Submerged Aquatic Vegetation Shifts in Oligo-mesotrophic Lakes

Charophytes are the typical submerged vegetation of most oligo-mesotrophic hardwater lakes and play a crucial role in their functioning. Despite protection by the European Union Habitats and Species Directive, charophyte populations have continued to decline across many European lakes. Increased nutrient loading has been proposed to be one of the main causes of charophyte loss, leading to a replacement by submerged vascular plants. Yet, our mechanistic understanding of this process remains incomplete. In this project, we investigate how light availability at depth and nutrient concentrations in the water affect plant and periphyton growth, plant tissue stoichiometry and decomposition of charophytes and submerged vascular plants, thus, identifying critical conditions for shifts between species dominance and their respective effects on nutrient immobilization. 

We ran two consecutive enclosure experiments at the IGB LakeLab (NE Germany) in summer 2024. For the first experiment, shoots of the charophyte Nitellopsis obtusa and the vascular plant Ceratophyllum demersum were exposed for 4 weeks in sediment-filled cups in 8 enclosures along a gradient of depths (0.375, 0.75, 1.5, 3 and 6 m) and nutrient concentrations ranging from oligo-mesotrophic to eutrophic conditions. Plastic strips for periphyton growth were also deployed at the same depths. After harvesting, plant material was further used to determine plant mass loss as an indicator of microbial decomposition rates in a subsequent second experiment over 2 weeks. Preliminary results suggest that vascular plant performance is maintained across a gradient of light and nutrients whereas charophyte competitive ability increased with decreasing light availability. However, charophyte growth was negatively affected by the increase in nutrient concentrations, particularly nitrogen. Knowledge of light and nutrient concentration requirements for charophyte dominance is needed and critical prerequisite for establishing the right conditions to achieve restoration goals and proper management actions in oligo-mesotrophic lakes.