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*By Aquaculture Magazine Editorial Team
The aquaculture sector contributes significantly to methane (CH4) and nitrous oxide (N2O) emissions. This study compared the emissions of earthen aquaculture ponds and plastic-lined aquaculture ponds at a shrimp farm in the Beibu Gulf. Plastic-lined aquaculture ponds reduced CH4 emissions by 96% and N2O emissions by 79%, improving ecological sustainability and production.
The global aquaculture sector has expanded rapidly due to rising food demands and diminishing natural marine resources (FAO, 2022; Xu et al., 2023). There is an estimated total of ~180,000 km2 of aquaculture ponds globally (FAO, 2019; Zhang et al., 2023), with Asia accounting for an overwhelming 91.6% of the world’s aquaculture production, and over half of that comes from mainland China (FAO, 2022; Tian et al., 2024).
The vast majority of the shrimp farms in China consist of small-hold earthen ponds with minimal management intervention (Avnimelech and Ritvo, 2003; Pouil et al., 2019; Saraswathy et al., 2022). The pond sediment, influenced by excessive use of fertilizers, unconsumed feeds and animal debris, often acts as a reservoir of pollutants and pathogens. Some farmers install plastic liners in shrimp ponds to seal off the sediment, which proves to be an effective approach to prevent seepage and enhance water quality and animal health (Naranjo-Paramo et al., 2022; Satanwat et al., 2023; Saraswathy et al., 2022).
In addition to being a potential source of nutrient pollutants and pathogens, aquaculture ponds may act as hotspots for greenhouse gas (GHG) emissions including CH4 and N2O. To properly investigate the effectiveness of plastic liners in lowering GHG emissions, it was conducted a study in Beibu Gulf, China to explore the effects of plastic liners on CH4 and N2O emissions and their major driving factors in aquaculture ponds.
Materials and methods
The research was carried out in the Zhulin aquaculture farm in Beibu Gulf, Guangxi, China. In the study area, the prevalent pond types are earthen aquaculture ponds (EAPs) and plastic-lined aquaculture ponds (PLAPs) for monospecific culturing of Litopenaeus vannamei.
For this study, water and gas samples were collected from three EAPs and three PLAPs. In each of the ponds, there was one sampling site near the edge of the pond, one at the center of the pond, and one at the midway point between the two. Taking into account the three different stages (i.e. initial, middle and final) of the growout cycle as well as the logistics. In the laboratory, water samples were analyzed for dissolved CH4 and N2O concentrations.
Results
When comparing across farming stages, we observed generally lower salinity and dissolved oxygen (DO), but higher total organic carbon (TOC) and nitrogen substrates during the middle stage. While dissolved CH4 and N2O concentrations in the two aquaculture pond types were highly variable, ranging from 0.1 to 0.5 µmol L− 1 (Figure 1) and 1.5–87.5 nmol L− 1 (Figure 1c), respectively, and were always supersaturated with respect to the atmosphere (Figure 1b and 1d). Overall, the mean CH4 and N2O concentrations in EAPs were significantly higher than those in PLAPs (Figure 1a). CH4 and N2O concentrations also showed significant variations in time (Figure 1a and 1c), with considerably higher values during the middle farming stage.
The average CH4 flux in EAPs was significantly greater than that in PLAPs. Both aquaculture pond types showed similar temporal patterns, with much higher CH4 emission fluxes in the middle farming stage (Figure 2a). In EAPs, ebullitive CH4 flux was 144.2–2547.6 µg m− 2 h− 1 which accounted for 87.9–92.0% of the total CH4 emission (Figure 2b). In contrast, diffusion was the dominant transport pathway in PLAPs, accounting for 94.3–97.4% of the CH4 emission (Figure 2d).
Seasonally, the highest N2O emission fluxes were observed in the middle farming stage, with a mean value of 9.6±1.9 μg m− 2 h− 1 in EAPs and 1.7±0.3 μg m− 2 h− 1 in PLAPs which were about 2–10 times higher than the other stages (p<0.001).
