Shangbin Xiao; Weiguo Liu; Hong Yang; Defu Liu; Yuchun Wang; Feng Peng; Yingchen Li; Chenghao Wang; Cheng Zhang; Xianglong Li; Gaochang Wu; Li Liu; Kaihua Ouyang

Research Article

Austin Biom and Biostat. 2014;1(2): 6.

Extreme Methane Bubbling Emissions from a Subtropical Shallow Eutrophic Pond

Shangbin Xiao^1,2*, Weiguo Liu¹, Hong Yang³, Defu Liu⁴, Yuchun Wang⁵, Feng Peng², Yingchen Li², Chenghao Wang², Cheng Zhang², Xianglong Li², Gaochang Wu², Li Liu² and Kaihua Ouyang²

¹Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710075, China

²College of Hydraulic & Environmental Engineering,China Three Gorges University, Yichang, 443002, China

³Bryant University, Smithfield RI, 02917, USA;

⁴College of Resources and Environment Sciences, Hubei University of Technology, Wuhan, 430068, China

⁵Department of Water Environment, China Institute of Water Resources and Hydropower Research, Beijing, 100038, China

*Corresponding author: Shangbin XIAO, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 10075, China

Received: September 29, 2014; Accepted: October 27, 2014; Published: November 01, 2014

Abstract

For the first time we report on real time diel bubble and diffusion gas fluxes lasting for 48 hours of a subtropical shallow pond. The averaged diffusion fluxes of methane and carbon dioxide were 0.074 and 62.70 mg•m^-2•h^-1, and the averaged ebullition fluxes of methane and carbon dioxide were 24.726 and 1.92 mg•m^-2•h^-1 respectively. Bubble emissions of CH₄ andCO₂ accounted for 99.7% of the total CH₄ emission and only 3.0% of the totalCO₂ from the pond respectively. The CH₄ flux across the water-air interface of the pond was 595.20 mg•m^-2•h^-1 and equaledCO₂ flux of 14880.0 mg•m^-2•d^-1 by multiplying its global warming potential. Thus, the small pond added equivalent of 35.712 kg/dCO₂ emission by transferring CO₂ to CH₄ in the summer, in which processCO₂ was absorbed owing to alga propagation and CH₄ emission was derived from the anaerobic degradation of dead alga buried on its bottom.

Keywords: Methane; Carbon dioxide; Bubbling efflux; Diffusion; Eutrophic; Pond

Introduction

Freshwaters can be substantial sources of CO₂ and CH₄ [1-3]. Lakes, ponds, and impoundments cover 3% of the earth's surface. Of them, natural lakes and ponds are estimated to cover about 4.2x10⁶ km², whereas impoundments cover 2.6x10⁵ km², and farm ponds cover about 7.7x10⁴ km2 [4]. Greenhouse gases emitted from lakes and reservoirs received respective attention [5-10]. However, less attention was paid to ponds, which are small in area and relatively shallow in depth but with various physical geographical characteristics and eutrophic situation. The importance of small lakes and ponds in the global lake area/number and in the global carbon cycle might have been underestimated [2,4,11]. Small wetland lakes and ponds that are often abundant in peat land areas can have high CO₂ and CH₄ emissions [12,13]. Moreover, it has been shown that the small areas of high CH₄ emissions, such as wetland ponds, can largely contribute to the landscape-scale CH₄ budgets in wetland regions and create a major uncertainty in the areal CH₄ emission estimates [14].

The rotation of the earth results in diel biogeochemical cycles, which are in response to the solar photocycle, particularly during stable hydrological conditions [15]. The amplitude of some of these diel changes can be as large as the changes occurring on annual timescales [15]. However, reports about gas fluxes on the diel timescale are scarce compared to those on the seasonal timescale. The former has received attention only more recently. To date, most reported gas ebullition rates were measured with bubble collectors [16-19], which could represent an average flux rate of a long monitoring period. No report could be found about the real-time gas ebullition emission. Study of diel variations is helpful to reveal which biogeochemical processes occur relatively rapidly in natural waters, and therefore which processes play an integral and important role in the normal functioning of natural water systems [15]. Data of diel greenhouse gas fluxes reported previously were at intervals of 3-4 h [20,21], and not continuous. Here, for the first time we present data of greenhouse gas flux from a field campaign lasting for 48 hours in the summer of 2013 regarding a subtropical hypertrophic pond. We hope the real time results can give some knowledge for recognizing the greenhouse gases emitting from small and shallow ponds both by ebullition and by diffusion.

