Elsevier

Water Research

Volume 201, 1 August 2021, 117363
Water Research

Eutrophication decreased CO2 but increased CH4 emissions from lake: A case study of a shallow Lake Ulansuhai

https://doi.org/10.1016/j.watres.2021.117363Get rights and content

Highlights

  • Divergent effects of eutrophication on CO2 and CH4 emissions were found.

  • Eutrophication decreased pCO2 and CO2 flux via increasing primary production.

  • Eutrophication increased pCH4 and CH4 emissions due to the supply of substance.

  • Global warming potential was dominated by CH4 in the eutrophic lakes.

Abstract

Eutrophic lakes, especially shallow eutrophic lakes, disproportionately contribute to greenhouse gas (GHG) emissions. To investigate the effects of eutrophication on GHG dynamics, we conducted field measurements every three months from January 2019 to October 2019 in Lake Ulansuhai, a shallow eutrophic lake (mean depth of 0.7 m) located in a semi-arid region in Northern China. We found that Lake Ulansuhai was a predominantly source of atmospheric carbon dioxide (CO2); however, it converted to a CO2 sink in July due to eutrophication. It was also a strong source of methane (CH4) with a mean CO2 emission of 35.7 ± 12.1 mmol m−2 d−1 and CH4 emission of 5.9 ± 2.9 mmol m−2 d−1. The CO2 concentrations in most sites and CH4 concentrations in all sites were supersaturated, with the average partial pressure of CO2 (pCO2) being 654±34 μatm and the partial pressure of CH4 (pCH4) being 157±37 μatm. The partial pressures and emissions of the greenhouse gases exhibited substantial seasonal and spatial variations. The correlation analysis between the trophic level index and the partial pressure of the greenhouse gases indicated that eutrophication could significantly decrease the CO2 emissions but increase the CH4 emissions from the lake, resulting in a CH4 and CO2 emission ratio of approximately 2 in terms of global warming potential. Eutrophication decreased the pCO2 in the lake and subsequently increased the pCH4 due to nutrient input, thereby enhancing primary production. The results indicated that shallow eutrophic lakes in arid regions are strong sources of CH4 and that eutrophication could alter the greenhouse gas emission patterns.

Introduction

Inland waters play a critical role in the storage and emission of greenhouse gases (GHGs), and they are extremely active components of the local and global carbon cycles (Butman et al., 2016; Davidson et al., 2015; Saunois et al., 2020; Vachon et al., 2010). Global inland waters emitted 0.13 Pg C yr−1 of methane (CH4) (Stanley et al., 2016) and 2.1 Pg C yr−1 of carbon dioxide (CO2) (DelSontro et al., 2018; Raymond et al., 2013). Lakes are a vital component of the inland water system concerning carbon cycle and climate regulation because they store, transport, and transform carbon (Tranvik et al., 2009). Global lakes only cover 3.7% of the non-glaciated land area on Earth (Verpoorter et al., 2014); however, they emit large amounts of CO2 and CH4. While previous research has estimated global GHG emissions from lakes, certain differences have been observed between the studies (Raymond et al., 2013; Holgerson and Raymond, 2016; Li et al., 2018; Bastviken et al., 2011; Wik et al., 2016; Saunois et al., 2020).

Shallow lakes (average depth of less than 3 m) are the water bodies that account for the largest area globally (Downing et al., 2006; Verpoorter et al., 2014), while functioning as important sources of carbon efflux (Bastviken et al., 2011; Tranvik et al., 2009). Low water levels lead to reduced water hydrostatic pressure and shorter gas transport pathways (DelSontro et al., 2011). This can reduce CH4 oxidation and lead to rapid CH4 emission into the atmosphere, thereby resulting in greater proportions of GHG emissions from shallow lakes than those observed from deep lakes (Li et al., 2020). Furthermore, shallow lakes are affected by turbulence (Margalef, 1997; Zhu et al., 2018). Wind-driven turbulence can lead to thermal destratification and hypoxia reduction, thereby enhancing organic matter aerobic decomposition and inhibiting CH4 production. Thus, turbulence can increase CO2 emissions and decrease CH4 emissions (Jalil et al., 2018; Jung et al., 2014; Zhu et al., 2018). Compared to deep lakes, shallow lakes are more vulnerable to eutrophication due to high nutrient loadings and poor self-cleaning capacity (Havens et al., 2001; Li et al., 2020). In addition, their nutrient levels and biotic interactions can affect GHG production and biogeochemical processes (Davidson et al., 2015). The high nutrient loadings in shallow lakes stimulate mineralisation and provided more liable organic substrates to enhance CH4 production, causing an increase in GHG emissions (Li et al., 2020; Xiao et al., 2020). In contrast, nutrient enrichment can enhance primary production to promote the carbon fixation efficiency of lake sediments (Gu et al., 2011), resulting in a decrease in CO2 emissions. Additionally, aquatic plants and phytoplankton drive the primary production in shallow lakes and therefore reduce CO2 emissions due to their CO2 uptake from the water (Engel et al., 2019). However, the dynamics of CO2 and CH4 under different eutrophication levels in a lake remain unclear. Therefore, it is necessary to study the impact of eutrophication on GHG emissions in shallow lakes.

