Increased nutrient loading threatens many freshwater ecosystems. Elevated temperatures may increase the sensitivity to eutrophication in these ecosystems. Higher concentrations of possibly toxic reduced nitrogen (NHx) in the water layer may be expected as production and anaerobic breakdown rates will increase. Apart from temperature, NHx and its effect on aquatic macrophytes will also depend on pH and light. We examined the interactive effects of NHx, temperature, pH and light on Elodea canadensis in a full factorial laboratory experiment. Results demonstrate that high NHx and high temperature together with low pH and low light causes the strongest toxic effects regarding relative growth rate and leaf tissue mortality. The adverse effects of high temperature and low light are most likely caused by increased metabolic activity and reduced photosynthesis, respectively. Severe toxicity at low pH compared to high pH can be ascribed to the ability of E. canadensis to induce a specialised bicarbonate-concentrating pathway at high pH, resulting in much higher carbon availability, needed for detoxification of NHx. We conclude that NHx toxicity will become more pronounced under higher temperatures, but that effects on aquatic macrophytes will strongly depend on pH of the water layer and specific metabolic adaptations of different species.
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