Growth and production of isoetids in oligotrophic Lake Kalgaard, Denmark

Summary

Leaf turnover rates of Littorella uniflora and Lobelia dortmanna in Lake Kalgaard, Denmark, determined by leaf marking, showed a unimodal seasonal pattern with maxima in June—July at about 0.83 and 0.56% day⁻¹, respectively. Renewal of leaf biomass per year was 150% for Littorella and 80% for Lobelia. Root turnover rate of Lobelia was equal to that of the leaves and the same probably applied for Littorella.

¹⁴C-experiments of Littorella corresponded well to the leaf marking experiments. In July—August Littorella photosynthesis was light saturated most of the day and assimilation in short-term experiments under ambient CO₂ concentrations was limited by the carbon supply. Consequently, daily production should be determined by integration of the results in brief experiments performed throughout the day. ¹⁴CO₂ assimilated was primarily taken up by the roots (about 95 %). Therefore, isoetid production experiments in which ¹⁴CO₂ is added to the leaves only are invalid.