Leaving aquaculture ponds ‘out-to-dry’ has implications for nitrous oxide budgets

Publication: Yang, P., H. Yang, D. Y. F. Lai, Q. Guo, Y. Zhang, C. Tong, C. Xu, and X. Li. 2020. Large contribution of non-aquaculture period fluxes to the annual N2O emissions from aquaculture ponds in Southeast China. Journal of Hydrology 582:124550. DOI: https://doi.org/10.1016/j.jhydrol.2020.124550

Cover image source: MyLifeStory https://www.flickr.com

Researchers from China and the UK have demonstrated that the dormant period of shrimp farming, when aquaculture ponds are drained and left ‘out-to-dry’ for half a year, is associated with increased nitrous oxide emissions. The global aquaculture industry is currently growing at a rate that surpasses most other food sectors, yet the potential climatic impacts from increased greenhouse gas production has implications for the sustainability of this farming system. Through an extended field campaign of nitrous oxide measurements, emissions from the dry-period of aquaculture ponds made up the vast majority of total nitrous oxide emissions. As a previously unaccounted for part of the pond farm greenhouse gas budget, this dry phase of the aquaculture period is too large to be ignored.

The rise of aquaculture

Aquaculture ecosystems represent an indispensable part of global agriculture, especially to the future of sea food supply with a rapidly growing world population. Pond-based aquaculture farming consists of engineered ponds created in freshwater or saltwater environments to farm a range of aquatic organisms for human consumption. Aquaculture industrialisation, termed the ‘Aquaculture revolution’, is on the rise and has reshaped vast stretches of land in countries such as China, Myanmar, and Bangladesh. With the industry now supplying over half of the fish we eat, aquaculture farming has become such a lucrative business that rice paddy fields are being converted to ponds (Yuan et al., 2019). While aquaculture farming offers a means to preserve the wild fish population by reducing overfishing, the environmental impacts are deeply concerning. One area of concern is the potential greenhouse gas impact aquaculture has when you account for the clearing of carbon-rich mangrove forests and the amount of climate-warming gases produced during intensive fish farming.

Greenhouse gas implications yet to be uncovered

In a climate-warming world, it is imperative to get a handle on the greenhouse gas ‘footprint’ of rapidly developing industries such as aquaculture farming. Studies on greenhouse gas budgets of worldwide aquaculture production remain scarce. As it stands, nitrous oxide emissions from aquaculture are cautiously estimated to contribute to 5.72% of global anthropogenic nitrous oxide emissions. The climatic impacts of industrial-scale aquaculture has been highlighted for freshwater systems in a recent global synthesis of aquatic methane and nitrous oxide measurements. However, estimates are limited by not including salt water aquaculture ponds and only accounting for the period where these ponds actually hold water (‘aquatic fluxes’). At least 20% of global aquaculture ponds undergo dry periods where water is drained after harvest to remove effluent and excess nutrients. This dry period can last for five to six months yet the contribution of these sediments to the total aquaculture nitrous oxide budget remained an overlooked component of the budget until now. In an effort to address this knowledge gap, a team of scientists from China and the UK took field measurements of nitrous oxide fluxes in three aquaculture shrimp ponds over the course of one year.

Emissions high while ponds are dry

The study took place in the Shanyutan Wetland of the Min River Estuary, southeast China, where shrimp ponds now make up the dominant part of the landscape. The authors measured nitrous oxide fluxes from the water and sediments and collected a range of other water and sediment parameters to gather information on conditions controlling nitrous oxide production. The ponds are filled with seawater and shrimp production starts in June and ends in November. After shrimp harvest, water is discharged and the pond sediments are left exposed until the next shrimp production cycle starts.

The study found that nitrous oxide was emitted at a rate over 30 times greater during the dry period compared to when the pond contained water. Over the entire year, this resulted in the non-aquaculture phase contributing 97% of total nitrous oxide emissions produced by the aquaculture pond system. This ‘dry flux’ contribution is too large to be ignored when considering greenhouse gas budgets and is becoming an increasing trend across other aquatic environments that go through a dry stage (Obrador et al., 2018).

So why such a big difference, especially when the flooded aquaculture phase is receiving fertilizer nitrogen inputs and the dry phase does not? It comes down to these two phases representing very different environments: one dominated by water (pond) and the other by sediment. Nitrous oxide gas is produced by micro-organisms that break down organic matter and consume nitrogen substrates. The rate of this microbial activity is mostly controlled by the amount of oxygen available. In pond sediments exposed to the air, more oxygen is able to penetrate into the sediment which fuels microbial activity and the subsequent production of nitrous oxide. Just like with us people; the more oxygen we breathe in the more energy we can expend. When the sediments are flooded with water during the aquaculture period, there exists a physical barrier between the sediment and atmosphere and not as much oxygen can pass through.

Over time the input of shrimp feed and shrimp effluent leads to an accumulation of nitrogen in the pond sediment, until the system is flushed by drainage at the end of the growing cycle. Any unremoved nitrogen remains on the sediment surface, and is at the mercy of microbial organisms with their newly supercharged metabolism. As with many other nitrogen polluted ecosystems, this high nitrogen content enables large amounts of  nitrous oxide to be emitted and likely explains the two times greater emissions found in aquaculture ponds compared to natural estuarine sites.

Aquaculture ponds should not be overlooked in nitrous oxide budgets

Findings from this study revealed aquaculture ponds to be ‘hotspots’ for nitrous oxide emissions and deserve consideration as potentially important contributors to the anthropogenic N2O budget. Management opportunities exist to make aquaculture a cleaner and sustainable practice, such as the use of aerators, intensity of production, and reducing feeding waste (Yuan et al., 2019). However, there are many different types of aquaculture systems producing a variety seafood across a range of environments (freshwater to saltwater) which complicates the management of these systems for greenhouse gas mitigation. The importance of field-based studies in this industry is critical for the development of greenhouse gas reduction strategies, as demonstrated in this study which identified a part of aquaculture farming that was contributing more to the nitrous oxide budget than realised.

Other references

Yuan, J., J. Xiang, D. Liu, H. Kang, T. He, S. Kim, Y. Lin, C. Freeman, and W. Ding. 2019. Rapid growth in greenhouse gas emissions from the adoption of industrial-scale aquaculture. Nature Climate Change 9:318-322. DOI: https://doi.org/10.1038/s41558-019-0425-9 

Obrador, B., D. von Schiller, R. Marcé, L. Gómez-Gener, M. Koschorreck, C. Borrego, and N. Catalán. 2018. Dry habitats sustain high CO2 emissions from temporary ponds across seasons. Scientific Reports 8:3015. DOI: https://doi.org/10.1038/s41598-018-20969-y