Notes: 1. Lobelia dortmanna is a common representative of the small isoetid plants dominating the vegetation in nutrient-poor lakes in Europe and North America. Because of large permeable root surfaces and continuous air lacunae Lobelia exchanges the majority of O2 and CO2 during photosynthesis across the roots. This leads to profound diel pulses of O2 and CO2 in sandy sediments with low microbial O2 consumption rates. The ready radial root loss of O2 may, however, make Lobelia very susceptible to more reducing sediments. Therefore, we grew Lobelia for 6 months on natural and organically enriched sandy sediments to test how: (i) root oxygenation influenced degradation of organic matter and depth profiles of N and C; (ii) Lobelia and microbial O2 consumption rates influenced pool size and depth penetration of O2 in the sediments; and (iii) sediment enrichment influenced growth and mineral nutrition of Lobelia.2. Naturally low-organic sediments (0.32% DW) accumulated organic C and N during the experiment as a result of growth of Lobelia and surface micro-algae. In contrast, surface layers of enriched sediments (0.58, 0.87 and 2.46% DW) lost organic C and N because of enhanced mineralisation rates because of oxygen availability. In deeper layers of enriched sediments no significant differences in organic C and N pools were found between plant-covered and plant-free sediments probably because faster organic degradation because of root oxygenation was balanced by release of organic matter from the plants and because short roots with dense Fe-Mn coatings in the most enriched sediments constrained O2 release.3. Depth-integrated O2 pools were much higher in light than darkness, higher in plant-covered than plant-free sediments and higher in sandy than in organically enriched sediments. All sediments had a primary O2 maximum 1–2 mm below the sediment surface in light because of photosynthesis of micro-algae. Plant-covered sediments of low organic content (0.32 and 0.58% DW) also had a secondary deep maximum (2–4 cm) because of higher O2 release from Lobelia roots than microbial O2 consumption. Nitrification occurred here resulting in depletion of NH〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1382:FWB_1382_mu1" location="equation/FWB_1382_mu1.gif"/〉 and accumulation of NO〈inlineGraphic alt="inline image" href="urn:x-wiley:00465070:FWB1382:FWB_1382_mu2" location="equation/FWB_1382_mu2.gif"/〉. In low organic sediments, oxygen pools increased with higher plant biomass both in light and darkness. The deep O2 and NO3 maxima disappeared in high organic sediments of greater O2 consumption rates and smaller O2 release rates.4. Lobelia was stressed by increasing O2 consumption rate of the sediments. Plant weight and leaf number declined twofold and maximum root length declined fourfold suggesting severe problems maintaining sufficient axial O2 transport to the root tips because of rapid radial O2 loss. Despite markedly higher nutrient concentrations in the enriched sediments, leaf-N declined twofold and leaf-P declined fourfold to growth-limiting levels. These responses can be explained by constrains on mycorrhisal activity, root metabolism and vascular transport because of O2 depletion. Management efforts to stop the decline and ensure the recovery of the isoetid vegetation should therefore focus on improving water quality as well as sediment suitability for growth.