Extreme Methane Bubbling Emissions from a Subtropical Shallow Eutrophic Pond

Research Article

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

Extreme Methane Bubbling Emissions from a Subtropical Shallow Eutrophic Pond

Shangbin Xiao1,2*, 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


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


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.