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

Conclusions and implicationsConclusions- In natural populations of Lobelia and Littorella the entire rhizosphere is oxidized by extensive oxygenloss from roots during daytime whereas anoxia occurs in sediments late in the night at summertemperatures.- Oxygen loss from roots prevents formation of reduced compounds in the pore-water and facilitates lossof nitrogen, use of inorganic carbon and precipitation of P in the sediment, hence, maintainingoligotrophic conditions.- Symbiosis with arbuscular mycorrhizal fungi mediates nutrient supply to isoetids by increasing thesurface area for uptake and sediment contact in oligotrophic sediments.- Addition of even small amounts of organic matter to sediments has long-lasting effects on sedimentbiogeochemistry and affects growth of isoetid species. Isoetids seem to benefit from small enrichmentsdue to increased nutrient and CO 2 availability but high organic additions lead to plant stress whensediment O 2 demand exceeds root oxygenation capacity.- High organic additions leads to anoxia, ankalinization and accumulation of NH 4 + and Fe 2+ in the sedimentpore-water and increased efflux of inorganic carbon and NH 4 + to the water column, hence, favoringfaster growing plant species and phytoplankton.- When organic enrichment deprives sediments of O 2 Lobelia experience prolonged tissue anoxia duringnighttime forcing this low productive species to use low energy yielding anaerobic metabolism to coverthe respiratory demand. Littorella avoids plant anoxia due to higher gas permeability of leaves whichmediates O 2 diffusion from the water column across leaf surfaces during the night.- Organic addition also results in low arbuscular mycorrhizal colonization in roots of isoetids but isprobably not responsible for the low nutrient content and plant stress observed at high additions.Implications- Isoetids counteracts eutrophication and maintain oligotrophic sediment conditions by controlling thesediment biogeochemistry. Oxidation of sediments prevents accumulation of organic matter andmediates denitrification and precipitation of P in the sediments unless O 2 demand of sediments exceedsthe oxygenation capacity of the isoetids. However, when isoetids are lost from lakes, hostile sedimentscan prevent re-colonization.- O 2 availability in sediments dictates the sediment biogeochemistry and is thereby the single mostimportant parameter since permanent anoxic sediments hamper growth of isoetids.- To prevent further loss of isoetid populations it is important to keep sedimentation of labile organicmatter low and water transparency high since isoetids otherwise lose control of sedimentbiogeochemistry.- Littorella can by its higher tolerance to eutrophication be used for restoration by improving sedimentconditions for less tolerant isoetid species such as Lobelia and preventing P release from sediments.25