Materials and Methods

Study area and monitoring sites

The pond (111°20'50.16"E, 30°44'30.978"N) is located at a suburban district of Yichang city, Hubei province, Central China. Yichang city is of a subtropical continental monsoon climate with a largely changing temperature in the spring, heavy rain and drought in the summer, wet weather in autumn and humid and snowy weather in the winter. T?he average annual temperature is 16.9 0C, and the average rainfall is 1215.6 mm. The pond is about 2,500 m² with a maximum and mean water depth of 2.5 and 1.5 m respectively. It is surrounded by a small, locally known restaurant, and is a popular fishing area. T?he total nitrogen and phosphorus concentrations in waters are about 0.067 and 0.020 mg/L respectively. The pound bottom is covered with soft organic sediments (the total organic carbon content is 27.92 mg/g). The waters appeared green color owing to a great deal of the growing algae(mainly Fragilaria, Scenedesmus quadricanda, and Oocystis). We carried out diel flux measuring at two sites, which are 2 and 5 m away from its bank, and with depths of 1.2 and 1.5 m respectively (here marked as Site N and F). The field campaign lasted 48 hours from July 22 to 24, 2013. Here, we only report the results of Site N because of its representative water depth and location.

In situ sampling measurements and analysis

Surface and bottom water temperature (Ts and Tb), pH, air temperature (Ta), air pressure (Pa), intensity of illumination (Ii) and wind speed (Swi) were measured at the sites. Water temperature, the pH of water, and dissolved oxygen concentration (DO) in water were measured using the multi-parameter instrument Orion Star A329 (the United States). Water samples were taken from depths of 0.1 m below the water surface and 0.1 m above the bottom respectively for analysis of chlorophyll a concentration (Chl-a), and dissolved CH₄ and CO₂. Water samples (350 mL) were collected and transported to the laboratory for Chl-a analysis using the national standard method [22]. A headspace equilibration technique was used to quantify dissolved gas concentrations in water, and dissolved gas concentration in water is calculated according to the equation given by Johnson et al. [23].

Water-to-air fluxes

A dynamic closed floating chamber system was used to measure diel CH₄ and CO₂ flux across the water-air interface. The chambers are non-transparent, thermally insulated tubes with a volume of 43.30 L and a surface area of 0.096 m2 (diameter and height are 0.35 and 0.45 m respectively). Fans inside the chambers were applied to obtain better mixing of the air inside the chamber headspace. When we measured the emission flux of CH₄ and CO₂ across the waterair interface, one chamber was connected to a Los Gatos Research's Greenhouse Gas Analyzer (DLT-100) (Los Gatos Research, USA), which could monitor the CH₄ and CO₂ concentration inside the chamber continuously with 1Hz frequency. The DLT-100 is a cavity ring down spectrometer with high resolution (0.1 ppb) and precision (1% of reading the accuracy) and was already described in detail and used by previous researchers [8,9,24-28]. A single flux measurement was usually finished in 30 minutes. The chamber was then taken off the water surface to ensure adequate exchanging and mixing between gas inside the chamber and the environmental air. A separate chamber at the other site was connected to the DLT-100 Analyzer to continue the f?lux measuring. Thus, we alternated between monitoring sites N and F.

When there is no or little gas bubbles present in the chamber, concentrations of CH₄ and CO₂ gradually curve over time in a straight line due to the increase or decrease of gas concentrations in the chamber (Figure1a). Under this situation, a simple linear regression method is used to calculate the releasing rate and the flux of gases, which was described in detail by Lambert and Fréchette [29]. The gas concentration in the chamber will increase abruptly when bubbling occurs (Figure1b). We can separate the bubble and diffusion flux with data acquired by the DLT-100 Analyzer thanks to its high sampling frequency (See the captions of Figure 1b) in (b): AB & CD- diffusing, BC-bubbling. The equation (Y = 0.00320*X + 2.41913) is acquired by linear fitting based on data of AB; Ct is the measured CH₄ concentration at the monitored endpoint; Cd is the calculated CH₄ concentration at the monitored endpoint according to the fitted equation; and the surplus CH₄ concentration in the chamber resulted from bubble is equal to Ct minus Cd. The figure above displays the calculated averages derived from original data. Each data point is derived from 20 individual data entries.

