Introduction
The global warming potential (GWP) of the net carbon and greenhouse gas (GHG) balance associated with food production is a high priority for many consumers, who are increasingly adopting a more vegetarian diet, reducing demand for meat and seafood options. "Seafood" (from fresh or marine water) typically has a lower GWP than terrestrial animal protein options, but there is high variability. The net GHG balance of seafood production forms a further aspect of environmental impact, but has so far not been considered consistently, particularly direct emissions from aquaculture ponds. The main GHGs of interest from food production are methane and nitrous oxide which both have significant impacts on global warming. Carbon dioxide is mostly biogenic in farming systems which is considered to be neutral because it is sequestered in feed ingredients and then released again by the cultured animal through respiration. Usually of more concern are methane and nitrous oxide that have 34 and 298 times the radiative forcing effect of CO2 respectively. Early papers concerning GHG emissions from aquaculture relied on assumptions with direct measurements reported only in recent years and there is still a significant gap in our knowledge of the greenhouse gas balance of different aquaculture systems, because of the huge variability of species and systems. This paper seeks to add to the discourse on the generation of GHGs from shrimp aquaculture facilities. In shrimp ponds, methanogenesis may not be the most energetically favourable pathway (Rowe et al., 2022). In saline environments, anaerobic decomposition preferentially uses sulphate as an intermediary leading to H2S production, but reverts to methanogenesis once sulphate is depleted or, if sediment organic loading is high and there is no competition for substrate, methanogenesis can co-exist with sulphate reduction (Sela-Adler et al., 2017). Therefore the relationship between salinity and organic loading in shrimnp ponds was hypothesized to be a key determinant of methane emissions.
Methods
We took 291 separate flux measurements and corresponding data on salinity and organic matter content from shrimp farms in Ecuador and Hoinduras, spanning a range of salinities and organic loading. Chambers were deployed at various points within ponds to achieve representative sampling. Gas samples were collected using a floating chamber with a plastic tube attached, from which gas samples could be extracted. Samples were collected using a syringe and injected into pre-evacuated exetainers for later analysis by gas chromatography. Gas samples were analysed using a Hewlett Packard 5890 series II gas chromatograph (GC) fitted with Flame Ionisation and Electron Capture Detectors, both at 350°C. Gases were separated on a porous polymer column (HayeSep�� Q, Valco Instruments, Houston, USA). Samples were run against standards of CO2, CH4 and N2O of known concentrations. The rate of GHG production was calculated on the basis of the linear increase in amounts of the respective gases in the volume of the gas collection chamber.
Results and dsicussion
Clear relationships were observed between both salinity and organic matter (Fig 1) with methane emissions rising steadily with organic loading. However, higher salinity inhibited methane emissions considerably with highest salinities demonstrating little methane generation, even at high organic loading. It was estimated that for every 10ppt rise in salinity, there was an approximately 60% lowering of methane emissions. The analysis confirmed that salinity is a major determinanat of methane generation, although with high uncertainty. Seasonality and fluctuatoing affects of rainfall may have an effect on salinity and organic loading, or other water quality parameters, which are an important area for further research. Very few nitrous oxide emissions were observed at any rate of organic loading or sailinity.
Figure 1. Methane emissions from tropical shrimp ponds related to a) organic matter loading and b) salinity
Acknowledgment
We would like to thank Laurence Massaut and Louis Cattini for their diligence and hard work collecting gas samples from Ecuador and Honduras respectively.
References
Rowe, A., Urbanic, M., Trutschel, L., Shukle, J., Druschel, G. & Booth, M. (2022) Sediment Disturbance Negatively Impacts Methanogen Abundance but Has Variable Effects on Total Methane Emissions. Front Microbiol, 13, 796018.10.3389/fmicb.2022.796018
Sela-Adler, M., Ronen, Z., Herut, B., Antler, G., Vigderovich, H., Eckert, W. & Sivan, O. (2017) Co-existence of Methanogenesis and Sulfate Reduction with Common Substrates in Sulfate-Rich Estuarine Sediments. Front Microbiol, 8, 766.10.3389/fmicb.2017.00766