Oxygen dynamics and plant-sediment interactions in isoetid ...

Paper 3Higher gas permeability of leaves provides greater tolerance of Littorella uniflorathan Lobelia dortmanna to sediment organic enrichmentClaus Lindskov Møller and Kaj Sand-JensenFreshwater Biological Laboratory, Biological Institute, University of Copenhagen, Helsingørsgade 51, DK-3400 Hillerød,DenmarkKey words: Permeability,isoetids, eutrophication,biogeochemistry, anoxia,oxygen, stress.Summary● Lobelia dortmanna and Littorella uniflora are prominent in nutrientpoor,softwater lakes because of efficient root exchange of CO 2 and O 2 .We hypothesize that higher gas permeability of Littorella than Lobelialeaves ensures O 2 uptake from water and greater tolerance to organicenrichment and anoxia of sediments.● We studied O 2 dynamics in plants and sediments and leaf photosynthesisupon organic sediment enrichment in long-term laboratory and fieldstudies.● Addition of organic matter triggered O 2 depletion and accumulation ofNH 4 + , Fe 2+ and CO 2 in sediments. O 2 in Lobelia’s leaf lacunae fluctuatedfrom air saturation in the light to anoxia late at night on natural sediments,but organic enrichment prolonged night anoxia. Littorella’s leaf surfaceswere 13-16 times more permeable and O 2 in the lacunae remained above10 kPa on anoxic sediments due to uptake from the water. Leaf N, P andphotosynthesis dropped to critically low levels under prolonged sedimentanoxia for Lobelia but not for Littorella.● High gas permeability of Littorella leaves improves performance andsurvival on organically enriched, anoxic sediments compared with Lobelia.Littorella changes between gas impermeable aerial leaves and permeableaquatic leaves whereas Lobelia uses the same impermeable leaves in airand water which is cost-effective in ultra-oligotrophic environments.IntroductionOligotrophic, softwater lakes in Europe andNorth America are inhabited by a taxonomicallydiverse group of small isoetid species all havingleaf rosettes, large root systems and extensiveaerenchyma as adaptations to acquire CO 2 andnutrients from sediments (Smolders et al. 2002).The isoetids are key species in internationalevaluations of pristine nature quality such asEU’s Habitat Directive and Water FrameworkDirective (Stelzer et al. 2005) and oligotrophic,softwater lakes are named Lobelia-lakes afterthe conspicuous flowering species, Lobeliadortmanna. Despite the same growth form andrequirement of relatively unpolluted habitats ofall isoetids, the species vary substantially inanatomy and physiology which may account fordifferences in their distributions (Sand-Jensen &Søndergaard 1979). For example, Littorellauniflora has a wider ecological range andtolerates greater eutrophication than Lobeliadortmanna which has gone extinct in many NWEuropean lakes and it is now confined to a fewpristine oligotrophic lakes (Arts et al. 1989;Sand-Jensen et al. 2000; Pedersen et al. 2006).We here test if Littorella is more tolerant thanLobelia to organic enrichment and anoxia of55