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Aquatic macrophytes have during the last decades faced a massive decline of coverage and repeated die‐off events have been detected for freshwater (Sand‐Jensen et al., 2000) and marine plants (Waycott et al., 2009). Increased nutrient loading has been suggested to play a central role, because high nutrient levels facilitate the formation of pelagic alga blooms which lead to poor light conditions (Nielsen et al., 2002). However, the lack of re‐colonization after reduced nutrient loading for Zostera marina and other seagrasses indicates that other factors influence the pattern. Sedimentation and degradation of dying alga blooms accelerate the microbial activity and thereby intensify oxygen consumption in sediment and water column. This oxygen consumption may accelerate further by increasing temperature caused by warmer climate. This thesis examines how low sediment and water column oxygen levels in combination with high temperature affect internal oxygen concentrations, growth and survival of aquatic macrophytes. Measurements of internal oxygen levels were made on several north temperate and tropical marine seagrass species exposed to a range of water column oxygen concentrations. The combined effects of eutrophication and temperatures were clarified for the temporal seagrass Zostera marina. Furthermore, the direct effect of sediment enrichment with labile organic matter was examined for four freshwater species with different growth strategies (isoetids: Lobelia dortmanna and Littorella uniflora, and elodeids: Potamogeton perfoliatus and Potamogeton crispus). There was a strong interaction between water column oxygen and temperature on internal oxygen concentrations in Z. marina, implying that Z. marina is rather resistant to unfavourable oxygen conditions in winter but becomes increasingly vulnerable in summer, especially at high temperatures. Results also showed that growth of Z. marina plants at higher temperatures will be reduced at gradually l

Eelgrass (Zostera marina L.) occasionally experiences severe die-offs during warm summer periods with variable water column oxygen partial pressures (pO). Eelgrass is known to be very intolerant to tissue anoxia with reduced growth and increasing mortality after ≤12h anoxia in the dark at temperatures of ≥25°C. In the present study we experimentally examine the impact of combined water column oxygen and temperature on oxygen dynamics in leaf meristems of seedlings and adult shoots to better understand how stressful environmental conditions affect eelgrass oxygen dynamics and subsequent growth and mortality. There was a strong interaction between water column oxygen and temperature on meristem pO implying that eelgrass is rather resistant to unfavorable oxygen conditions in winter but becomes increasingly vulnerable in summer, especially at high temperatures. At 25°C meristems became anoxic at a water column pO of 12kPa corresponding to 60% of air saturation, and leaf growth was negatively affected after 24h exposure to water column pO below 7kPa. Temperature had in itself a substantial impact on meristem pO both in light and in the dark as a result of increasing tissue respiration. Seedlings were less prone to meristem anoxia than adult shoots suggesting that seedlings would better tolerate unfavorable combinations of water column oxygen conditions and temperature. The results suggest that eelgrass die-offs will occur more frequently in eutrophied coastal areas with the more frequent periods of unusually high temperatures expected in the future as a result of global warming.

Eelgrass (Zostera marina L.) occasionally experiences severe die-offs during warm summer periods with variable water column oxygen partial pressures (pO). Eelgrass is known to be very intolerant to tissue anoxia with reduced growth and increasing mortality after ≤12h anoxia in the dark at temperatures of ≥25°C. In the present study we experimentally examine the impact of combined water column oxygen and temperature on oxygen dynamics in leaf meristems of seedlings and adult shoots to better understand how stressful environmental conditions affect eelgrass oxygen dynamics and subsequent growth and mortality. There was a strong interaction between water column oxygen and temperature on meristem pO implying that eelgrass is rather resistant to unfavorable oxygen conditions in winter but becomes increasingly vulnerable in summer, especially at high temperatures. At 25°C meristems became anoxic at a water column pO of 12kPa corresponding to 60% of air saturation, and leaf growth was negatively affected after 24h exposure to water column pO below 7kPa. Temperature had in itself a substantial impact on meristem pO both in light and in the dark as a result of increasing tissue respiration. Seedlings were less prone to meristem anoxia than adult shoots suggesting that seedlings would better tolerate unfavorable combinations of water column oxygen conditions and temperature. The results suggest that eelgrass die-offs will occur more frequently in eutrophied coastal areas with the more frequent periods of unusually high temperatures expected in the future as a result of global warming.

