Effects of turbulence on carbon emission in shallow lakes

Abstract

Turbulent mixing is enhanced in shallow lakes. As a result, exchanges across the air–water and sediment–water interfaces are increased, causing these systems to be large sources of greenhouse gases. This study investigated the effects of turbulence on carbon dioxide (CO₂) and methane (CH₄) emissions in shallow lakes using simulated mesocosm experiments. Results demonstrated that turbulence increased CO₂ emissions, while simultaneously decreasing CH₄ emissions by altering microbial processes. Under turbulent conditions, a greater fraction of organic carbon was recycled as CO₂ instead of CH₄, potentially reducing the net global warming effect because of the lower global warming potential of CO₂ relative to CH₄. The CH₄/CO₂ flux ratio was approximately 0.006 under turbulent conditions, but reached 0.078 in the control. The real-time quantitative PCR analysis indicated that methanogen abundance decreased and methanotroph abundance increased under turbulent conditions, inhibiting CH₄ production and favoring the oxidation of CH₄ to CO₂. These findings suggest that turbulence may play an important role in the global carbon cycle by limiting CH₄ emissions, thereby reducing the net global warming effect of shallow lakes.

Introduction

Global warming has intensified extreme weather events, such as hurricanes, droughts, heat waves, and floods (Fedorov et al., 2010, Min et al., 2011, Tollefson, 2012). The greenhouse gases carbon dioxide (CO₂) and methane (CH₄) are two main drivers of global warming, contributing 63% and 18%, respectively (Hu et al., 2017, Smith et al., 2013, Tu and Li, 2017). Current atmospheric CH₄ and CO₂ concentrations are approximately 2.5 and 2 times, respectively, pre-industrial levels (Kirschke et al., 2013, Hartmann et al., 2013). Inland waters are considered to be hotspots of CH₄ and CO₂ emissions, which are estimated at 0.65 Pg C/year (CO₂ eq) and 1.2–2.1 Pg C/year, respectively (Bastviken et al., 2011, Raymond et al., 2013). Thus, CH₄ and CO₂ emissions from inland waters have received considerable research attention.

In aquatic systems, CH₄ and CO₂ have different production pathways. CH₄ is primarily produced in surface sediment by microbial communities as part of anaerobic respiration, usually within the first few centimeters of the sediment layer (Conrad et al., 2007). CO₂ is produced by aerobic respiration and acetoclastic methanogenesis throughout the lake and lake sediment (Casper et al., 2000). Microbial CH₄ production (methanogenesis) is achieved by specific groups of Archaea in anoxic environments, where anaerobic degradation of organic matter occurs as a result of fermentation (Nazaries et al., 2013). CO₂ production is driven largely by the aerobic decomposition of organic matter (Jung et al., 2014, Sobek et al., 2005). Under aerobic conditions, CH₄ can be oxidized to CO₂ by methanotrophic Proteobacteria (Bastviken et al., 2008, Xiao et al., 2013). Thus, environmental conditions strongly influence organic matter mineralization and CH₄ and CO₂ emissions from inland waters.

The hydrodynamics of waves created by the wind play an important role in aquatic ecosystems, and turbulence is a ubiquitous hydrodynamic feature of all inland waters, especially shallow lakes (Margalef, 1997). Waves are generated by wind passing over a water surface. As long as the waves propagate more slowly than the wind speed just above the waves, there is an energy transfer from the wind to the waves; however, there is little space for wave energy to dissipate in shallow lakes, enhancing turbulent kinetic-energy and turbulent shear forces (G-Tóth et al., 2011). Lake Taihu is a shallow eutrophic lake in China, with an average depth of 1.9 m. The lake's turbulence is mainly determined by wind velocity, wind direction, and prevailing wind (Qin et al., 2007). Physical turbulence can strongly affect the stability of lake water and sediment. Specifically, wind-induced turbulence controls the exchange of material across both air–water and sediment–water interfaces. Previous studies have shown that disturbance can accelerate the rate at which oxygen diffuses from the atmosphere into the water column (Chatelain and Guizien, 2010, You et al., 2007). By changing the dissolved oxygen (DO) concentration of water, turbulent mixing can shift the mineralization pathways of organic matter, altering CO₂ and CH₄ emissions to the atmosphere (Li et al., 2012, Stoliker et al., 2016), particularly in small or shallow lakes (Holgerson and Raymond, 2016). These studies have focused on the impacts of turbulence and mixing on physical and chemical properties of aquatic ecosystems; however, the fundamental relationship between turbulence and greenhouse gas emissions from lakes has not been fully addressed.

This study used simulated mesocosms to investigate the effects of turbulence on lake CH₄ and CO₂ emissions. CH₄ and CO₂ fluxes were measured using a static chamber method throughout the experiment. Methanogen and methanotroph abundances were also analyzed using real-time quantitative PCR (qPCR), to illustrate the associated molecular mechanism. We hypothesized that under turbulent conditions, (1) water column oxygenation is enhanced; (2) the heterotrophic microbial community shifts towards taxa tolerant of this oxygenation; and (3) the degradation of organic carbon is changed, decreasing CH₄ and increasing CO₂ emissions.