* By Aquaculture Magazine Editorial Team
Aquatic food production, including shrimp farming, contributes significantly to global greenhouse gas (GHG) emissions. A study on Litopenaeus vannamei shrimp ponds in China explored strategies to reduce emissions, such as liming pond water to alter CO2 levels. While shrimp ponds emit lower methane and nitrous oxide than other aquaculture systems, the processes controlling GHG fluxes remain poorly understood, hindering the development of effective emission-reduction strategies in aquaculture.
To limit global warming to below 1.5-2.0°C, carbon dioxide removal strategies must offset unavoidable greenhouse gas (GHG) emissions, which include emissions from food production, contributing 23% of global GHG emissions from 2007 to 2017.
Aquatic food production is a critical part of the global food system, providing essential proteins and micronutrients. By 2016, aquaculture accounted for 47% of global aquatic food production with shrimp farming, particularly Litopenaeus vannamei, becoming increasingly important in China.
Despite its growing significance, aquaculture, especially shrimp farming, has a higher environmental impact, with significant GHG emissions due to energy use and nitrous oxide (N2O) production. In fact, shrimp farming is characterized by a high GHG emission intensity compared to other aquaculture products.
To address this, recent studies have focused on better understanding and reducing GHG emissions in aquaculture. A study on a L. vannamei pond in China explored how liming, which raises the pH on pond water, could potentially convert a CO2 emitting pond into a CO2 sink. However, the environmental impacts of these treatments remain uncertain due to the emissions associated with their application.
The study also examined the low methane (CH4) and N2O emissions in the pond, but the processes controlling these emissions are still not fully understood. These findings highlight the need for further research to develop effective strategies to reduce GHG emissions from aquaculture.
Material and Methods
The study was conducted at a plastic lined L. vannamei shrimp pond in Wenchang, China, between April 14 and May 4, 2016. The pond, managed by Hainan Guangtai Marine Animal Breeding Ltd., had an area of 1296 m2 and a depth of 1.5 m. Water was a mix of farm-treated and seawater (70:30 ratio).
Paddle wheel aerators provided oxygen for the shrimp, which were in their grow-out phase. During the 21-day experiment, shrimp were fed a protein-rich diet three daily. The study monitored greenhouse gas (GHG), emissions, including CO2, CH4, and N2O, using a sophisticated system comprising a Ferry Box and a Fourier Transform Infrared Trace Analyzer (FTIR) (Figure 1).
The Ferry Box measured various environmental factors like wind speed, temperature, salinity, pH, and dissolved oxygen, while the FTIR analyzed gas concentrations from water samples. Data were recorded at 5-minute intervals, with some gaps due to equipment failure.
The study also utilized floating chamber experiments to calculate the gas transfer velocity (kw) for CO2 fluxes. A box model was developed to simulate gas exchange, primary production, and oxygen dynamics in the pond. The model incorporated various biochemical processes, including respiration, feeding, and phytoplankton growth, to estimate changes in dissolved inorganic carbon (DIC) and total alkalinity (TA).
Results and Discussion
Physical eater parameters and dissolved gases
The pond water was brackish with salinities ranging from 20.1 to 27.2, pH fluctuating between 7.5 and 8.2 with a mean of 7.9, aligning with conditions in the eastern tropical Pacific Ocean where L. vannamei is native. Water temperatures from 26.5 °C to 31.2°C, rising by about 1.5°C during the study. Wind speeds were lower in later periods, resulting in reduced gas exchange rates (kw), especially in periods 3 and 4 (Figure 2).
The calculated wk values, following the Wannimkhof and McGillis (1999) method, showed a strong diurnal cycle. Similarly, pH and gas concentrations (N2O, CH4, CO2) exhibited daily cycles, with varying amplitudes, between early and later.
Diurnal cycle of N2O
In aquaculture ponds, the mechanisms controlling N2O concentrations are not fully understood. In the ocean, nitrification and denitrification are primary N2O sources. Nitrification, which is inhibited by light, peaks at night. However, night-time N2O concentrations were lower than expected, suggesting that nitrification was not the main source.
Denitrification, also N2O source, may act as a sink when oxygen is low, but during this study, oxygen levels were generally above the threshold that would promote denitrification. Hence, denitrification in pond sediments or the shrimp´s gut could explain the diurnal N2O cycle, with feeding in the morning raising N2O concentrations.
Night-time N2O concentrations below atmospheric levels in later periods suggest a possible N2O decomposition exceeding atmospheric input. This shift could be related to reduced wind speeds and changes in oxygen levels, which influenced microbial activity.
