Tropical red alga Compsopogon caeruleus: an indicator of thermally polluted waters of Europe in the context of temperature and oxygen requirements

The red alga Compsopogon caeruleus can generally be found in tropical and subtropical waters worldwide. In addition to its natural habitats, this species may be found in waters that receive abnormally hot water, e.g., from powerhouses. To date, the presence of C. caeruleus has not been observed in thermally polluted lacustrine ecosystems in Poland, which has a moderate climate. The thalli of this red alga were found growing on Vallisneria spiralis in Lichenskie Lake. Impor-tantly, this paper presents a previously unknown relationship between the temperature (20, 25, 30, 35, and 40 degrees C) and oxygen requirements of C. caeruleus (based on ex situ measurements of O2 consumption by thalli). Surprisingly, 35 degrees C can be the optimum temperature for C. caeruleus, and this temperature is higher than the values reported by some previous thermal analyses by approximately 10 degrees C. Additionally, we reviewed and mapped the distribution of this nonnative and mesophilic red alga in natural / seminatural water ecosystems in Europe. Finally, we propose that the occurrence of C. caeruleus mature thalli can be a novel, simple and easy-to-recognize bioindicator of artificially and permanently heated waters in moderate climate zones by a regular discharge of postindustrial water.