Double drought phases characterize the high-elevation temporary pond hydroperiod

• First definition of the "double dry phase" in high-mountain ponds • First evidence of two distinct drought periods (summer/winter) • Winter dry phase is complete freezing (functional drought) • Biological activity is limited to few months when liquid water is present • Crucial baseline for alpine pond conservation in climate change Temporary ponds are among the most widespread lentic ecosystems on Earth, capable of sustaining remarkably high levels of biodiversity. Their defining feature is the hydroperiod, the time span during which water remains in a liquid state. Owing to this ephemeral nature, temporary ponds host rare and specialized communities adapted to the alternation between wet and dry phases. However, high-mountain temporary ponds exhibit a distinctive hydrological cycle. For the first time, we define this pattern as a “double dry phase”, consisting of two distinct dry periods within the same annual cycle. The first is a “classic” summer dry phase, when rising temperatures and scarce precipitation cause complete desiccation. The second is a “frozen” winter dry phase, when ponds, unlike most lakes, freeze entirely due to their shallow depth (typically <8 m). Complete freezing makes these systems biologically inactive, creating a functional drought despite the physical presence of ice. This study provides the first empirical evidence of such a dual-phase hydrological cycle in high-elevation ponds of Central Italy, highlighting their extreme environmental intermittency. These unique ecosystems remain biologically active only for a few months each year, during which liquid water is available. In the context of ongoing climate change, with expected increases in both summer temperatures that could further shorten the liquid-water period, our findings offer a crucial baseline for the recognition and conservation of alpine temporary ponds as singular and vulnerable habitats.