Ane-Marie Løvendahl Raun:
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 lower oxygen concentrations. The tropical seagrasses were surprisingly tolerant to oxygen depletion in the water column and none of them showed reduced growth within the 72 hours of anoxic treatment in the dark, although signs of stress on the photosynthesis apparatus were observed in some treatments just after anoxic exposure. All investigated freshwater species were heavily affected by high concentrations of labile organic matter in the sediment. They all showed decreased root formation and elodeid plants, furthermore, had reduced leaf formation. Higher levels of bicarbonate were unable to alleviate the negative impact of organic enrichment of sediment for all the tested species.
No doubt that both eutrophication and global warming are challenging to the aquatic macrophyte and their distribution, although some species are more tolerant than others. In addition, the combined effect of both factors seems to cause synergetic stress on Z. marina and that is, most likely, also the case for other aquatic macrophyte species. The future increase in temperature will reduce the tolerance to oxygen depletion and further reduce areal distribution and re‐colonization potential. If we are unable to stop climate warming, we will have to work harder to avoid water column oxygen depletion.