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.