Das Projekt "Interactive effects of global warming and nutrient enrichment on litter decomposition in freshwater marshes" wird vom Umweltbundesamt gefördert und von Eawag - Das Wasserforschungsinstitut des ETH-Bereichs durchgeführt. Many human activities have repercussions that are profoundly altering natural ecosystems at large-scales. The physical interference by humans with the climate systems has been intensively studied and has become increasingly clear. Significant progress has also been made in assessing range shifts of species and various other ecological responses such as altered species interactions. However, the consequences for ecosystem functioning and possible feedbacks on climate are still poorly understood. This is particularly true for aquatic ecosystems. In addition, information is exceedingly scant on the interactive effects of multiple environmental factors that are changing simultaneously. The proposed project builds on our previous experiments in a freshwater marsh to assess the combined effects of elevated temperature and nutrient loading on litter decomposition, a critical component of carbon cycling at the local and global scale in a variety of ecosystems. The general hypothesis is that impacts of elevated temperature, nutrient loading and the interaction of both are not readily predictable from current theory or through simple laboratory experiments, and that the magnitude of effects is such that important ramifications for elemental flows both within wetlands and across their boundaries are likely to result. I propose testing at this stage a series of specific hypotheses derived from our previous findings and relating to the effects of temperature and nutrients on decomposition. The focus will be on three priorities: (1) testing whether the lack of stimulation of litter decomposition by nitrogen enrichment that we observed is due to the limiting role of phosphorus in the marsh; (2) testing whether a shift in life-history patterns of specific detritivore taxa is responsible for the observed dramatic acceleration of litter decomposition in spring under simulated global warming; and (3) assessing whether the observed lack of effects by elevated temperature and nutrient loading on stem litter decomposition may be related to changes in O2 regimes during long-term operation of enclosures. The backbone of the proposed experiments to address these questions is a unique manipulative field experiment in enclosures installed in a littoral marsh dominated by Phragmites australis. Enclosures are heated to 4 C above ambient water temperature or enriched with Ca(NO3)2 or both. The experiment is set up as a randomized block design (N = 4) with two factors (temperature and nitrate enrichment), each with two levels. An open-marsh control is also included. The first two questions will be addressed in combined field enclosure and laboratory microcosm experiments, while the third question will be addressed in a field enclosure experiment only. The processes considered in different experiments include litter decomposition, nutrient immobilization, microbial respiration and productivity, enzymatic activity, and various measures of detritivore performance.