Millennial-scale dynamics of submerged macrophytes (Isoëtes spp.) reflect long-term environmental changes in mountain lakes situated in Central Europe

Submerged macrophytes serve as important water-quality indicators due to their sensitive response to modern human-induced landscape changes . Paleoecological records demonstrate that submerged macrophytes can be used to understand long-term natural aquatic ecosystem dynamics such as nutrient cycling or water-level fluctuations. Here, we aim to investigate long-term dynamics of Isoëtes spp. which are a type of submerged macrophytes highly adapted to clear water and nutrient poor ecosystems. Isoëtes spp. are considered to be the main colonizers of deglaciated lakes in Europe. Furthermore, we aim to elucidate how local environmental factors may have impacted their distribution and abundance during the Holocene. To complete our research aims, we used a multi-proxy approach utilizing plant macrofossils, macrocharcoal, pollen, diatoms and chironomids from four lakes (Prášilské jezero, Plešné jezero, Černé jezero, Rachelsee) located within the Bohemian/Bavarian Forest mountain range in Central Europe. Our results show regional, synchronous patterns, with Isoëtes spp. colonization beginning between 10,300-9,300 cal yr BP, and substantially declining around 6,500 cal yr BP. Results from Prášilské jezero imply that Isoëtes spp. dynamics were mainly driven by terrestrial processes connected with changes in vegetation cover and fire activity around 6,500 cal yr BP. Specifically, Fagus expansion coinciding with decreasing fire activity likely affected podzolization and accelerated the accumulation of organic acids in the catchment, and in turn, caused the lake to became dystrophic and depleted of oxygen. This is reflected by a shift in diatom composition towards more acidophilous and dystrophic species and a distinctive decrease in chironomid abundance. The lack of light availability in dystrophic lake resulted in the decline in Isoëtes spp. abundance. Our study demonstrates that millennial-scale dynamics of submerged macrophytes are important indicators reflecting environmental changes within the land-water ecotone.