Claus Lindskov Møller:
● Isoetids occupy littoral zones of the most pristine oligotrophic softwater lakes due to their unique adaptations. Intrinsic low growth rates, use of sediment CO2 for photosynthesis and mycorrhizal symbiosis enable them to grow where other plants cannot. Eutrophication of many former oligotrophic lakes has caused a dramatic decline in the occurrence and distribution of isoetids throughout northern Europe and North America within the last 100 years since degradable organic matter builds up in sediments creating a hostile environment for isoetids. The objective of this thesis was to investigate effects of changes in sediment biogeochemistry following organic enrichment of sediments on growth and survival of isoetid species with focus on O2 dynamics, plant nutrition and mycorrhizal associations.
● A combination of laboratory and in-situ experiments was performed to investigate plant and sediment response to organic enrichment. Pore-water samples were used to track changes in sediment biogeochemistry while O2 electrodes were used to measure O2 availability and dynamics in plants and sediment. Plant morphology and nutrition were used as stress indicators. The response of mycorrhizal fungi to organic enrichment was also addressed.
● The studies showed that addition of even small amounts of labile organic matter to sediments had profound and long-lasting effects on sediment biogeochemistry and affected isoetid growth and survival. Moderate additions sometimes resulted in increased growth and no apparent plant stress, but higher additions resulted in widespread anoxia in sediments and plants, decreased plant nutrition and photosynthesis and accumulation of Fe2+, NH4+ and dissolved inorganic carbon in pore-water of sediments. Addition of organic matter also resulted in reduced arbuscular mycorrhizal fungi colonization of roots.
● Sediment composition is therefore a key issue in rehabilitation of isoetid populations, since hostile sediments cause reduced growth and survival. Littorella was able to cope with higher organic additions than Lobelia agreeing with its wider distribution presumably because it avoids tissue anoxia during the night by O2 diffusion across permeable leaves from the water column whereas Lobelia experiences many hours of anoxia and low energy yield in anaerobic respiration. Littorella could due to higher eutrophication tolerance, higher growth rates and higher oxygenation capacity of sediments be used in restoration projects to creating suitable sediment conditions for less tolerant species such as Lobelia.