In terms of environmental drivers of GHG concentrations and fluxes, between the two aquaculture pond types, the variations in CCH4 (or CN2O) and FCH4 (or FN2O) correlated positively with air temperature (TA), water temperature (TW), TOC and nitrogenous substrates (e.g., total dissolved nitrogen [TDN], NH4+-N) (p<0.01), but negatively with DO and salinity (SAL) (p < 0.05). According to SEM analysis, substrate availability had a direct positive effect on dissolved GHG concentrations, subsequently impacting GHG emissions (Figure 3). Conversely, DO and salinity negatively impacted FCH4 and FN2O, respectively (Figure 3).
Discussion
The two sets of shrimp ponds in this study were virtually identical in terms of physical dimensions and farming practice, with the only difference being the installation of plastic liners in PLAPs in the initial setup, thereby allowing a direct comparison between the two to assess the environmental benefits of plastic liners.
Despite the fact that both EAPs and PLAPs had the same stocking density and feed application, TOC, TDN and inorganic N contents were all significantly higher in EAPs, particularly during the middle farming stage when farmers increased feed quantity to boost shrimp growth (Yang et al., 2020). In the case of EAPs, some of the organic debris would be decomposed by sediment microbes, returning carbon and nitrogen to the water column as TOC, TDN, NO3–N and NH4+-
N, and the higher microbial activities would have consumed more oxygen, as reflected in the lower DO levels in EAPs. In PLAPs, feed and waste that had settled out of the water column would not come into contact with the sediment, and the shrimp could still pick food particles off of the liner surface, resulting in less waste and higher yield, as indicated by the lower carbon and nitrogen levels and lower feed conversion ratio (FCR) in PLAPs.
Microbial production of CH4 within the anoxic sediment still required organic input from the overlying water in the form of unconsumed feed and animal debris. By sealing off the sediment, the plastic liners in PLAPs effectively prevented sediment methanogenesis such that the ebullitive flux was reduced to a negligible amount, and the total CH4 emission was decreased by 96% (Figure 2c).
In aerated earthen shrimp ponds, anoxic N2O production would occur predominantly in the sediment. Therefore, installation of plastic liners would eliminate N2O output from anoxic sediment. Between EAPs and PLAPs, the data showed that plastic liners lowered the N2O concentration by 2-fold (initial and final stages) to as much as 7-fold (middle stage), suggesting that sediment microbial processes contributed 50–86% of the dissolved N2O in the ponds (Figure 1c). Accordingly, N2O emission was reduced by 79% in PLAPs relative to EAPs.
This study results showed that the plastic liners in PLAPs significantly lowered both CH4 and N2O emissions relative to EAPs across all three farming stages. Based on the average flux values in PLAPs, it was calculated that plastic liners decreased GHG emission by 6910 mg CH4 m− 2 and 20 mg
N2O m− 2 over the entire farming cycle (245 days per year). Considering the warming potentials of CH4 and N2O in a century time scale, this is equal to a combined reduction of 199.4 g CO2-eq m− 2 per year.
Conclusions and recommendations
This study showed that installation of plastic liners offers a simple and effective way to cut down GHG emissions from earthen aquaculture ponds with minimal intervention, although government subsidy may be needed to encourage a wider implementation among small-hold farmers.
This article is sponsored by: REFF INDUSTRIES INC.
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “PLASTIC LINERS AS A SIMPLE AND EFFECTIVE APPROACH TO REDUCE CH4 AND N2O EMISSIONS FROM AQUACULTURE PONDS” developed by: YANG, P. – Fujian Normal University, SU, Z. – Guangxi Mangrove Research Center, TANG, K. – Swansea University and Texas A&M University, YANG, H. – University of Reading, TANG, L. – Fujian Normal University, ZHANG, L. – Fujian Normal University. The original article was published, including tables and figures, on JULY, 2024, through AGRICULTURE, ECOSYSTEMS AND ENVIRONMENT. The full version can be accessed online through this link: https://doi.org/10.1016/j.agee.2024.109191
