Drivers of CO ₂ emissions during the dry phase in 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 of CO2. However, CO2 emissions during the dry phases of ponds remain underrepresented in global studies, despite growing evidence that climate change-driven shifts in temperature and precipitation are likely to increase the frequency and duration of these dry periods. Here we assess CO2 fluxes 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 fluxes from bare air-exposed sediments using closed static chambers equipped with internal mini-loggers in 30 ponds across Mediterranean and Temperate regions. Ponds acted as sources of CO2 during dry phases, with emissions ranging from 127 to 4889 mgCm-2d-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 the interaction between sediment temperature and water content. The highest fluxes occurred at approximately 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.