Näringsretention i återskapad våtmark på betesmark : studier av en mad vid Bornsjön

Abstract: It is important to construct or recreate different types of wetlands and study their retention of nutrients, since knowledge of their effectiveness in this matter is poor. In 2003 a wetland was constructed on an old meadow on the western shore of Lake Bornsjön in central Sweden. The main purpose of the wetland was to reduce the amount of phosphorus entering the lake, which is the back-up water supply for Stockholm when the city cannot obtain water from Lake Mälaren. Large areas around Lake Bornsjön consist of agricultural land and the nutrient concentrations in the inflows to the lake are usually relatively high (approx. 1 mg/l total nitrogen and 0.05-0.1 mg/l total phosphorus). The inflow to the wetland is also affected by sewage water from private houses. The purpose of this study was to investigate how this fairly newly constructed wetland functioned during spring high flood and whether the uptake or outflow of nutrients changed when the growing season started. The study was performed between 29 March and 24 May 2005. The flow was measured continuously at two points and water samples were collected at least once and at most four times a week. The wetland received the highest nutrient load during the early part of April. This was also the period when some nutrient retention was estimated to occur. The retention of phosphorus in the wetland over the period was calculated to be 10 % (0.16 kg P/ha) of that reaching it in inflow and the retention of nitrogen 8 % (1.34 kg N/ha). On 3 May, it was raining heavily and the nutrient concentration in the inflow was about 10 times higher than normal. When these extreme values were included in the calculations, the retention was 45 % of phosphorus and 50 % of nitrogen. In the subsequent part of the study period the wetland became a source of nutrients, even after the growing season had started. Decreasing oxygen concentration and increased concentration of total organic carbon in the outflow suggested that decomposition of organic matter was taking place in the wetland. Since the area has not been harvested since the 1980s, organic material has probably accumulated in the wetlands. When the soil thaws in spring, old organic material starts decomposing and this process most likely negates the effects of nitrogen and phosphorus uptake by plants. No studies have been done on this type of wetland during the conditions of middle Sweden. Wetlands and ponds of southern Sweden with a high nutrient load have been found to reduce 560 kg N/ha/yr and 37 kg P/ha/yr in average. Another study on wetlands with overland flow and high nutrient load showed net retention of nitrogen (0-13 %) and sometimes of phosphorus (-48-+20 %). These numbers seems comparable to the results for the present wetland. Wetlands are complicated and dynamic systems that are difficult to control and investigate, and they can frequently become a source of nutrients as well as a sink. The biological diversity and the recreation value of an area often increase when a wetland is created. The wetland in this study seemed to be very much appreciated by people using the area for recreation since the number of birds had increased. It was concluded that wetlands might be regarded as more valuable if other aspects in addition to nutrient retention were considered.