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Under a warming scenario many permanent inland water bodies in the Mediterranean region have become temporary ones, so the persistence of submerged macrophytes depends on the timely production of drought‐resistant propagules. Phenological research measures the timing of ecological processes and allows the consequences of disturbances such as climate change to be evaluated. Among macrophytes, the charophytes are widely distributed benthic macroalgae and are particularly sensitive to environmental variations. The systems in which they live constitute valuable, threatened, and protected habitats (European Habitats Directive code 3140). This study established a methodological basis for determining which environmental drivers trigger the sexual reproductive phenology of charophytes. For the first time, circular statistics were applied to describe and compare seasonal patterns for submerged macrophytes. Over a period of 2 years, three charophyte species growing in two Mediterranean brackish ponds were monitored. Most of the sexual reproduction occurred during spring and summer, including the production of drought‐resistant propagules, before the harshest conditions arrived. Each species showed a distinctive phenological pattern, according to specific breeding systems, life cycles, and environmental tolerances and requirements. Deeper conditions and higher salinities delayed reproduction. Different rainfall intensities in autumn produced distinctions between years, as stronger rainfall reduced the salinity stress and stimulated the reproduction of plants. Interacting factors act as cues for charophyte reproduction and cause antagonistic effects on propagule production. Among them, the heat required for each phenophase peak is essential for understanding the phenology of a population and for predicting its long‐term persistence. Charophytes appear to be good candidates for tracking climate change in shallow ecosystems. Further phenological studies should consider more species, populations, and long‐term observations in order to predict climate‐change effects on water bodies, and to develop effective plans for management and conservation. Circular statistics is a potential tool for analysing phenology in the context of global warming, as well as for many other conservation issues.

Corresponding author: Sara.Calero@uv.es life events. In addition, studying phenology is the simplest procedure to track current global warming and its effects on the success and survival of different populations of the same species. Little is known about the effect of water temperature and its corresponding accumulated heat on charophytes' phenology. We compared differences in water temperature and sexual reproductive phenology of Chara hispida in two ponds of two countries located at different latitudes (Spain and Switzerland) over the same year. We estimated the accumulated heat required to develop from one phenophase to another (unripe/ripe gametangia and oospores). Curve fitting techniques on water temperature showed an advance of 26 days in the Spanish spring 2 onset. All phenological events happened for the first time around 40 days earlier in the Spanish pond, agreeing with the Hopkins' Bioclimatic Law prediction. C. hispida sexually reproduced in a daily mean temperature (DMT) range of 10-25°C and needed 600 growing degree-days (GDD) to ripen gametangia in the Spanish pond. The Swiss population required a higher DMT (15°C) to begin to reproduce, and ~700 GDD to initiate gametangia ripening. Temperature (as well as radiation) is one of the most important drivers of reproductive phenology, and accumulated heat is a better predictor than DMT for charophyte phenology. In the foreseeable warming scenario, we assume that C. hispida sexual events would advance by more than one month in Switzerland and expand at the end of the season, considerably lengthening its reproductive period.

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