Since the re-establishment of a more natural water regime is considered by water management in wetlands with artificially stable water levels, the biogeochemical and ecological effects of water level fluctuation with different nutrient loads should be investigated. This is particularly important for biodiverse mesotrophic fens, sensitive to acidification and eutrophication. Mesocosm experiments were conducted to study the interactive effects of water level fluctuation and P-enrichment under controlled summer conditions, using peat cores including vegetation from three fens differing in biogeochemical characteristics.The effects of fluctuating water levels on biogeochemistry and vegetation appeared to be highly dependent on peat chemistry, and more important than the effects of P-enrichment. Only when plant growth was stimulated by a favorable water level regime, P-enrichment led to increased P-consumption by plants. In rich fens with a high soil Ca-content, 7 weeks of lowered water table (−15 cm) did not lead to a drop in pH. However, soil subsidence, increased N-availability and decline of the rich fen bryophyte Scorpidium scorpioides give cause to concern. 7 weeks of inundation (+15 cm) offered possibilities for restoration in these fens, since alkalinity and Ca-concentrations increased, while soil P-mobilization did not occur. Even P-enrichment did not result in increased P-availability, presumably due to Ca-related precipitation of P. In rich fens with a high soil Fe-content, water table lowering should be avoided as well, because of soil subsidence, increased N-availability, decline of the rich fen bryophyte Calliergon giganteum, plus acidification due to Fe-oxidation. Shallow inundation, however, is also harmful, especially after mowing and with P-rich water, because plant growth was hampered, presumably by toxicity of NH 4 + and/or Fe(II). In mineral-poor fens with a high soil P-and S-content, shallow inundation should be avoided, because of tremendous internal P-mobilization. Vitality of the dominant bryophyte Sphagnum palustre, however, was not affected. Low water tables affected neither vegetation, nor biogeochemistry, showing resistance to short-term drought in these fens.Given the strong mediating effect of soil chemistry, risks and benefits of re-establishment of fluctuating water levels with clean or P-rich water need to be considered for different fen types separately in water and nature management.

Symbiotic relationships are common in nature, and are important for individual fitness and sustaining species populations. Global change is rapidly altering environmental conditions, but, with the exception of coralmicroalgae interactions, we know little of how this will affect symbiotic relationships. We here test how the effects of ocean acidification, from rising anthropogenic CO 2 emissions, may alter symbiotic interactions between juvenile fish and their jellyfish hosts. Fishes treated with elevated seawater CO 2 concentrations, as forecast for the end of the century on a business-as-usual greenhouse gas emission scenario, were negatively affected in their behaviour. The total time that fish (yellowtail scad) spent close to their jellyfish host in a choice arena where they could see and smell their host was approximately three times shorter under future compared with ambient CO 2 conditions. Likewise, the mean number of attempts to associate with jellyfish was almost three times lower in CO 2 -treated compared with control fish, while only 63% (high CO 2 ) versus 86% (control) of all individuals tested initiated an association at all. By contrast, none of three fish species tested were attracted solely to jellyfish olfactory cues under present-day CO 2 conditions, suggesting that the altered fish-jellyfish association is not driven by negative effects of ocean acidification on olfaction. Because shelter is not widely available in the open water column and larvae of many (and often commercially important) pelagic species associate with jellyfish for protection against predators, modification of the fish-jellyfish symbiosis might lead to higher mortality and alter species population dynamics, and potentially have flow-on effects for their fisheries.