Global warming shifts climate zones to the north, resulting in changes in biodiversity. One of the species adaptation strategies to survive is to change their phenology. In temperate climate, most aquatic plants show their peak of biomass in summer. In our study, we hypothesised that the rare charophyte Lychnothamnus barbatus and the partially invasive vascular plant Myriophyllum spicatum may apply a different seasonality and cause a decrease in the biodiversity of submerged macrophytes. The object of our study was a mesotrophic temperate lake in Western Poland with abundant submerged vegetation, where macrophyte seasonality was studied from November 2015 to November 2019 (autumn, spring, summer). Altogether, 29 taxa of plants were recorded, including 13 species of charophytes (40 % of the Polish charophyte flora) but only four species co-dominated, two eutrophic vascular plants, Ceratophyllum demersum and M. spicatum, and two charophytes, the rare mesotrophic L. barbatus and common Nitellopsis obtusa. M. spicatum due to its expansiveness in the shallow littoral decreased biodiversity in the study lake. Moreover, C. demersum negatively affected the endangered L. barbatus by direct competition for habitats in the lake. L. barbatus and M. spicatum dominated in vegetation in autumn while the further two species co-dominated in summer. L. barbartus and M. spicatum seem to avoid high competition in summer, which proves a high adaptive potential with respect to climate warming. However, further study in other lakes is needed to verify the relevance of our findings in terms of the conservation of rare macrophyte species.
