1. Littorella uniflora and Lobelia dortmanna are prominent small rosette species in nutrient-poor, soft-water lakes because of efficient root exchange of CO2 and O2. We hypothesise that higher gas exchange across the leaves of L.similar to uniflora than of L.similar to dortmanna ensures O2 uptake from water and underlies its greater tolerance to sediment anoxia following organic enrichment. 2. We studied plant response to varying sediment O2 demand and biogeochemistry by measuring photosynthesis, gas exchange across leaves and O2 dynamics in plants during long-term laboratory and field studies. Frequent non-destructive sampling of sediment pore water was used to track changes in sediment biogeochemistry. 3. Addition of organic matter triggered O2 depletion and accumulation of , Fe2+ and CO2 in sediments. Gas exchange across leaf surfaces was 1316 times higher for L.similar to uniflora than for L.similar to dortmanna. Oxygen in the leaf lacunae of L.similar to uniflora remained above 10 kPa late at night on anoxic sediments despite organic enrichment. Leaf content of N and P of L.similar to uniflora remained sufficient to keep up photosynthesis despite prolonged sediment anoxia, whereas nutrient content was too low for long-term survival of L.similar to dortmanna. 4. High gas exchange across L.similar to uniflora leaves improves its performance and survival on anoxic sediments compared with L.similar to dortmanna. Lobelia dortmanna uses the same gas-tight leaves in air and water, which makes it highly susceptible to sediment anoxia but more cost-effective in ultra-oligotrophic environments because of slow leaf turnover.