Aquatic macrophytes have during the last decades faced a massive decline of coverage and repeated die‐off events have been detected for freshwater (Sand‐Jensen et al., 2000) and marine plants (Waycott et al., 2009). Increased nutrient loading has been suggested to play a central role, because high nutrient levels facilitate the formation of pelagic alga blooms which lead to poor light conditions (Nielsen et al., 2002). However, the lack of re‐colonization after reduced nutrient loading for Zostera marina and other seagrasses indicates that other factors influence the pattern. Sedimentation and degradation of dying alga blooms accelerate the microbial activity and thereby intensify oxygen consumption in sediment and water column. This oxygen consumption may accelerate further by increasing temperature caused by warmer climate. This thesis examines how low sediment and water column oxygen levels in combination with high temperature affect internal oxygen concentrations, growth and survival of aquatic macrophytes. Measurements of internal oxygen levels were made on several north temperate and tropical marine seagrass species exposed to a range of water column oxygen concentrations. The combined effects of eutrophication and temperatures were clarified for the temporal seagrass Zostera marina. Furthermore, the direct effect of sediment enrichment with labile organic matter was examined for four freshwater species with different growth strategies (isoetids: Lobelia dortmanna and Littorella uniflora, and elodeids: Potamogeton perfoliatus and Potamogeton crispus). There was a strong interaction between water column oxygen and temperature on internal oxygen concentrations in Z. marina, implying that Z. marina is rather resistant to unfavourable oxygen conditions in winter but becomes increasingly vulnerable in summer, especially at high temperatures. Results also showed that growth of Z. marina plants at higher temperatures will be reduced at gradually l

Udgivelsesdato: September 2010, 1. Lake eutrophication has increased phytoplankton blooms and sediment organic matter. Among higher plants, small, oligotrophic rosette species (isoetids) have disappeared, while a few tall, eutrophic species (elodeids) may have persisted. Despite recent reduction of nutrient loading in restored lakes, the vegetation has rarely regained its former composition and coverage. Patterns of recovery may depend on local alkalinity because HCO3- stimulates photosynthesis of elodeids and not of isoetids. In laboratory growth experiments with two isoetids (Lobelia dortmanna and Littorella uniflora) and two elodeids (Potamogeton crispus and P. perfoliatus), we test whether organic enrichment of lake sediments has a long-lasting influence by: (i) reducing plant growth because of oxygen stress on plant roots and (ii) inhibiting growth more for isoetids than elodeids. We also test whether (iii) increasing alkalinity (from 0.17 to 3.20 meq. L-1) enhances growth and reduces inhibition of organic sediment enrichment for elodeids but not for isoetids.2. In low organic sediments, higher oxygen release from roots of isoetids than elodeids generated oxic conditions to greater sediment depth for Lobelia (4.3 cm) and Littorella (3.0 cm) than for Potamogeton species (1.6-2.2 cm). Sediment oxygen penetration depth fell rapidly to 0.4-1.0 cm for all four species at even modest organic enrichment and oxygen consumption in the sediments. Roots became shorter and isoetid roots became thicker to better supply oxygen to apical meristems.3. Growth of elodeids was strongly inhibited across all levels of organic enrichment of sediments being eight-fold lower at the highest enrichment compared to the unenriched control. Leaf biomass of isoetids increased three-fold by moderate organic enrichment presumably because of greater CO2 supply from sediments being their main CO2

Udgivelsesdato: September 2010, 1. Lake eutrophication has increased phytoplankton blooms and sediment organic matter. Among higher plants, small, oligotrophic rosette species (isoetids) have disappeared, while a few tall, eutrophic species (elodeids) may have persisted. Despite recent reduction of nutrient loading in restored lakes, the vegetation has rarely regained its former composition and coverage. Patterns of recovery may depend on local alkalinity because HCO3- stimulates photosynthesis of elodeids and not of isoetids. In laboratory growth experiments with two isoetids (Lobelia dortmanna and Littorella uniflora) and two elodeids (Potamogeton crispus and P. perfoliatus), we test whether organic enrichment of lake sediments has a long-lasting influence by: (i) reducing plant growth because of oxygen stress on plant roots and (ii) inhibiting growth more for isoetids than elodeids. We also test whether (iii) increasing alkalinity (from 0.17 to 3.20 meq. L-1) enhances growth and reduces inhibition of organic sediment enrichment for elodeids but not for isoetids.2. In low organic sediments, higher oxygen release from roots of isoetids than elodeids generated oxic conditions to greater sediment depth for Lobelia (4.3 cm) and Littorella (3.0 cm) than for Potamogeton species (1.6-2.2 cm). Sediment oxygen penetration depth fell rapidly to 0.4-1.0 cm for all four species at even modest organic enrichment and oxygen consumption in the sediments. Roots became shorter and isoetid roots became thicker to better supply oxygen to apical meristems.3. Growth of elodeids was strongly inhibited across all levels of organic enrichment of sediments being eight-fold lower at the highest enrichment compared to the unenriched control. Leaf biomass of isoetids increased three-fold by moderate organic enrichment presumably because of greater CO2 supply from sediments being their main CO2

Udgivelsesdato: 2009, Individer kan godt opnå succes, selv om de ikke lever op til kravet om hurtig vækst, stort ressourceforbrug og effektiv konkurrence. Nøjsomhed og samarbejde har længe været underkendte, men de er essensen af strategien "lev stille og bliv gammel", som har succes i ressourcefattige miljøer.

