1. Oligotrophic softwater lakes represent a special type of aquatic ecosystem with unique plant communities where generalisations from other aquatic plant communities to rising CO₂ in the water column may not apply.

2. In the present study, we set up large in situ mesocosms and supporting laboratory experiments with isoetid vegetation (Littorella uniflora) where water column CO₂ and light could be manipulated in order to test whether (i) light and CO₂ availability affect nutrient concentrations in isoetid vegetation, and (ii) if changes in light and CO₂ climate affect fluxes of inorganic nitrogen (N) and phosphorus (P) from sediment to water column, which potentially could result in increased growth of epiphytic algae.

3. The results showed that the standing stocks of phosphorus and nitrogen in the L. uniflora vegetation were significantly influenced by CO₂ concentration and light intensity. Both standing stocks of P and N were significantly higher in the mesocosm treatments with high CO₂ concentration than in those at low CO₂ concentration. Similarly, standing stocks of P and N enhanced with increasing light intensity.

4. Measurements of nutrient fluxes both in the field and the laboratory did not show any significant release of nutrients to the water column from plants or sediments at any of the light or CO₂ treatments. However, mats of epiphytic algae developed from the beginning of June to late September and caused a light reduction for the isoetid vegetation.

5. Increasing CO₂ concentrations in the water column may over time potentially result in a change in soft water plant communities.