Figure 1: Curve Patterns of Changes in CH₄ concentration without bubble (a) and with bubble (b) during a single flux monitoring period.

    
    
    Figure 1:  Curve Patterns of Changes in CH₄ concentration without bubble (a) and with bubble (b) during a single flux monitoring period.

Results

Variations in environmental factors

During the monitoring period, air temperature experienced a large net change of 12.3 °C (Table 1). However, surface and bottom water temperature changed only 5.7 and 4.4 °C respectively, and they showed a synchronal change (Figure 2). The differences between them were less than 2.0 °C due to the shallow water depth at the site. Surface and bottom water temperature had a similar average value, and was greater than the averaged air temperature.

Table 1: Statistical description of main environmental factors.




  
    
    Ta
      (°C)
    Pa
      (KPa)
    Swi
      (m/s)
    L
      (Lux)
    DO in water
      (mg/L)
    pH of water
    water� temperature
      (°C)
    Chl-a (mg/L)
  
  
    DOs
    DOb
    pH-s
    pH-b
    Ts
    Tb
    Chl-as
    Chl-ab
  
  
    Max
    37.1
    98.97
    0.99
    78770
    8.57
    6.19
    7.13
    7.01
    34.7
    33.3
    76.578
    92.236
  
  
    Min
    24.8
    98.52
    0
    0
    2.44
    2.23
    6.07
    5.52
    29.0
    29.9
    48.724
    45.232
  
  
    Average
    29.7
    98.79
    0.09
    23412.88
    5.02
    3.99
    6.60
    6.63
    31.1
    31.1
    60.021
    64.640
  
  
    Stdev
    3.65
    0.11
    0.23
    29538.56
    1.61
    0.98
    0.23
    0.27
    1.17
    0.82
    7.50
    11.28
  
  
    CV
    0.12
    0.00
    2.58
    1.26
    0.32
    0.24
    0.03
    0.04
    0.04
    0.03
    0.12
    0.17
  
  
    Stdev: standard deviation; CV:    coefficient of variation. Dos and DOb: dissolved oxygen concentration in surface and bottom    water; pHs and pHb: pH of surface and bottom water; Ts and Tb: surface and    bottom water temperature; Chl-as and Chl-ab: Chl-a concentration in surface    and bottom water respectively.



Table 1:  Statistical description of main environmental factors.

Figure 2: Diel CH₄ and CO₂ flux and environmental factors' changes during July 22-24, 2011in (a): dash-dotted and dashed lines represent the averaged diel diffusive CH₄ and CO₂ fluxes.

    
    
    Figure 2:  Diel CH₄ and CO₂ flux and environmental factors' changes during July 22-24, 2011in (a): dash-dotted and dashed lines represent the averaged diel diffusive CH₄ and CO₂ fluxes.

Most environmental factors except the wind speed, the DO content in bottom water, and the pH of water appeared obviously diel changes, which corresponded to the earth's evolution and the solar radiation (Figure 2). Significant correlations between them were observed here.

An excess of nutrients, hot weather and motionless water resulted the algal bloom in the pond (Figure 2 and Table 1). Chl-a concentration in surface water was significantly positively correlated with Ii, which indicated the dependence of photosynthesis on solar radiation. DO in surface water showed the similar changing tendency with the Chl-a concentration in surface water which resulted from the alternative of phytoplankton respiration of algae (Figure 2). Wind speed had the biggest coefficient of variation of all environmental factors, which might result from obstruction of low speed wind from the surrounding buildings.

Variations in dielCH₄ and CO₂ flux

Both the diel diffusionCH₄ and CO₂ flux varied greatly during the monitoring period (Table 2). The diel diffusion CO₂ flux showed obviously synchronal changes with air temperature and surface temperature. The diel diffusionCH₄ flux fluctuated and changed gradually. The maximum values of diel diffusionCH₄ and CO₂ flux were 7.70 and 7.51 times of the respective minimum values. The natural variability of gas fluxes means that the number and time period of flux measurements is an important factor in obtaining an accurate average [30]. Here, the averaged values of bothCH₄CH₄ and CO₂ flux occurred synchronously at around 9:00 and 20:00-21:00 of a day, which might give a referred time for single sampling when carrying out gas flux investigation in eutrophic water.

