Sphaerochara canadensis (Charophyceae): A circumpolar species with a high temperature optimum.

Sphaerochara canadensis, an aquatic macrophyte belonging to the Characeae, is described as a species with a circumpolar distribution and occurs in the polar (to boreal) zonobiomes, suggesting that it is cold-stenothermic. A recent report of an occurrence in Lake Wolfgangsee, Austria, contradicted this assumption, prompting this study to investigate the species' physiological adaptation capabilities and, consequently, ability to survive in non-polar environments. Field measurements at Lake Torneträsk, Sweden, indicated that S. canadensis is more adapted to low light compared to the co-occurring charophytes, with water temperatures in the lake ranging from 10 to 12°C during the experiment. Cultivation experiments also revealed clear temperature effects on growth, photosynthetic performance, and pigment composition at 5, 10, 15, and 20°C, with higher temperatures having a positive impact. Furthermore, it was shown that the species can adapt to different light intensities. A published occurrence of the species in Austria is probably erroneous. The photographic material of the original report was misidentified, and eDNA analyses of water samples taken from Lake Wolfgangsee and surrounding water bodies failed to confirm the presence of the species. Consequently, a factor other than temperature limits its distribution. Identifying the factor is crucial for conservation under climate change.