Fluxes of greenhouse gases at two different aquaculture ponds in the coastal zone of southeastern China
Introduction
Carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) are key radiatively active greenhouse gases (GHGs) in the atmosphere that have been recognized as contributing to global warming by 60, 25 and 5%, respectively (Mosier, 1998). The current global atmospheric concentrations of the three GHGs are approximately 390.5 ppm, 1803 ppb and 324.2 ppb in 2011, and these concentrations have increased steadily over the past century by approximately 0.50, 1.10 and 0.26%, respectively, per year (IPCC, 2013). Increasing atmospheric concentrations of GHGs have stimulated research to determine their production and emissions in different ecosystems (Selvam et al., 2014).
Global freshwater environments based on estimates from recent research could be emitting 1.2–2.1 Pg of C (CO2 equiv.) year−1 as CO2 (Aufdenkampe et al., 2011) and 0.65 Pg of C (CO2 equiv.) year−1 in the form of CH4 (Bastviken et al., 2011), i.e., more than 2 Pg C (CO2 equiv.) year−1 in total. It has been suggested that CO2 and CH4 emissions from freshwater environments counterbalance a large portion of the global land carbon sink of 2.6 Pg of C year−1 (Selvam et al., 2014). Freshwater aquatic ecosystems play an important role in global carbon biogeochemical cycles and are considered significant emission sources of GHGs (Natchimuthu et al., 2014). Therefore, GHG emissions from aquatic ecosystems have been studied widely in recent years because of their contribution to global warming (Selvam et al., 2014). At present, research on GHG emissions from aquatic ecosystems is focused on inland freshwaters, including natural lakes (Huttunen et al., 2003, Xing et al., 2005), rivers (Aufdenkampe et al., 2011, Clough et al., 2011), ditches (Schrier-Uijl et al., 2011) and reservoirs (Soumis et al., 2004, Diem et al., 2012). These reports suggested that the magnitude and pattern of spatiotemporal variations in GHG emission are influenced by weather, water thermal regime, nutrient content, hydrodynamic condition and biological activity (Zhu et al., 2010, Palma-Silva et al., 2013, Natchimuthu et al., 2014). However, very few studies have presented GHG fluxes from aquatic environments in coastal zones, especially fluxes caused by anthropogenic disturbances on aquatic environments, including aquaculture ponds.
Currently, large coastal wetlands areas around the world have been converted to aquaculture ponds because of economic benefits and the expansion of human populations (Cao et al., 2011). In contrast to natural wetlands, aquaculture ponds receive large amounts of nutrient inputs from aquaculture activities (Serrano-Grijalva et al., 2011). Excess availability of nutrients facilitates aquaculture pond primary production but can also have significant effects on microbial processes, which can directly or indirectly affect carbon and nitrogen biogeochemical processes. However, information on global GHG emissions from coastal aquaculture ponds is very limited, and the potential effects of exogenous nutrient loading on producing and emitting gaseous carbon (CH4 and CO2) and nitrogen (N2O) into the atmosphere have remained poorly documented until now. Additionally, microbial processes affecting GHG production are regulated by many parameters, including temperature, salinity, pH, dissolved oxygen and substrate availability (Xing et al., 2005, Schrier-Uijl et al., 2011). Evaluating the influence of different environmental factors on GHG emissions from coastal aquaculture ponds is important.
Aquaculture ponds cover a large and significant area in China's entire coastal zone; the area of aquaculture ponds was 1260 km2 (Zuo et al., 2013). Therefore, it is critical to quantify GHG fluxes and controlling factors in aquaculture ponds to accurately evaluate coastal ecosystem GHG budgets and scientifically manage coastal wetlands. We investigated GHG fluxes and environmental variables in this paper for two coastal aquaculture ponds in the Min River estuary. The aims of this study are to (1) quantify and compare GHG fluxes at the water-atmosphere interfaces of a shrimp pond and mixed culture pond and (2) investigate the temporal variations in GHG fluxes and influencing factors.
Section snippets
Study area and sampling sites
The study region is located on the Shanyutan wetland (26°00′36″–26°03′42″ N, 119°34′12″–119°40′40″ E) in the Min River estuary, southeastern China; this wetland covers 3120 ha. The climate is typically subtropical monsoon with a mean annual temperature of 19.6 °C and a mean annual precipitation of 1350 mm (Zheng et al., 2006). Within the Shanyutan wetland there is a tidal marsh vegetation landscape dominated by native species Phragmites australis and Cyperus malaccensis and the invasive plant
Environmental variables and trophic conditions
The daily air temperature during the sampling period ranged from 21.4 to 33.4 °C in the shrimp pond and from 18.8 to 32.0 °C in the mixed culture pond (Fig. 2a). The water temperature at a depth of 10 cm ranged from 20.3 to 33.3 °C and from 17.4 to 31.3 °C in the two ponds (Fig. 2b). Daily atmospheric pressure varied from 999.2 to 1016.4 Pa and from 1007.5 to 1018.7 Pa in the shrimp pond and mixed culture pond, respectively (Fig. 2c).
The water in both ponds was slightly alkaline, with a mean pH
CO2 flux and environmental factors
The dominate factor controlling CO2 production and emission in different aquatic environments is different. Tank et al. (2009) reported that the algal activity and the mineralization of organic matter might play a more important role in CO2 flux dynamics in the Mackenzie Delta lakes within the Arctic Landscape. Zhu et al. (2010) found that the respiration and the mineralization of organic matter in sediments were important factors influencing CO2 emission in Lakes Tuanjie and Mochou in the
Conclusion
The results in this study indicate that aquaculture ponds in the coastal zone are strong CH4 emitters and weak N2O emitters during the culture period and are a source of GHG emissions during the drained period. GHG fluxes are regulated by pH, thermal regime, trophic status and metabolic activity of algae in aquaculture ponds during the culture period. GHG emissions from the aquaculture ponds of coastal zones in China play a more important role in global GHG budgets because of their higher CH4
Acknowledgements
This research was supported financially by the National Science Foundation of China (No.41071148, 41371127), the Program for Innovative Research Team of Fujian Normal University (IRTL1205) and the Key Sciences and Technology Project of the Fujian Province (2014R1101). We would like to thank Hui-Xu and Yong-Yue Ma of the School of Geographical Sciences, Fujian Normal University for their field assistance and Liu-Ming Yang and Yuan-Zhen Peng of the School of Geographical Sciences, Fujian Normal
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