposedly caused by the anomalous amount of atmospheric precipitation in August 2012. Since detailed inter-annual observations of sedimentation processes are not often found in limnology, we hope that our results will be useful for reconstructing regional climate and, further, for understanding the dynamics of stratified temperate water bodies.