Udgivelsesdato: July 2009, gra

Udgivelsesdato: March 2009, Nostoc pruniforme is a freshwater cyanobacterium forming large spherical colonies of up to several centimeters in diameter. The size and shape result in low surface area to volume (SA/V) ratios that potentially put severe constraints on resource acquisition. In the present study we have specifically examined how N. pruniforme copes with the acquisition of dissolved inorganic carbon (DIC) for photosynthesis. The results showed that N. pruniforme elevates its functional SA/V considerably by locating the cyanobacterial trichomes primarily in the outer shell, while the inner mucus of polysaccharides contains declining densities of trichomes with increasing colony size. N. pruniforme is a very efficient bicarbonate (HCO3-) user and, in addition, actively accumulates large pools of DIC that can support net photosynthesis for >22 h without a supply of external DIC. Both the efficient HCO3- utilization and the large internal DIC pools greatly reduce N. pruniforme dependence on the immediate availability and species of external DIC. The location of the trichomes in the shell, the HCO3- utilization and the internal DIC pools mean that colony size only has a minor effect on photosynthetic rates as a function of external DIC availability.

Udgivelsesdato: 2009, Individer kan godt opnå succes, selv om de ikke lever op til kravet om hurtig vækst, stort ressourceforbrug og effektiv konkurrence. Nøjsomhed og samarbejde har længe været underkendte, men de er essensen af strategien "lev stille og bliv gammel", som har succes i ressourcefattige miljøer.

Udgivelsesdato: March 2009, Nostoc pruniforme is a freshwater cyanobacterium forming large spherical colonies of up to several centimeters in diameter. The size and shape result in low surface area to volume (SA/V) ratios that potentially put severe constraints on resource acquisition. In the present study we have specifically examined how N. pruniforme copes with the acquisition of dissolved inorganic carbon (DIC) for photosynthesis. The results showed that N. pruniforme elevates its functional SA/V considerably by locating the cyanobacterial trichomes primarily in the outer shell, while the inner mucus of polysaccharides contains declining densities of trichomes with increasing colony size. N. pruniforme is a very efficient bicarbonate (HCO3-) user and, in addition, actively accumulates large pools of DIC that can support net photosynthesis for >22 h without a supply of external DIC. Both the efficient HCO3- utilization and the large internal DIC pools greatly reduce N. pruniforme dependence on the immediate availability and species of external DIC. The location of the trichomes in the shell, the HCO3- utilization and the internal DIC pools mean that colony size only has a minor effect on photosynthetic rates as a function of external DIC availability.

Udgivelsesdato: July 2009, gra

Lobelia dortmanna leads a group of small, highly-valued rosette species that grow on coarse, nutrient-poor soils in temperate softwater lakes. They acquire most CO2 for photosynthesis by root uptake and efficient gas transport in large air channels to the leaves. Lobelia is the only species that releases virtually all photosynthetic oxygen from the roots and generates profound day-night changes in oxygen and CO2 in the sediment pore-water. While oxygen release from roots stimulates decomposition and supports VA-mycorrhiza fungi, the ready gas exchange presents a risk of insufficient oxygen supply to the distal root meristems as sediments accumulate organic matter from lake pollution. So the plant with the greatest oxygen release from roots is also the most sensitive to oxygen depletion in sediments and it dies or losses anchorage by shortening the roots from 10 to 2 cm at even modest contents (2.4%) of degradable organic matter. Coatings of oxidized iron on roots in organically enriched sediments reduce radial oxygen loss and, thereby, increase internal concentrations and supply of oxygen to root tips. Oxidized iron is also a redox buffers which may prevent the ingress of sulfides and other reduced toxic solutes during nights. Controlled experiments are under way to test if iron enrichment can help survival of rosette species threatened by lake pollution or whether removal of organic surface sediments is required.

Lobelia dortmanna leads a group of small, highly-valued rosette species that grow on coarse, nutrient-poor soils in temperate softwater lakes. They acquire most CO2 for photosynthesis by root uptake and efficient gas transport in large air channels to the leaves. Lobelia is the only species that releases virtually all photosynthetic oxygen from the roots and generates profound day-night changes in oxygen and CO2 in the sediment pore-water. While oxygen release from roots stimulates decomposition and supports VA-mycorrhiza fungi, the ready gas exchange presents a risk of insufficient oxygen supply to the distal root meristems as sediments accumulate organic matter from lake pollution. So the plant with the greatest oxygen release from roots is also the most sensitive to oxygen depletion in sediments and it dies or losses anchorage by shortening the roots from 10 to 2 cm at even modest contents (2.4%) of degradable organic matter. Coatings of oxidized iron on roots in organically enriched sediments reduce radial oxygen loss and, thereby, increase internal concentrations and supply of oxygen to root tips. Oxidized iron is also a redox buffers which may prevent the ingress of sulfides and other reduced toxic solutes during nights. Controlled experiments are under way to test if iron enrichment can help survival of rosette species threatened by lake pollution or whether removal of organic surface sediments is required.