Spatial and Seasonal Variations of C, Nutrient, and Metal Concentration in Thermokarst Lakes of Western Siberia Across a Permafrost Gradient

Rinat M Manasypov,Sergey N Vorobyev,Oleg S Pokrovsky,Ivan V Kriсkov,Sergey N Kirpotin,Liudmila S Shirokova,Artem G Lim

doi:10.3390/w12061830

Abstract

Thermokarst lakes and ponds formed due to thawing of frozen peat in high-latitude lowlands are very dynamic and environmentally important aquatic systems that play a key role in controlling C emission to atmosphere and organic carbon (OC), nutrient, and metal lateral export to rivers and streams. However, despite the importance of thermokarst lakes in assessing biogeochemical functioning of permafrost peatlands in response to climate warming and permafrost thaw, spatial (lake size, permafrost zone) and temporal (seasonal) variations in thermokarst lake hydrochemistry remain very poorly studied. Here, we used unprecedented spatial coverage (isolated, sporadic, discontinuous, and continuous permafrost zone of the western Siberia Lowland) of 67 lakes ranging in size from 102 to 105 m2 for sampling during three main hydrological periods of the year: spring flood, summer baseflow, and autumn time before ice-on. We demonstrate a systematic, all-season decrease in the concentration of dissolved OC (DOC) and an increase in SO4, N-NO3, and some metal (Mn, Co, Cu, Mo, Sr, U, Sb) concentration with an increase in lake surface area, depending on the type of the permafrost zone. These features are interpreted as a combination of (i) OC and organically bound metal leaching from peat at the lake shore, via abrasion and delivery of these compounds by suprapermafrost flow, and (ii) deep groundwater feeding of large lakes (especially visible in the continuous permafrost zone). Analyses of lake water chemical composition across the permafrost gradient allowed a first-order empirical prediction of lake hydrochemical changes in the case of climate warming and permafrost thaw, employing a substituting space for time scenario. The permafrost boundary shift northward may decrease the concentrations and pools of dissolved inorganic carbon (DIC), Li, B, Mg, K, Ca, Sr, Ba, Ni, Cu, As, Rb, Mo, Sr, Y, Zr, rare Earth elements (REEs), Th, and U by a factor of 2–5 in the continuous permafrost zone, but increase the concentrations of CH4, DOC, NH4, Cd, Sb, and Pb by a factor of 2–3. In contrast, the shift of the sporadic to isolated zone may produce a 2–5-fold decrease in CH4, DOC, NH4, Al, P, Ti, Cr, Ni, Ga, Zr, Nb, Cs, REEs, Hf, Th, and U. The exact magnitude of this response will, however, be strongly seasonally dependent, with the largest effects observable during baseflow seasons.

Highlights

The impact of ongoing climate change on the functioning of aquatic ecosystems and biogeochemical cycles of chemical elements poses the main environmental threat in the arctic and subarctic regions [1].Western Siberia is a key region and the most convenient platform to study the fundamental issues of climate–permafrost interaction, examine the applied aspects of these changes, and assess their possible social impacts [2,3,4]
We addressed the following questions: (1) What are the relationships between element concentration and water surface area in different permafrost zones?; (2) How do the concentration and stock of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), nutrients, and trace elements in lake waters respond to the change in the main hydrological seasons?; (3) How do the concentrations and stocks of DOC, nutrients, and trace elements in thermokarst lakes change throughout the latitudinal profile of Western Siberia, and how may the elemental composition of the lake water respond to active layer thickness (ALT) increase and the permafrost boundary shift?
The concentration of dissolved chemical elements, DOC, CO2, and CH4 strongly depends on lake surface area [13,19,40,42], as it is known in various regions of the northern hemisphere [66,67]

Summary

Introduction

The impact of ongoing climate change on the functioning of aquatic ecosystems and biogeochemical cycles of chemical elements poses the main environmental threat in the arctic and subarctic regions [1].Western Siberia is a key region and the most convenient platform to study the fundamental issues of climate–permafrost interaction, examine the applied aspects of these changes, and assess their possible social impacts [2,3,4]. Thawing of frozen peatlands under increasing air and water temperature and greenhouse gas (GHG) concentrations are the most crucial issues related to behavior of aquatic ecosystems in the north of Western Siberia, the largest permafrost peatland in the world. The majority of lakes here are of thermokarst origin, formed during thawing of frozen peat bogs, similar to other arctic and subarctic regions of the northern hemisphere. Thermokarst lakes in Western Siberia are the key components of GHG exchange between surface waters and the atmosphere [7]. Such lakes represent highly dynamic water systems [8,9,10,11], which are substantially fed by atmospheric precipitation [12,13]. The surface of frozen peat bogs in the north of Western Siberia is smooth and flat; the water surface area of lakes is almost equal to the catchment area [13]

Objectives

Results

Conclusion