Drivers of CO₂ emissions during the dry phase of Mediterranean and Temperate ponds

Abstract. Pond ecosystems play an important role in the global carbon cycle with the potential to act as both sinks and sources. Emissions of CO₂ during their dry phases remain largely overlooked, despite growing evidence that climate change-induced shifts in temperature and precipitation will likely result in longer and more frequent dry periods. Here we assess CO₂ emissions from dry pond sediments in relation to climatic region, seasonal changes, and hydroperiod duration. Specifically, we aimed to identify the key environmental drivers shaping CO₂ fluxes during the dry phase. We measured CO₂ emissions from air-exposed sediments in 30 ponds across Mediterranean and Temperate regions. Ponds acted as sources of CO₂ during dry phases, with emissions ranging from 127 to 4889 mg C m⁻² d⁻¹ (mean ± SD = 1398 ± 1201). Although mean emissions did not differ significantly between climate regions, hydroperiod length interacted with climate and season, showing a significant effect in summer, particularly in Mediterranean ponds, where longer hydroperiods led to higher emissions. Emissions were considerably higher in summer than in autumn, primarily driven by an interaction between sediment temperature and water content. The highest fluxes occurred at c. 27 °C and sediment water content between 27 % and 44 %. Additionally, ponds in better conservation status and with lower carbonate content emitted more CO₂. Our findings improve understanding of CO₂ emissions during increasingly common dry phases and highlight how climate modulates local sediment conditions, thereby influencing the magnitude of these emissions. This underscores the need for comprehensive assessments of carbon fluxes that incorporate dry-phase emissions, accounting for climate, hydroperiod, and both direct and indirect effects of local environmental drivers.