Could rising aquatic carbon dioxide concentrations favour the invasion of elodeids in isoetid‐dominated softwater lakes?

1. During the past century, isoetid vegetation types in softwater lakes have often been invaded by faster‐growing elodeids. In these C‐limited systems, this may be related to rising aquatic CO₂ levels. 2. In a laboratory experiment we tested the growth response of two elodeid species, Myriophyllum alterniflorum and Callitriche hamulata, at four different CO₂ levels, ranging from 20 to 230 μmol L⁻¹. In addition, we tested the effect of the nutrient status of the sediment on the growth of C. hamulata at the different CO₂ levels. 3. Shoot and root growth increased with rising CO₂ availability. Irrespective of sediment type, growth was minimal to negative at the lowest CO₂ treatment level, while becoming positive at CO₂ levels around 40–50 μmol L⁻¹. Substantial growth was only obtained when the macrophytes were growing on mesotrophic sediments. The plants reached close to maximal growth at CO₂ levels of c. 100 μmol L⁻¹. 4. Within this experiment, the growth of C. hamulata at CO₂ levels above 90 μmol L⁻¹ may have been limited by N and P availability in both sediment types. The growth rate of M. alterniflorum did not seem to be limited by N and P availability, most likely due to its much higher relative root production. 5. The experimental results show that neither M. alterniflorum nor C. hamulata is able to invade isoetid‐dominated softwater lakes at very low aquatic CO₂ concentrations. However, if the sediments contain enough nutrients, a rise in aquatic CO₂ could allow the invasion of elodeid species leading to the subsequent disappearance of slow‐growing isoetids.