Carolin Waldemer

• Freshwater aquaculture is increasingly recognized as a significant source of climate-relevant gases.Emissions of greenhouse gases (GHGs) from these systems are controlled by complex biogeochemical and physical interactions involving production, transformation, and transport processes, which are influenced by diverse factors including climate and management practices. To date, research has focused on tropical and subtropical aquaculture, whereas extensive to semi-intensive freshwater fish ponds are widespread in the temperate zone and constitute a traditional management form in Europe. This research provided the first assessment of the climate impact of these systems by quantifying diffusive and ebullitive emissions of CO2, CH4, and N2O from twelve ponds in Eastern Germany during the fish growing season. Particular emphasis was placed on the long-overlooked ebullition pathway and its pronounced spatiotemporal heterogeneity. The overarching goal was to advance a mechanistic understanding of ebullition to improve its quantification and reduce uncertainties in regional and global GHG budgets, as well as to identify management strategies for mitigating the climate impact of temperate freshwater fish ponds based on my research findings and existing literature. While the natural-looking ponds acted as weak N2O sources, their CO2 and CH4 emissions were comparable to those of tropical and subtropical aquaculture. CH4 ebullition represented a major transport pathway, showing significant intra-system variability and spatiotemporal heterogeneity comparable to the mean flux magnitude. It was primarily driven by labile, nitrogen-rich sediment organic matter and was largely independent of annual characteristics of fish cultivation. Massive input of easily biodegradable protein-feed at the Gerstenteich led, however, to the development of bioreactor-like conditions, resulting in the highest CH4 ebullition rates reported to date in both natural and aquaculture systems. Advanced molecular analysis further revealed how the protein-rich feed influences sedimentary organic matter composition and degradation processes. Ammonium in surface water served as a robust and easily measurable proxy for CH4 ebullition. As one of the few studies investigating diel dynamics, a distinct diurnal ebullition pattern was observed - likely linked to the activity of benthivorous fish. By introducing a novel approach for determining representative bubble sizes, this study overcame major limitations of existing methods and, for the first time, provided insights into the spatiotemporal variability of bubble sizes in ponds, revealing a nonlinear relationship between bubble size and ebullition intensity. The results demonstrated that increasing anaerobic degradation increases both the contribution of ebullition to total CH4 emissions and the bubble size, thereby enhancing the overall efficiency of CH4 release disproportionally. In addition, bubble-induced stripping from the water column was identified as an effective, yet overlooked ecosystem oxygen sink. In summary, this work identifies temperate freshwater fish ponds as overlooked but climate-relevant systems and underscores ebullition as a major emission pathway. Beyond improving the understanding and quantification of ebullition in such systems, this research established a basis for identifying key research priorities and outlines a first mitigation framework for a more climate-friendly management. Mitigating eutrophication and labile organic matter accumulation, together with optimized feeding and fewer and shorter drainages, could markedly reduce CH4 ebullition and total GHG emissions while enhancing the pond carbon balances. Future research should cover the entire production cycles and include all major transport pathways to further refine climate impact estimates. It should assess species-specific net effects of submerged macrophytes and bioturbation, optimize feed formulations, and rigorously evaluate mitigation strategies. In light of climate change and the growing global demand for protein, climate-friendly aquaculture production adapted to local ecological and socioeconomic conditions will be essential, requiring both targeted research and effective legislative action.