Extreme drought–accelerated dissolved carbon metabolism triggers pulsed CO₂ outgassing in karst lakes

Abstract. Karst aquatic ecosystems are important reservoirs of dissolved carbon (C), supporting dynamic CO₂ fluxes through the biological C pump. However, our current understanding of how sophisticated interactions between aquatic microbiomes and dissolved C turnover constrain the timing of CO₂ sequestration and emission remains limited. Here we capture an extreme drought event and the ensuing relatively wet conditions from systematic investigations in karst lakes, demonstrating that temporally distinct microbiomes are tuned to the metabolic patterns of dissolved C and thereby modulate CO₂ emissions. Specifically, we find that the extreme drought accelerates respiration of dissolved organic C, sharply increasing the CO₂ evasion rate. Wet conditions stimulate photosynthetic uptake of dissolved inorganic C, consuming lake CO₂ while promoting labile organic C formation. We therefore propose that pulses of CO₂ emissions from karst lakes occur after wet conditions end, as a consequence of rapid remineralization of newly produced bioavailable organic C, especially during extreme droughts. Our findings highlight the crucial importance of managing periodic CO₂ outgassing from karst waters under drought conditions for the implementation of region-specific C neutrality strategies.