High sensitivity of Lobelia dortmanna to sediment oxygen depletion following organic enrichment

Summary• Lobelia dortmanna thrives in oligotrophic, softwater lakes thanks to O 2 and CO 2 exchange across roots and uptake of sediment nutrients. We hypothesize that low gas permeability of leaves constrains Lobelia to pristine habitats because plants go anoxic in the dark if O 2 vanishes from sediments. • We added organic matter to sediments and followed O 2 dynamics in plants and sediments using microelectrodes. To investigate plant stress, nutrient content and photosynthetic capacity of leaves were measured. • Small additions of organic matter triggered O 2 depletion and accumulation of NH 4 + , Fe 2+ and CO 2 in sediments. O 2 in leaf lacunae fluctuated from above air saturation in the light to anoxia late in the dark in natural sediments, but organic enrichment prolonged anoxia because of higher O 2 consumption and restricted uptake from the water. Leaf N and P dropped below minimum thresholds for cell function in enriched sediments and was accompanied by critically low chlorophyll and photosynthesis. • We propose that anoxic stress restricts ATP formation and constrains transfer of nutrients to leaves. Brief anoxia in sediments and leaf lacunae late at night is a recurring summer phenomenon in Lobelia populations, but increased input of organic matter prolongs anoxia and reduces survival.