A field facility to simulate climate warming and increased nutrient supply in shallow aquatic ecosystems

Global warming and excess nitrogen deposition can exert strong impacts on aquatic populations, communities, and ecosystems. However, experimental data to establish clear cause-and-effect relationships in naturally complex field conditions are scarce in aquatic environments. Here, we describe the design and performance of a unique outdoor enclosure facility used to simulate warming, increased nitrogen supply, and both factors combined in a littoral freshwater wetland dominated by common reed, Phragmites australis. The experimental system effectively simulated a 2.8 °C climate warming scenario over an extended period, capturing the natural temperature variations in the wetland at diel and seasonal scales with only small deviations. Excess nitrogen supply enhanced nitrate concentrations especially in winter when it was associated with increased concentration of ammonium and dissolved organic carbon. Nitrogen also reduced dissolved oxygen concentrations, particularly in the summer. Importantly, by stimulating biological activity, warming enhanced the nitrogen uptake capacity of the wetland during the winter, emphasizing the need for multifactorial global change experiments that examine both warming and nitrogen loading in concert. Establishing similar experiments across broad environmental gradients holds great potential to provide robust assessments of the impacts of climate change on shallow aquatic ecosystems.