Table 2: Variation of diel CH₄ and CO₂ fluxes.




  
     
    Diffusive-CH₄
    Bubble-CH₄
    Diffusive-CO₂
    Bubble-CO₂
  
  
    Maximum (mg•m^-2•h^-1)
    0.177
    424.284
    119.84
    19.37
  
  
    Minimum (mg•m^-2•h^-1)
    0.023
    0.0
    15.95
    0.00
  
  
    Average (mg•m^-2•h^-1)
    0.074
    24.726
    62.70
    1.92
  
  
    Stdev
    0.039
    63.164
    26.47
    5.01
  
  
    CV
    0.525
    2.555
    0.42
    2.61



Table 2:  Variation of diel CH₄ and CO₂ fluxes.

CH₄ and CO₂ atmospheric fluxes

Both total CH₄ and CO₂fluxes across the water-air interface at Site F were very high, and were 595.20 and 1550.79 mg•m^-2•d^-1respectively. The CH₄ flux is 75.06 times of the overall average value across the water-air interface of the Three Gorges Reservoir [31] located in the same climate zone, and also much higher than the highest value (~150 mg•m^-2•d^-1) of multitude reservoirs documented from Lake Wohlen, a run-of-river hydropower reservoir located in Switzerland [32].

Bubble emissions of CH₄ accounted for 99.7% of the total CH₄ emission, which implied that CH₄ emitted into the air mainly through bubbles in the shallow eutrophic pond. However, bubble emissions of CO₂ only accounted for 3.0% of the total, which showed that the ebullition had no significance in the release of CO₂ from the pond. The result verifies that ebullition does not make a significant contribution to the release of CO₂ from shallow eutrophic freshwater systems to the atmosphere, in contrast to the situation for CH₄ where ebullition from them was significant [19]. The phenomenon was also observed in Petit Saut reservoir, where gas bubbles contained 50 to 80% CH₄ with few amounts (<1%) of CO₂ [16]. In the small hypertrophic freshwater Priest Pot, most CH₄ (96%) was also lost by ebullition, and most CO₂ (99%) by diffusive processes [33]. The ebullition CH₄ efflux was 593.42 mg•m^-2•d^-1, which is much bigger than reported lakes, reservoirs and ponds except that from the exposed Reservoir Lokka (seasonal average up to 656 mg•m^-2•d^-1) [34].

Discussion

Diel CO₂ and CH₄ diffusion flux

DielCO₂ diffusion flux during the observing period was significantly positively correlated to the air pressure, and negatively correlated to the air temperature, intensity of illumination, DO in surface water, surface and bottom water temperature, and Chl-a in surface and bottom water (Table 3).

Extreme Methane Bubbling Emissions from a Subtropical Shallow Eutrophic Pond

Abstract

Introduction

Materials and Methods

Study area and monitoring sites

In situ sampling measurements and analysis

Water-to-air fluxes

Results

Variations in environmental factors

Variations in dielCH₄ and CO₂ flux

CH₄ and CO₂ atmospheric fluxes

Discussion

Diel CO₂ and CH₄ diffusion flux

Diel CH₄ ebullition flux

C cycling mode of tropic shallow eutrophied impoundments

Acknowledgment

Author Contributions

References

Extreme Methane Bubbling Emissions from a Subtropical Shallow Eutrophic Pond

Abstract

Introduction

Materials and Methods

Study area and monitoring sites

In situ sampling measurements and analysis

Water-to-air fluxes

Results

Variations in environmental factors

Variations in dielCH4 and CO2 flux

CH4 and CO2 atmospheric fluxes

Discussion

Diel CO2 and CH4 diffusion flux

Diel CH4 ebullition flux

C cycling mode of tropic shallow eutrophied impoundments

Acknowledgment

Author Contributions

References

Variations in dielCH₄ and CO₂ flux

CH₄ and CO₂ atmospheric fluxes

Diel CO₂ and CH₄ diffusion flux

Diel CH₄ ebullition flux