Constructed Wetlands and Deconstructed Discourses : Greenhouse gas fluxes and discourses on purifying capacities

Sammanfattning: This thesis is a study within the sciences ofconstructed wetlands. It has two sections. Section one is a natural scienrific study ofgas fluxes from construcred wetlands and section two is an analysis ofhow natural sciences describe constructed wetlands.The aim of section one was to determine methane and nitrous oxide fluxes between the atmosphere and constructed wetlands. The result from these gas flux studies showed that methane fluxes ranged between -375 and 1739 mg m-2 day1, whereas the nitrous oxide fluxes ranged from consumption at -350 to emissions at 1791 Jlg m-2 h-1. The annual inregrated methane fluxes were 87 mg m-2 day1, whereas the inregrated nitrous oxide fluxes resulred in an overall average of 126 µg N2O m-2 h-1. There were large temporal and spatial variations in gas fluxes but no differences between night and day. The methane rates were lower than the average rate, 225 mg m-2 day1, found at a constructed pilot scale wetland treating dairy wastewater in New Zealand (Tanner, 1997), but the cited measuremenrs were only taken during one monrh in mid-summer. The average and range of methane flux values in my study also showed that the methane emission rates were of the same magnitude as those observed from natural wetlands with similar vegetational composition. The nitrous oxide emissions observed in the Nykvarn study were ca. three to ten times higher than in previous studies of constructed wetlands treating wastewater, whereas the nitrous oxide consumption rate was approximately 80 times higher. The nitrous oxide fluxes were 3 to 180 times higher compared to other wetland systems. Emission factors were calculated according to the methods presenred by the Inrergovernmenral Panel of Climate Change (IPCC). The calculated nitrous oxide emission factors from Nykvarn were always lower (0.02 to 0.27%) compared to the default factor provided by the IPCC (0.75%).The aim of section two was to analyse descriptions on purifying capacities in constructed wetlands in selected scienrific articles. This was made with help of a discourse analysis, which can be explained as a study of communication. The results from this analysis showed that the wetlands were described in terms of self-purifying, biochemical cycles. The cycle concept is used in all three discourses (case I to 3) but it is comprehended differently. In case I the cycling processes are treated as inrrinsic and unproblematic in relation to purification. In case 2, the purifying cycles are treated as dynamic, complex and problematic, but humans could influence them and develop their inrrinsic ability. In case 3 th~ cycling processes became a tool, allowing us to grasp, handle, act upon and influence the wetlands. In the thesis these differences are discussed with respecr to scienrific ideals including language in general and metaphors and concepts in parricular as weil as to the ambiguous relationship between humans and wetlands. Finally, the thesis conrain a reflection on the studies discussed in section I and II. The reflection aims at developing new paths for of gas flux investigations (with biosemiotic perspectives) in research fields related to constructed wetlands.