Eutrophication is a serious environmental problem in inland lakes (Anderson et al., 2014; Sinha et al., 2017), and the eutrophic status plays a critical role in influencing the amount of GHG emissions from inland lakes (Davidson et al., 2015; Xiao et al., 2017). Studies have found that eutrophication can stimulate GHGs emissions. For example, a laboratory incubation experiment demonstrated that hypereutrophic lakes exhibited higher CH4 emissions than those of oligotrophic lakes (Sepulveda-Jauregui et al., 2015). Studies on GHG emissions from continuous eutrophication lakes and impoundments illustrated that higher levels of eutrophication can increase GHG emissions into the atmosphere (DelSontro et al., 2018). In contrast, Balmer and Downing found that eutrophication enhanced primary production, thereby decreasing CO2 emissions (Balmer and Downing, 2011). Although the impact of eutrophication on GHG emissions has been investigated, the results of previous studies have been contradictory (Davidson et al., 2015; Sepulveda-Jauregui et al., 2015; Xiao et al., 2020). Furthermore, it is not clear how different levels of eutrophication affect GHG dynamics. In China, there are approximately 185,000 lakes, that exhibit a total surface area of approximately 82,232 km2 (Yang and Lu, 2014). Eutrophication has become one of the most prominent problems faced by freshwater shallow lakes (Zhao et al., 2012). Therefore, more studies are required to study the impact of eutrophication on GHG emissions.

Here, we conducted a field study to explore the effect of eutrophication on GHG dynamics in an arid shallow lake. The specific objectives of this study were (1) to evaluate the spatial and seasonal variations and possible influence factors associated with the partial pressures and emissions of CO2 and CH4, and (2) to clarify the impact of eutrophication on CO2 and CH4 emissions in shallow lakes.

Section snippets

Study area and sampling sites

Lake Ulansuhai (40°46′–41°08′N, 108°40′–108°57′E) is located in Urad Qianqi, Bayannaoer City of the Inner Mongolia Autonomous Region, China (Fig. 1). It is a typical furiotile lake with a total area of 293 km2 and a storage capacity of 2.5–3 × 108 m3 (Ma et al., 2013). The depth of the lake ranges from 0.5 to 3 m, with an average depth of 0.7 m (Köbbing et al., 2014). It lies in a temperate continental climate with four distinct seasons, and the ice-free period of the lake is from April to

Temporal and spatial variation of environmental variables and trophic states

Lake Ulansuhai exhibited significant temporal variations in its biological and chemical properties (Fig. 2, Fig. 3). Furthermore, there were significant differences in mean value of pH, DO, DOC, TN and NO3 amongst the seven sites; however, there were no significant spatial variations in the mean water temperature, Chl-a, DIC, TP, DTP and NH4+. The pH value changed from 7.05 to 10.00 with a mean value of 8.82±0.13; the pH value at S1 was slightly lower than that of the other sites (Fig. 2b).

CO2

Water environments affect the balance of aquatic CO2 production and consumption. pCO2 exhibited spatial and temporal variations in Lake Ulansuhai, suggesting that the CO2 dynamics were affected by ambient parameters that also demonstrated spatial and temporal variations. The correlation analysis results demonstrated that pCO2 was negatively correlated with water temperature (R2=0.31, p<0.01) and pH (R2=0.26, p<0.01) (Fig. 8a and b), which was similar to the results reported in previous studies (

Conclusions

In this study, we investigated GHG (CO2 and CH4) emissions and environmental parameters in Lake Ulansuhai from January 2019 to October 2019 by conducting trimestral sampling campaigns. This study explored the dynamics of CO2 and CH4 in a hypereutrophic lake, demonstrating large spatial and temporal variations in the partial pressures and emissions of CO2 and CH4. Our results indicate that the lake is a source of CO2 but a stronger source of CH4, and that an increase in the trophic state (e.g.

CRediT authorship contribution statement

Heyang Sun: Visualization, Methodology, Formal analysis, Writing – original draft, Writing – review & editing. Xixi Lu: Visualization, Writing – original draft, Writing – review & editing. Ruihong Yu: Visualization, Writing – original draft, Writing – review & editing. Jie Yang: Visualization, Writing – original draft. Xinyu Liu: Writing – review & editing. Zhengxu Cao: Methodology, Formal analysis. Zhuangzhuang Zhang: Methodology, Formal analysis. Meixia Li: Methodology, Formal analysis. Yue

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have influenced the work reported in this paper.

Acknowledgments

This study was funded by the Major Science and Technology Projects of Inner Mongolia Autonomous Region (grant nos. 2020ZD0009 and ZDZX2018054), National Natural Science Foundation of China (grant no. 51869014), Key Scientific and Technological Project of Inner Mongolia (grant no. 2019GG019), National Key Research and Development Program of China (grant no. 2016YFC0500508), and Open Project Program of the Ministry of Education Key Laboratory of Ecology and Resources Use of the Mongolian Plateau

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