Diurnal cycle of CH4
CH4 exhibited a less pronounced diurnal cycle compared to N2O. Its concentrations were consistently higher than atmospheric levels, a phenomenon not fully understood but likely linked to sediment processes and organic matter breakdown. Possible sources include microbial activity in oxygen-rich waters and zooplankton grazing. The higher CH4 concentrations in later periods may reflect increased sediment accumulation, though sediment data are lacking to confirm this.
Diurnal cycles of CO2, dissolved oxygen, and pH
The pronounced daily cycles of CO2, dissolved oxygen, and pH indicate the importance of primary production and respiration. Photosynthesis during the day fixes CO2 and raises pH, while respiration at night increases CO2 and lowers pH. These cycles affect oxygen availability, with oxygen saturation higher during the day and lower at night. Despite lower nighttime oxygen levels, it remained above the hypoxic threshold, ensuring sufficient oxygen for L. vannamei. Paddle wheel aerators were used to maintain oxygen levels, especially at night.
Primary production
Primary producers, such as phytoplankton, are not the main focus in shrimp ponds but are beneficial dure to their roles in oxygen production, shading shrimp, and ammonia removal. Phytoplankton biomass serves as a low-cost food source for L vannamei, and its uptake of ammonia helps mitigate its toxic effects. Primary production rates in shrimp ponds typi-cally range from 2-12 g C m2d-1, with the study pond potentially reaching up to 27.8 µmol C L-1hr-1, comparable to other well-managed ponds.
Quantitative estimates
A numerical model was developed to simulate GHG production and emission. The model used measured data to predict CO2, N20, and CH4 fluxes. Although the model simplified some aspects, it captured the general trends and allowed for comparisons between observed and calculated cycles. The calculated primary production rate in periods 1 & 2 peaked at 30 µmol C L-1hr-1, consistent with field measurements. The model also replicated diurnal cycles of gas concentrations, with minor discrepancies in timing and peak values, likely due to the model’s simplifications of anaerobic processes.
Periods 1 & 2. During these periods, the model predicted a steadystate for oxygen, N2O, and CH4 after approximately two days, with slight underestimation of N2O and CH4 peaks. The model also indicated that CO2 concentrations would increase over time due to food respiration, However, with lime addition, CO2 levels could be reduced, shifting the pond from a CO2 sink to a CO2 source after day 6.
Periods 3 & 4. For periods 3 & 4, the model indicated lower wind speeds and gas exchange rates, leading to different dynamics in CO2, N2O, and CH4 production. The model´s results were consistent with the observed transition from CO2 sink to source, even without lime addition. Increased N2O production rates and a switch from a night-time N2O source to a sink were likely influenced by reduced oxygen levels, which affected microbial processes.
GHG fluxes
Net CO2 uptake was observed at 0.25 ± 0.36 mmol C m2 hr-1, increasing with paddle wheel aerators. However, the impact of lime addiction on CO2 fixation and pH made it uncertain whether the pond could be considered a true CO2 sink, GHG emissions were quantified at 9.05 ± 11.34 μg CH4 m2 hr-1 and 2.43 ± 3.74 μg N2O m2 hr-1, These fluxes were lower than those observed in estuarine or earthen ponds, suggesting that plastic-lined shrimp ponds may emit less GHG. Overall, this study provides insights into biogeochemical processes affecting GHG fluxes in intensive shrimp farming.
Conclusion
The data from our GHG monitoring system, aligned with model results, suggest that liming could convert a pond from a CO2 source to a CO2 sink.
However, CO2 emissions from lime production, delivery, and water treatment raise questions about whether liming results in a net CO2 sink. Transforming mangroves into aquaculture sites may act as a CO2 source by reducing organic carbon burial. While CH4 and N2O emissions were low compared to other aquaculture systems, the underlying processes remain unclear. The use of plastic to shield pond waters from ambient environments could reduce emissions. However, the limited understanding of these processes hinders the development of strategies to lower GHG emissions in shrimp cultures.
This informative version of the original article is sponsored by: REEF INDUSTRIES INC.
This is a summarized version developed by the editorial team of Aquaculture Magazine based on the review article titled “GREENHOUSE GAS CONCENTRATIONS AND EMISSIONS FROM A PLASTIC-LINED SHRIMP POND ON HAINAN, CHINA” developed by: RIXEN, T.- Leibniz Centre for Tropical Marine Research University of Hamburg; DREWS, M.- Leibniz Centre for Tropical Marine Research; VAN ASPEREN, H. – Institute for Environmental Physics; DAORU, W. – Hainan Academy for Ocean and Fishery Science; KLEMME, A. – Institute for Environmental Physics; THORSTEN, W. – Institute for Environmental Physics. The original article, including tables and figures, was published on FEBRUARY, 2023, through ESTUARINE, COASTAL AND SHELF SCIENCE. The full version can be accessed online through this link: https://doi. org/10.1016/j.ecss.2023.108278
