The balance of carbon emissions versus burial in fish ponds: The role of primary producers and management practices

In the global carbon cycle, ponds can play a dual role: they have the capacity to store carbon through sedimentation, but also to be important greenhouse gas emitters. Of all the pond landscapes in Europe, many are managed by Humans for fish production and little is known today about their role as carbon sinks or sources. We monitored 20 fish ponds from the Dombes region (France) during a production season in 2022. We measured both sedimentation rates, the diffusion of CO 2 , CH 4 and N 2 O and ebullition of CH 4 over three different seasons (spring, summer and autumn) in order to provide an estimate of the carbon balance for each pond. Five were dried-out in 2023 as part of the management cycle, and were monitored for dry flux emissions during this period. On average, our measurements suggest that fish ponds were carbon sinks (6 fish ponds as sources and 14 as sinks). There was on average a net sequestration of 4.16 (+/- 10.00) tonnes of CO 2 e per hectare over 6 months. Measurements from the dry year, indicate that the ponds were carbon sources (mean value of 23.89 tonnes of CO 2 eq emitted). However, whilst the drying out phase directly increases emissions, it also encourages the development of macrophytes in the subsequent years which improves carbon storage through sedimentation, and appears to be an important driver of the observed balances. These data are consistent with the fact that fish ponds have the capacity to be either sources or sinks of carbon for the atmosphere. Moreover, by promoting good levels of primary productivity, the presence of aquatic plants and by improving carbon storage, management practices appear to have the capacity to turn fish ponds into carbon sinks. These agroecosystems could thus play an important role in the context of climate change mitigation. • Fish ponds vary greatly in their sedimentation rates and emissions (CO 2 , CH 4 , N 2 O). • Carbon sedimentation is higher than emissions in fish ponds under water. • A dry year is characterized by important C emissions. • Drying-out for a year also appears to be a lever of C sequestration.